Slide 1 ___________________________________ ___________________________________ Intermolecular Forces Love & Hate in the Molecular Realm ___________________________________ ___________________________________ 1 ___________________________________ ___________________________________ ___________________________________ Slide 2 ___________________________________ If I put 2 molecules into a sealed flask, what could happen? 1. 2. 3. They ignore each other. They LOVE each other – they’re attracted to each other They HATE each other – they repel each other ___________________________________ ___________________________________ ___________________________________ 2 ___________________________________ ___________________________________ ___________________________________ Slide 3 ___________________________________ REMEMBER: MOLECULES MOVE! ___________________________________ ___________________________________ (except at 0 K) ___________________________________ 3 ___________________________________ ___________________________________ ___________________________________ Slide 4 ___________________________________ If they LOVE each other, what would that look like? ___________________________________ ___________________________________ Initially Later ___________________________________ 4 ___________________________________ ___________________________________ ___________________________________ Slide 5 ___________________________________ If they HATE each other, what would that look like? ___________________________________ ___________________________________ Initially Later ___________________________________ 5 ___________________________________ ___________________________________ ___________________________________ Slide 6 ___________________________________ If they IGNORE each other, what would that look like? ___________________________________ ___________________________________ Initially Later ___________________________________ 6 ___________________________________ ___________________________________ ___________________________________ Slide 7 ___________________________________ What determines LOVE or HATE? The structure of the molecule. ___________________________________ What is the structure of a molecule? ___________________________________ H Br e- What’s in the nuclei? Protons! ___________________________________ 7 ___________________________________ ___________________________________ ___________________________________ Slide 8 ___________________________________ Molecular structure is all about… POSITIVE & NEGATIVE CHARGES! ___________________________________ So Love & Hate is all about… ___________________________________ Opposites attract, like repel! ___________________________________ 8 ___________________________________ ___________________________________ ___________________________________ Slide 9 ___________________________________ Types of Intermolecular Forces 1. 2. 3. 4. 5. London Dispersion forces, aka Van der Waal’s forces, aka Instantaneous dipoleinduced dipole forces. Dipole-Dipole interactions Hydrogen bonding – particularly strong case of dipole-dipole interaction Ionic forces Mixed forces ___________________________________ ___________________________________ ___________________________________ 9 ___________________________________ ___________________________________ ___________________________________ Slide 10 London Dispersion forces, aka Van der Waal’s forces, aka Instantaneous dipoleinduced dipole forces. ___________________________________ This is NOT the strongest, but it is the primary intermolecular force. ___________________________________ All atoms or molecules with electrons have Van der Waal’s forces – so ALL atoms or molecules have Van der Waal’s forces ___________________________________ ___________________________________ 10 ___________________________________ ___________________________________ ___________________________________ Slide 11 ___________________________________ Instantaneous dipole-induced dipole forces ___________________________________ Induced dipole Br δ- Instantaneous dipole δ+ Br Br δ+ δBr ___________________________________ The electron cloud is mobile. Charge density is constantly moving around ___________________________________ 11 ___________________________________ ___________________________________ ___________________________________ Slide 12 ___________________________________ How Great is THAT!?!?!? ___________________________________ Induced love Br δ- Instantaneous love δ+ Br Br δ+ ___________________________________ δBr ___________________________________ Because the induced love is ALWAYS a mirror image of the instantaneous love, dispersion forces are ALWAYS attractive 12 ___________________________________ ___________________________________ ___________________________________ Slide 13 ___________________________________ Dispersion Forces are ALWAYS ATTRACTIVE All molecules like each other, at least a little bit. So all molecules stick together, at least a little bit. If they didn’t… …the universe would be a much more chaotic place! Occasional repulsion would have things flying apart all over the place! ___________________________________ ___________________________________ ___________________________________ 13 ___________________________________ ___________________________________ ___________________________________ Slide 14 ___________________________________ Van der Waal’s forces Van der Waal’s forces get stronger as the temporary dipole gets stronger. ___________________________________ The temporary dipole is caused by electron mobility, so the more electrons the stronger the Van der Waal’s forces. ___________________________________ # electrons increases as # protons, so the heavier the molecule the stronger the Van der Waal’s forces. ___________________________________ 14 ___________________________________ ___________________________________ ___________________________________ Slide 15 ___________________________________ Alkanes Methane – CH4 Ethane – CH3CH3 Propane – CH3CH2CH3 Butane - CH3CH2CH2CH3 Pentane - CH3CH2CH2CH2CH3 Hexane - CH3CH2CH2CH2CH2CH3 Heptane - CH3CH2CH2CH2CH2CH2CH3 Octane - CH3CH2CH2CH2CH2CH2CH2CH3 ___________________________________ ___________________________________ ___________________________________ 15 ___________________________________ ___________________________________ ___________________________________ Slide 16 ___________________________________ What do they look like? Here’s propane (C3H8). CH3CH2CH3 ___________________________________ ___________________________________ ___________________________________ 16 ___________________________________ ___________________________________ ___________________________________ Slide 17 ___________________________________ When you include the electrons… “Space-filling model” CH3CH2CH3 ___________________________________ ___________________________________ ___________________________________ 17 ___________________________________ ___________________________________ ___________________________________ Slide 18 Here’s pentane (C5H12): CH3CH2CH2CH2CH3 ___________________________________ ___________________________________ ___________________________________ ___________________________________ 18 ___________________________________ ___________________________________ ___________________________________ Slide 19 ___________________________________ Space-filling model of pentane ___________________________________ ___________________________________ ___________________________________ 19 ___________________________________ ___________________________________ ___________________________________ Slide 20 ___________________________________ Pentane is just a “longer caterpillar” than propane. ___________________________________ That makes it easier to compare these molecules, they are homologues. ___________________________________ ___________________________________ 20 ___________________________________ ___________________________________ ___________________________________ Slide 21 ___________________________________ What do you know about these molecules? Methane – CH4 Ethane – CH3CH3 Propane – CH3CH2CH3 Butane - CH3CH2CH2CH3 Pentane - CH3CH2CH2CH2CH3 Hexane - CH3CH2CH2CH2CH2CH3 Heptane - CH3CH2CH2CH2CH2CH2CH3 Octane - CH3CH2CH2CH2CH2CH2CH2CH3 ___________________________________ ___________________________________ ___________________________________ 21 ___________________________________ ___________________________________ ___________________________________ Slide 22 ___________________________________ What do you know about these molecules? Methane – gas at standard T & P Ethane – gas at standard T & P Propane – gas at standard T & P – Liquid under slight pressure Butane - gas at standard T & P – Liquid under slight pressure Pentane - Liquid Hexane - Liquid Heptane - Liquid Octane - Liquid ___________________________________ ___________________________________ ___________________________________ 22 ___________________________________ ___________________________________ ___________________________________ Slide 23 ___________________________________ Solids, Liquids, and Gases What is the difference between a solid, a liquid, and a gas microscopically? ___________________________________ How tightly stuck together the molecules are!!! Solids are stuck together more than liquids that are stuck together more than gases ___________________________________ ___________________________________ 23 ___________________________________ ___________________________________ ___________________________________ Slide 24 Sticking together is a function of TWO things: ___________________________________ How much you like each other…and how much you are trying to get away from each other. ___________________________________ In the context of molecules, this is a question of intermolecular forces vs. kinetic energy. ___________________________________ ___________________________________ 24 ___________________________________ ___________________________________ ___________________________________ Slide 25 ___________________________________ ALL MOLECULES MOVE! ___________________________________ ___________________________________ (except at 0 K) ___________________________________ 25 ___________________________________ ___________________________________ ___________________________________ Slide 26 Motion = Kinetic Energy = Temperature ___________________________________ Kinetic energy is energy of motion. ___________________________________ Temperature is a measure of the “mean kinetic energy of molecules”. Temperature reflects your desire to escape… ___________________________________ ___________________________________ 26 ___________________________________ ___________________________________ ___________________________________ Slide 27 ___________________________________ Solids, Liquids, and Gases & Heat What happens when you heat up a solid? ___________________________________ Eventually it melts – why? Adding heat adds energy to the molecules, when they have enough energy they can escape their attraction to their neighbors! ___________________________________ ___________________________________ 27 ___________________________________ ___________________________________ ___________________________________ Slide 28 ___________________________________ When 2 molecules like each other.. ___________________________________ ___________________________________ ___________________________________ 28 ___________________________________ ___________________________________ ___________________________________ Slide 29 ___________________________________ ___________________________________ ___________________________________ ___________________________________ 29 ___________________________________ ___________________________________ ___________________________________ Slide 30 ___________________________________ But, they are moving… ___________________________________ ___________________________________ ___________________________________ 30 ___________________________________ ___________________________________ ___________________________________ Slide 31 ___________________________________ If they are HOT HOT HOT! ___________________________________ ___________________________________ ___________________________________ 31 ___________________________________ ___________________________________ ___________________________________ Slide 32 ___________________________________ If they are very COLLLLDDD ___________________________________ ___________________________________ ___________________________________ 32 ___________________________________ ___________________________________ ___________________________________ Slide 33 ___________________________________ Intermolecular forces… …only depend on distance between the molecules. ___________________________________ I can’t change the structure of the molecule. But the farther apart they are, the smaller the force they feel. ___________________________________ [Think gravity and distance from the center of the earth.] ___________________________________ 33 ___________________________________ ___________________________________ ___________________________________ Slide 34 ___________________________________ I can add heat… “Hot” and “Cold” are relative… ___________________________________ Phase is a balance between the temperature (kinetic energy) of the molecules that is trying to separate them and the intermolecular forces which are trying to hold them together. Melting point or boiling point is the kinetic energy where the balance tips. ___________________________________ ___________________________________ 34 ___________________________________ ___________________________________ ___________________________________ Slide 35 ___________________________________ Van der Waal’s Forces are… …the first consideration – but not the last! ___________________________________ But ALL the intermolecular forces are about CHARGE! (Opposites attract.) ___________________________________ ALL intermolecular forces are ATTRACTIVE in the end. ___________________________________ 35 ___________________________________ ___________________________________ ___________________________________ Slide 36 ___________________________________ Dipole – Dipole Interactions ___________________________________ Br Permanent dipole δ- Permanent dipole δ+ H H δ+ ___________________________________ δBr ___________________________________ 36 ___________________________________ ___________________________________ ___________________________________ Slide 37 ___________________________________ Seems REPULSIVE – but it’s really not Permanent dipole H Permanent dipole δ+ H ___________________________________ Br δ+ δ- ___________________________________ δBr MOLECULES ARE MOBILE. They always align themselves. That’s why I say that in the end all intermolecular forces are attractive. ___________________________________ 37 ___________________________________ ___________________________________ ___________________________________ Slide 38 ___________________________________ Dipole – Dipole interactions A molecule with a permanent dipole is called a “polar molecule”. ___________________________________ All polar molecules have Dipole-Dipole interactions in ADDITION TO Van der Waal’s forces. ___________________________________ ___________________________________ 38 ___________________________________ ___________________________________ ___________________________________ Slide 39 ___________________________________ Dipole – Dipole interactions Dipole-Dipole interactions are in ADDITION TO Van der Waal’s forces. ___________________________________ They are generally weaker and just add on to VDW forces with ONE EXCEPTION. ___________________________________ ___________________________________ 39 ___________________________________ ___________________________________ ___________________________________ Slide 40 ___________________________________ Hydrogen Bonding – just a special case of dipole-dipole interactions Hydrogen bonding is a dipole-dipole interaction that occurs when hydrogen is bonded to something very electronegative like F, O, or N. ___________________________________ ___________________________________ It is just a very strong dipole-dipole interaction because of the very polar nature of the H-F, H-O, or H-N bond. ___________________________________ 40 ___________________________________ ___________________________________ ___________________________________ Slide 41 ___________________________________ Hydrogen Bonding – just a special case of dipole-dipole interactions ___________________________________ O Strongly polar dipole H δ-- δ++ Strongly polar dipole δ++ H ___________________________________ δ-O ___________________________________ 41 ___________________________________ ___________________________________ ___________________________________ Slide 42 ___________________________________ Compare H2O to H2S Which would you expect to have the higher boiling point? ___________________________________ H2O has a molar mass of 18 g/mol H2S has a molar mass of 34 g/mol ___________________________________ Based on Van der Waal’s forces alone, H2S should have the higher boiling point. ___________________________________ 42 ___________________________________ ___________________________________ ___________________________________ Slide 43 ___________________________________ Compare H2O to H2S The boiling point of water is 373 K. ___________________________________ The boiling point of H2S is 213 K. ___________________________________ H2S is a gas at room temperature while water is a liquid! No FON, no Hydrogen bonding ___________________________________ 43 ___________________________________ ___________________________________ ___________________________________ Slide 44 ___________________________________ Ion – Ion Interactions ___________________________________ Actual separation of charge Cl Na + - Actual separation of charge + Na ___________________________________ Cl ___________________________________ 44 ___________________________________ ___________________________________ ___________________________________ Slide 45 ___________________________________ Ion-Ion interactions The strongest possible interaction. ___________________________________ The complete charge separation makes it a HUGE dipole-dipole type interaction. This is why most ionic compounds are solids at room temperature. ___________________________________ ___________________________________ 45 ___________________________________ ___________________________________ ___________________________________ Slide 46 ___________________________________ Permanent dipole-induced dipole forces ___________________________________ Induced dipole Br δ- Permanent dipole δ+ H Br δ+ ___________________________________ δBr ___________________________________ 46 ___________________________________ ___________________________________ ___________________________________ Slide 47 ___________________________________ Dipole – Induced Dipole interactions This is a special case of a Dipole – Dipole interaction where there are 2 different molecules involved and only 1 of them is polar. ___________________________________ ___________________________________ Generally weaker than a permanent DipoleDipole interaction, it is still IN ADDITION TO Van der Waal’s forces. ___________________________________ 47 ___________________________________ ___________________________________ ___________________________________ Slide 48 ___________________________________ All the forces… 1. 2. 3. 4. Van der Waal’s/Dispersion forces – FIRST consideration. Weakest for single bond BUT it is a more global force. Heavier molecules have bigger VDW forces. Dipole-Dipole forces – add on to VDW forces (with ONE exception - #3). If the molecules have similar mass and shape. The one with a permanent dipole will have a higher boiling point. Hydrogen Bonding TRUMPS VDW Ionic forces TRUMP EVERYTHING ___________________________________ ___________________________________ ___________________________________ 48 ___________________________________ ___________________________________ ___________________________________ Slide 49 ___________________________________ ___________________________________ ___________________________________ ___________________________________ 49 ___________________________________ ___________________________________ ___________________________________ Slide 50 ___________________________________ NaF vs. F2 What do you know about these 2 molecules? ___________________________________ NaF is an ionic solid F2 is a gas at room temp ___________________________________ NaF has a molar mass of 42 g/mol, F2 has a molar mass of 38 g/mol. ___________________________________ 50 ___________________________________ ___________________________________ ___________________________________ Slide 51 ___________________________________ Ion-ion interactions are the strongest Based on Van der Waal’s forces, you’d expect NaF and F2 to be similar. ___________________________________ The powerful ionic forces of NaF make it a solid – trumping the Van der Waal’s interaction. ___________________________________ NaF melts at 1266 K and boils at 1968 K F2 melts at 53 K and boils at 85 K ___________________________________ 51 ___________________________________ ___________________________________ ___________________________________ Slide 52 ___________________________________ HBr vs. Cl2 What do you know about these 2 molecules? ___________________________________ HBr is a gas at room temp Cl2 is a gas at room temp HBr has a molar mass of 81 g/mol Cl2 has a molar mass of 71 g/mol ___________________________________ HBr is polar, Cl2 is non-polar ___________________________________ 52 ___________________________________ ___________________________________ ___________________________________ Slide 53 ___________________________________ HBr vs Cl2 So HBr is heavier – more van der Waal’s forces ___________________________________ HBr is polar – dipole-dipole forces also So you would think that HBr has the higher boiling point…and so we go to wikipedia and find… ___________________________________ ___________________________________ 53 ___________________________________ ___________________________________ ___________________________________ Slide 54 ___________________________________ HBr boils at 207 K, Cl2 boils at 239 K WTWikipedia?!?!? ___________________________________ ___________________________________ ___________________________________ 54 ___________________________________ ___________________________________ ___________________________________ Slide 55 ___________________________________ So, why doesn’t it? Geometry is also an issue! ___________________________________ ___________________________________ ___________________________________ 55 ___________________________________ ___________________________________ ___________________________________ Slide 56 ___________________________________ Geometry ___________________________________ Br ___________________________________ H ___________________________________ 56 ___________________________________ ___________________________________ ___________________________________ Slide 57 ___________________________________ ___________________________________ Cl Cl ___________________________________ ___________________________________ 57 ___________________________________ ___________________________________ ___________________________________ Slide 58 ___________________________________ No separation of charge, no dipole (including VDW forces) ___________________________________ Cl Br Cl ___________________________________ H ___________________________________ 58 ___________________________________ ___________________________________ ___________________________________ Slide 59 This is a warning…your final warning… ___________________________________ There are limits to the easy comparisons. ___________________________________ If you have two structurally similar molecules, then the heavier one will have the higher boiling point. ___________________________________ ___________________________________ 59 ___________________________________ ___________________________________ ___________________________________ Slide 60 ___________________________________ Br2 boils at 332 K, Cl2 at 239 K ___________________________________ Cl Br Cl ___________________________________ Br Both are non-polar “bar-shaped” molecules ___________________________________ 60 ___________________________________ ___________________________________ ___________________________________ Slide 61 ___________________________________ Polar molecules are higher… Permanent dipoles are sort of a bonus. ___________________________________ Take two similarly shaped molecules with similar molar masses and the polar one will have a higher boiling point than the lower one. ___________________________________ But if the molar masses are different enough, the polar nature won’t save you. ___________________________________ 61 ___________________________________ ___________________________________ ___________________________________ Slide 62 ___________________________________ A few molecules Molecule Molar mass Polar/nonpolar Boiling point chloropropane 78.5 g/mol Weakly Polar 320 K Hexane 86 g/mol Non-polar 342 K Chlorine 70.9 g/mol Non-polar 239 K Calcium sulfide 72.1 g/mol ionic Melts at 2800 K Sulfur dioxide 64 g/mol Polar 263 K Water 18.02 g/mol Polar 373 K ___________________________________ ___________________________________ CH3CH2CH2Cl CH3CH2CH2CH2CH2CH3 ___________________________________ 62 ___________________________________ ___________________________________ ___________________________________ Slide 63 So if you are comparing 2 molecules: 1. 2. 3. 4. Look for ionic compounds – they have the strongest forces – trumps EVERYTHING Look for hydrogen bonding – hydrogen bonding is the 2nd strongest and will usually swamp van der Waal’s if the molecules are SIMILAR size Van der Waal’s forces – heavier wins Dipole-dipole forces – sort of a tiebreaker ___________________________________ ___________________________________ ___________________________________ ___________________________________ 63 ___________________________________ ___________________________________ ___________________________________ Slide 64 ___________________________________ Limits of hydrogen bonding: Octane – CH3CH2CH2CH2CH2CH2CH2CH3 Molar mass = 114.23 g/mol Non-polar molecule Boiling point = 399 K ___________________________________ ___________________________________ Ethanol – CH3CH2-OH Molar mass =46.07 g/mol Hydrogen Bonding Boiling point = 351 K ___________________________________ 64 ___________________________________ ___________________________________ ___________________________________ Slide 65 ___________________________________ Limits of hydrogen bonding: Octane – CH3CH2CH2CH2CH2CH2CH2CH3 Molar mass = 114.23 g/mol Non-polar molecule Boiling point = 399 K ___________________________________ Ethanol – CH3CH2-OH Molar mass =46.07 g/mol Hydrogen Bonding Boiling point = 351 K ___________________________________ Water – H2O Molar mass = 18.02 g/mol Hydrogen bonding Boiling point = 373 K ___________________________________ 65 ___________________________________ ___________________________________ ___________________________________ Slide 66 ___________________________________ Limits of hydrogen bonding: Octane – CH3CH2CH2CH2CH2CH2CH2CH3 Molar mass = 114.23 g/mol Non-polar molecule Boiling point = 399 K ___________________________________ ___________________________________ Octanol – CH3CH2CH2CH2CH2CH2CH2CH2OH Molar mass = 130.23 g/mol Hydrogen bonding Boiling point = 468 K Bigger ___________________________________ 66 ___________________________________ ___________________________________ ___________________________________ Slide 67 ___________________________________ Limits of hydrogen bonding: Ethanol – CH3CH2-OH Molar mass =46.07 g/mol Hydrogen Bonding Boiling point = 351 K ___________________________________ Ethane – CH3CH3 Molar mass = 30.07 g/mol Non-polar Boiling point = 185 K ___________________________________ Almost double! The hydrogen bond is a much bigger part of the smaller molecule ___________________________________ 67 ___________________________________ ___________________________________ ___________________________________ Slide 68 ___________________________________ Bottom Line The more similar the molecules are in size and shape the easier it is to determine the size of the relative forces. ___________________________________ If they are very different in size (ethanol vs. octane) or shape (HBr vs. Cl2) we are just making educated guesses. ___________________________________ ___________________________________ 68 ___________________________________ ___________________________________ ___________________________________ Slide 69 ___________________________________ In order of importance 1. 2. 3. 4. Ionic forces (biggest by a lot) Hydrogen bonding (special case of…) Dipole-Dipole Van der Waal’s ___________________________________ ___________________________________ But 3 and 4 are much weaker than 1 and 2. 3 only matters if the molecules are similar sizes. ___________________________________ 69 ___________________________________ ___________________________________ ___________________________________ Slide 70 ___________________________________ ___________________________________ ___________________________________ ___________________________________ 70 ___________________________________ ___________________________________ ___________________________________ Slide 71 ___________________________________ Here’s some… Physical properties that show “intermolecular forces”: 1. Boiling point 2. Melting point 3. Surface tension 4. Viscosity 5. Capillary action 6. Evaporation ___________________________________ ___________________________________ ___________________________________ 71 ___________________________________ ___________________________________ ___________________________________ Slide 72 ___________________________________ Phase changes Intermolecular Forces are attractions between molecules. ___________________________________ Temperature is a measure of kinetic energy. Boiling Point (or Freezing Point) are measures of the strength of intermolecular forces: the higher the temperature, the more kinetic energy required to separate the molecules. ___________________________________ ___________________________________ 72 ___________________________________ ___________________________________ ___________________________________ Slide 73 ___________________________________ Not just temperature… We mentioned TWO things that affected molecules and their interactions: 1. Energy 2. Space ___________________________________ ___________________________________ Another way of looking at “space” is pressure. ___________________________________ 73 ___________________________________ ___________________________________ ___________________________________ Slide 74 ___________________________________ What is “pressure”? Pressure = Force Area ___________________________________ Pressure is squeezing the molecules together! ___________________________________ ___________________________________ 74 ___________________________________ ___________________________________ ___________________________________ Slide 75 ___________________________________ Phase Changes You can create a phase change, by changing the temperature. ___________________________________ Consider a flask full of steam at 200 C. ___________________________________ If I start cooling it down, what happens? It condenses into liquid water. When? ___________________________________ NOT (necessarily) 100 C. 75 ___________________________________ ___________________________________ ___________________________________ Slide 76 ___________________________________ Normal Boiling Point 100 C is the “normal boiling point” of water. What’s the “normal” for? ___________________________________ Normal means at standard pressure, 1 atm. ___________________________________ One way to condense steam is to decrease the temperature, another way is to increase the pressure. ___________________________________ 76 ___________________________________ ___________________________________ ___________________________________ Slide 77 ___________________________________ It’s all about forces! ___________________________________ Kinetic Energy ___________________________________ Pressure Intermolecular Force ___________________________________ 77 ___________________________________ ___________________________________ ___________________________________ Slide 78 ___________________________________ Phase Diagrams A “phase diagram” collects all the P, T and phase information and displays it in one simple graph. ___________________________________ ___________________________________ ___________________________________ 78 ___________________________________ ___________________________________ ___________________________________ Slide 79 ___________________________________ Phase Diagram for CO2 ___________________________________ Liquid Pressure Solid What do you call this? Gas 1 atm ___________________________________ Sublimation! ___________________________________ -78 C Temp 79 ___________________________________ ___________________________________ ___________________________________ Slide 80 ___________________________________ Phase Diagram for H2O ___________________________________ Liquid Pressure 1 atm ___________________________________ Solid Gas 0 C Temp ___________________________________ 100 C 80 ___________________________________ ___________________________________ ___________________________________ ___________________________________ Phase Diagrams ___________________________________ Liquid 1 atm Pressure Slide 81 ___________________________________ What do you call this? Solid Gas Triple Point – solid, liquid and gas coexisting together! Temp ___________________________________ 81 ___________________________________ ___________________________________ ___________________________________ Slide 82 ___________________________________ Phase Diagrams ___________________________________ Liquid Pressure 1 atm ___________________________________ What do you call this? Solid Gas Critical point – No liquid beyond this – gas of liquid density Temp ___________________________________ 82 ___________________________________ ___________________________________ ___________________________________ Slide 83 ___________________________________ Energy of Phase Changes How do you define “boiling”? ___________________________________ Vapor pressure = atmospheric pressure ___________________________________ What’s vapor pressure? It’s the pressure exerted by the vapor above a liquid. ___________________________________ 83 ___________________________________ ___________________________________ ___________________________________ Slide 84 ___________________________________ As you raise T, you raise Pvap until Pvap = Patm ___________________________________ ___________________________________ ___________________________________ 84 ___________________________________ ___________________________________ ___________________________________ Slide 85 ___________________________________ As you raise T, you raise Pvap until Pvap = Patm ___________________________________ ___________________________________ ___________________________________ 85 ___________________________________ ___________________________________ ___________________________________ Slide 86 ___________________________________ Remember Ptot = P1 + P2 + The vapor crowds out the air above the solution since Ptot must always be Patm ___________________________________ ___________________________________ ___________________________________ 86 ___________________________________ ___________________________________ ___________________________________ Slide 87 ___________________________________ Remember Ptot = P1 + P2 + Ptot must always be Patm. When Pvap = Patm, it’s all water vapor and WE ARE BOILING! ___________________________________ ___________________________________ ___________________________________ 87 ___________________________________ ___________________________________ ___________________________________ Slide 88 ___________________________________ What do you have to do to become “vapor”? You have to go from a liquid to a gas! What do you need to do to go from a liquid to a gas? GAIN ENERGY! ___________________________________ ___________________________________ ___________________________________ 88 ___________________________________ ___________________________________ ___________________________________ Slide 89 ___________________________________ Remember, all molecules like each other. So the difference between a solid, a liquid and a gas… ___________________________________ ___________________________________ ___________________________________ Solid Liquid Gas 89 ___________________________________ ___________________________________ ___________________________________ Slide 90 ___________________________________ …is all relative to the Energy There are two different energies (or forces). The attraction between molecules, the individual energy of the molecules. ___________________________________ ___________________________________ ___________________________________ Solid Liquid Gas 90 ___________________________________ ___________________________________ ___________________________________ Slide 91 ___________________________________ Suppose I tie myself to one of you using a noodle. Could you escape? ___________________________________ ___________________________________ ___________________________________ 91 ___________________________________ ___________________________________ ___________________________________ Slide 92 ___________________________________ Of course you could. You just start walking away and the noodle breaks. ___________________________________ Suppose I tie myself to you using a piece of thread? ___________________________________ You may have to walk faster or pull harder but you can still break away. ___________________________________ 92 ___________________________________ ___________________________________ ___________________________________ Slide 93 ___________________________________ Suppose I tie myself to you using a piece of copper wire? ___________________________________ You may have to run or tug or get your friends to also tug, but you can break the wire. ___________________________________ ___________________________________ 93 ___________________________________ ___________________________________ ___________________________________ Slide 94 ___________________________________ Same for phases of matter. They like each other, you want to separate them you need to overcome the “like”. Easiest way: heat ‘em up so they are moving faster! ___________________________________ ___________________________________ ___________________________________ Solid Liquid Gas 94 ___________________________________ ___________________________________ ___________________________________ Slide 95 ___________________________________ Making a phase change… Suppose I start with 100 g of ice at -40 C (1 atm) and start heating it up, what happens? ___________________________________ The ice gets warmer and warmer until…melting point! Suppose I am ice at 0 C, do I just spontaneously melt? ___________________________________ Not exactly. I am warm enough, but I’m still a solid and my molecules are still “associated” with each other. I need to get ripped away from my brothers. ___________________________________ 95 ___________________________________ ___________________________________ ___________________________________ Slide 96 ___________________________________ ___________________________________ O H O H H HH O HH O H H O HH O H O O O H H O HH -40 C solid HH O HH O H H O H O H H H H HH O H H O HH O H O O O HH O H H O HH H HH O HH ___________________________________ H O H H ___________________________________ 0 solid 96 ___________________________________ ___________________________________ ___________________________________ Slide 97 ___________________________________ ___________________________________ O H O H HH O H O H HH O H H O HH O H H O HH H H H O HH O HH O O O O H H H H H O H O H H H H H O H H O H O ___________________________________ H O H ___________________________________ H 0 solid 0 liquid 97 ___________________________________ ___________________________________ ___________________________________ Slide 98 ___________________________________ At the phase transition temperature… …you still need energy to make the transition. Going from solid to liquid, this is called the “heat of fusion” (Hfus ) ___________________________________ ___________________________________ Going from liquid to gas, this is called “heat of vaporization” (Hvap ) Going from solid to gas, this is called the “heat of sublimation” (Hsub ) ___________________________________ 98 ___________________________________ ___________________________________ ___________________________________ ___________________________________ Phase Diagram for H2O The “ ” in the H means standard conditions. ___________________________________ Liquid 1 atm Pressure Slide 99 ___________________________________ Solid Gas ___________________________________ 0 C Temp 99 ___________________________________ ___________________________________ ___________________________________ Slide 100 ___________________________________ Heating/cooling curves ___________________________________ Temp gas ___________________________________ Liq/gas liquid solid Sol/liq ___________________________________ Heat added (time, if constantly heating) 100 ___________________________________ ___________________________________ ___________________________________ Slide 101 ___________________________________ Heating/cooling curves ___________________________________ Temp gas ___________________________________ Liq/gas liquid Sol/liq solid Heat removed (time, if constantly heating) ___________________________________ 101 ___________________________________ ___________________________________ ___________________________________ ___________________________________ Could we quantify the energy? If I have 50.0 g steam at 500 K, how much energy do I need to remove to get to 373 K? Q = mcT ___________________________________ 500 K gas Temp Slide 102 373 K Slowing the molecules down. Liq/gas ___________________________________ liquid Sol/liq Heat removed (time, if constantly heating) solid ___________________________________ 102 ___________________________________ ___________________________________ ___________________________________ Slide 103 ___________________________________ Could we quantify the energy? Once my steam is down to 373 K, how much energy do I need to remove to turn it into a liquid? ___________________________________ nHvap gas Temp “forming” the intermolecular forces ___________________________________ Liq/gas liquid Sol/liq solid ___________________________________ Heat removed (time, if constantly heating) 103 ___________________________________ ___________________________________ ___________________________________ Slide 104 ___________________________________ Just follow the curve. I start here with a gas (gas, gas) Temp 500 K Q=mcT For water: Csteam = 1.84 J/g c Cice = 2.09 J/g C Cwater = 4.18 J/g C Hfus =6.02 kJ/mol Hsub =46.72 kJ/mol Hvap =40.7 kJ/mol ___________________________________ ___________________________________ -nHvap 373 K Q=mcT -nHfus Q=mcT ___________________________________ Heat removed (time, if constantly heating) 104 ___________________________________ ___________________________________ ___________________________________ Slide 105 ___________________________________ O H ___________________________________ H ___________________________________ O H ___________________________________ H 105 ___________________________________ ___________________________________ ___________________________________ Slide 106 ___________________________________ Just follow the curve. For water: Csteam = 1.84 J/g c Cice = 2.09 J/g C Cwater = 4.18 J/g C Hfus =6.02 kJ/mol Hsub =46.72 kJ/mol Hvap =40.7 kJ/mol Temp 500 K 373 K ___________________________________ ___________________________________ -nHvap Q=mcT -nHfus Q=mcT Heat removed (time, if constantly heating) ___________________________________ 106 ___________________________________ ___________________________________ ___________________________________ Slide 107 ___________________________________ Did I cheat on Temperature? Why have I mixed K and ºC? ∆T not T Temp 500 K ___________________________________ A CHANGE in Temperature of 127 K is a change of 127 ºC 373 K ___________________________________ -nHvap Q=mcT -nHfus Q=mcT Heat removed (time, if constantly heating) ___________________________________ 107 ___________________________________ ___________________________________ ___________________________________ ___________________________________ Just follow the curve. For water: Csteam = 1.84 J/g c Cice = 2.09 J/g C Cwater = 4.18 J/g C Hfus =6.02 kJ/mol Hsub =46.72 kJ/mol Hvap =40.7 kJ/mol Temp Slide 108 ___________________________________ ___________________________________ -nHvap Q=mcT Now I’m here…cold…so cold. -nHfus Q=mcT Heat removed (time, if constantly heating) ___________________________________ 108 ___________________________________ ___________________________________ ___________________________________ Slide 109 ___________________________________ Cold gas O H ___________________________________ H ___________________________________ O H ___________________________________ H 109 ___________________________________ ___________________________________ ___________________________________ Slide 110 But I don’t want to be a cold gas…I want to be a liquid ___________________________________ ___________________________________ O H H ___________________________________ O H H ___________________________________ 110 ___________________________________ ___________________________________ ___________________________________ ___________________________________ Just follow the curve. For water: Csteam = 1.84 J/g c Cice = 2.09 J/g C Cwater = 4.18 J/g C Hfus =6.02 kJ/mol Hsub =46.72 kJ/mol Hvap =40.7 kJ/mol Temp Slide 111 Now, I’m here. Capture! ___________________________________ ___________________________________ Q=mcT -nHfus Q=mcT Heat removed (time, if constantly heating) ___________________________________ 111 ___________________________________ ___________________________________ ___________________________________ Slide 112 ___________________________________ Just follow the curve. Temp For water: Csteam = 1.84 J/g c Cice = 2.09 J/g C Cwater = 4.18 J/g C Hfus =6.02 kJ/mol Hsub =46.72 kJ/mol Hvap =40.7 kJ/mol ___________________________________ ___________________________________ Q=mcT -nHfus Now, I’m here. Q=mcT Heat removed (time, if constantly heating) ___________________________________ 112 ___________________________________ ___________________________________ ___________________________________ Slide 113 ___________________________________ YAY! I’M A LIQUID! ___________________________________ O H H ___________________________________ O H H ___________________________________ 113 ___________________________________ ___________________________________ ___________________________________ ___________________________________ Just follow the curve. For water: Csteam = 1.84 J/g c Cice = 2.09 J/g C Cwater = 4.18 J/g C Hfus =6.02 kJ/mol Hsub =46.72 kJ/mol Hvap =40.7 kJ/mol Temp Slide 114 ___________________________________ 373 K Q=mcT 273 K ___________________________________ -nHfus Now, I’m here. Instead of a cold gas, I’m a warm liquid Heat removed (time, if constantly heating) Q=mcT ___________________________________ 114 ___________________________________ ___________________________________ ___________________________________ Slide 115 ___________________________________ Just follow the curve. Temp For water: Csteam = 1.84 J/g c Cice = 2.09 J/g C Cwater = 4.18 J/g C Hfus =6.02 kJ/mol Hsub =46.72 kJ/mol Hvap =40.7 kJ/mol ___________________________________ ___________________________________ 373 K -nHfus 273 K Now, I’m here. Instead of a cold gas, I’m a warm liquid Heat removed (time, if constantly heating) Q=mcT ___________________________________ 115 ___________________________________ ___________________________________ ___________________________________ Slide 116 ___________________________________ Just follow the curve. Temp For water: Csteam = 1.84 J/g c Cice = 2.09 J/g C Cwater = 4.18 J/g C Hfus =6.02 kJ/mol Hsub =46.72 kJ/mol Hvap =40.7 kJ/mol ___________________________________ ___________________________________ 373 K -nHfus 273 K Now, I’m here. I’m a cold liquid Heat removed (time, if constantly heating) Q=mcT ___________________________________ 116 ___________________________________ ___________________________________ ___________________________________ Slide 117 ___________________________________ YAY! I’M A LIQUID! ___________________________________ O H H ___________________________________ O H H ___________________________________ 117 ___________________________________ ___________________________________ ___________________________________ Slide 118 ___________________________________ I don’t want to be a cold liquid. I want to be a warm solid! ___________________________________ O O O H O H H HH O H H O HH O H O O O HH O H H O HH H H H H H H O HH O HH H O O H H O H H O H H O H H H H ___________________________________ H O H ___________________________________ H 0 solid 0 liquid 118 ___________________________________ ___________________________________ ___________________________________ Slide 119 ___________________________________ Just follow the curve. Temp For water: Csteam = 1.84 J/g c Cice = 2.09 J/g C Cwater = 4.18 J/g C Hfus =6.02 kJ/mol Hsub =46.72 kJ/mol Hvap =40.7 kJ/mol ___________________________________ ___________________________________ 373 K -nHfus 273 K I’m here. I’m a cold liquid Heat removed (time, if constantly heating) I want to be here. Warm Q=mcT solid. ___________________________________ 119 ___________________________________ ___________________________________ ___________________________________ ___________________________________ Just follow the curve. For water: Csteam = 1.84 J/g c Cice = 2.09 J/g C Cwater = 4.18 J/g C Hfus =6.02 kJ/mol Hsub =46.72 kJ/mol Hvap =40.7 kJ/mol Temp Slide 120 ___________________________________ ___________________________________ 373 K Now I’m here. 273 K Q=mcT down to 0K Heat removed (time, if constantly heating) ___________________________________ 120 ___________________________________ ___________________________________ ___________________________________ Slide 121 ___________________________________ A little problem I have 50 g of ice at 100 K. How much energy would I need to add to get steam at 500 K? ___________________________________ ___________________________________ ___________________________________ 121 ___________________________________ ___________________________________ ___________________________________ Slide 122 ___________________________________ Just follow the curve. For water: Csteam = 1.84 J/g c Cice = 2.09 J/g C Cwater = 4.18 J/g C Hfus =6.02 kJ/mol Hsub =46.72 kJ/mol Hvap =40.7 kJ/mol ___________________________________ Q=mcT 500 K 373 K Q=mcT Hfus ___________________________________ Temp Hvap 273 K Q=mcT 100 K ___________________________________ Heat removed (time, if constantly heating) 122 ___________________________________ ___________________________________ ___________________________________ Slide 123 ___________________________________ Just follow the curve. For water: Csteam = 1.84 J/g c Cice = 2.09 J/g C Cwater = 4.18 J/g C Hfus =6.02 kJ/mol Hsub =46.72 kJ/mol Hvap =40.7 kJ/mol ___________________________________ Q=50(1.84)(127) 500 K (50/18.01)*40700 (50/18.01)6020 J/mol ___________________________________ Temp 373 K Q=50*4.18*100 273 K Q=50(2.09)(173) 100 K Heat removed (time, if constantly heating) ___________________________________ 123 ___________________________________ ___________________________________ ___________________________________ Slide 124 ___________________________________ Just follow the curve. For water: Csteam = 1.84 J/g c Cice = 2.09 J/g C Cwater = 4.18 J/g C Hfus =6.02 kJ/mol Hsub =46.72 kJ/mol Hvap =40.7 kJ/mol ___________________________________ Total Q = 1.81x105 J 1.17x104 J 373 K Q=2.09x104 J 1.67x104 J 500 K ___________________________________ Temp 1.13x105 J 273 K 1.81x104J 100 K Heat removed (time, if constantly heating) ___________________________________ 124 ___________________________________ ___________________________________ ___________________________________ Slide 125 ___________________________________ Phase Diagram for H2O For most phase transitions, temperature is more helpful than pressure…except when a gas is involved. ___________________________________ Liquid Pressure 1 atm ___________________________________ Solid Gas ___________________________________ 0 C Temp 125 ___________________________________ ___________________________________ ___________________________________ Slide 126 ___________________________________ Hence “vapor pressure”… …only really applies to sublimation or boiling. ___________________________________ And unsurprisingly, it depends on Temperature (how fast the molecules are moving) and Hvap (how much energy it takes to separate the molecules and make them into gases). ___________________________________ ___________________________________ 126 ___________________________________ ___________________________________ ___________________________________ Slide 127 ___________________________________ Vapor Pressure Vapor pressure depends on temperature. Vapor pressure also depends on Hvap ___________________________________ Clausius-Clapeyron equation: ___________________________________ Where C is a constant, R is the ideal gas constant. ___________________________________ 127 ___________________________________ ___________________________________ ___________________________________ Slide 128 ___________________________________ Not completely useful in this form Clausius-Clapeyron equation: ___________________________________ ___________________________________ If I want to calculate Pvap, I need to know Hvap , C, and T. Except for T, the other two parameters are specific to each compound measured. But math (as ALWAYS!) can Save The Day!! ___________________________________ 128 ___________________________________ ___________________________________ ___________________________________ Slide 129 ___________________________________ Why did I write it that way? Clausius-Clapeyron equation: ___________________________________ ___________________________________ I could have just written it as: ___________________________________ 129 ___________________________________ ___________________________________ ___________________________________ Slide 130 ___________________________________ Why did I write it that way? Clausius-Clapeyron equation: ___________________________________ ___________________________________ Looks like: y = mx+b ___________________________________ 130 ___________________________________ ___________________________________ ___________________________________ Slide 131 ___________________________________ If I’m doing an experiment Clausius-Clapeyron equation: ___________________________________ ___________________________________ If I plot ln Pvap vs. I should get a straight line with a y-intercept of C and a slope of ___________________________________ This is how you would find C and 131 ___________________________________ ___________________________________ ___________________________________ Slide 132 ___________________________________ There’s also the short cut Maybe you don’t want to do the whole experiment! And maybe someone else has already determined (you did the enthalpy lab!) ___________________________________ ___________________________________ Algebra is your BESTEST friend! ___________________________________ 132 ___________________________________ ___________________________________ ___________________________________ Slide 133 ___________________________________ Common trick Compare two values ___________________________________ The C and the Hvap depend a little bit on temperature but not much, so they should be the same in both equations. So, what do I do? Simply “compare” the two values by subtracting them! ___________________________________ ___________________________________ 133 ___________________________________ ___________________________________ ___________________________________ Slide 134 ___________________________________ Common trick Compare two values ___________________________________ ___________________________________ Doing a little algebra…the Cs cancel and we get… ___________________________________ 134 ___________________________________ ___________________________________ ___________________________________ Slide 135 ___________________________________ Vapor Pressure More helpful form – find the Pvap at 2 different temperatures: ___________________________________ ___________________________________ This is more helpful for a couple reasons. First of all…I lost “C”!!! That’s one less material specific variable to worry about! ___________________________________ 135 ___________________________________ ___________________________________ ___________________________________ Slide 136 ___________________________________ Vapor Pressure And then there’s “normal”: ___________________________________ ___________________________________ I usually know the “normal boiling point” of a material…which is? The boiling point at Patm = 1 atm. Since boiling occurs when Pvap = Patm, I know one set of Pvap and T! ___________________________________ 136 ___________________________________ ___________________________________ ___________________________________ Slide 137 ___________________________________ Sample problem: What is the vapor pressure of water at 50 C? ___________________________________ I say vapor pressure, you think… Clausius-Clapeyron! ___________________________________ ___________________________________ 137 ___________________________________ ___________________________________ ___________________________________ Slide 138 ___________________________________ Vapor Pressure ___________________________________ What do I know? Pvap1 = ? Pvap2 = ? Hvap, water = ? T2 = ? T1 = ? R=? ___________________________________ ___________________________________ 138 ___________________________________ ___________________________________ ___________________________________ Slide 139 ___________________________________ Vapor Pressure ___________________________________ What do I know? Pvap1 = 1 atm Pvap2 = ? ___________________________________ Hvap, water = 40.7 kJ/mol at boiling point (pg 472, Tro) = 44.0 kJ/mol at 25 C T2 = 50 C = 323.15 K T1 = 100 C = 373.15 K R = 8.314 J/(mol K) ___________________________________ 139 ___________________________________ ___________________________________ ___________________________________ Slide 140 ___________________________________ Plugging and chugging time… ___________________________________ What do I know? Pvap1 = 1 atm Pvap2 = ? ___________________________________ Hvap, water = 44.0 kJ/mol at 25 C T2 = 50 C = 323.15 K T1 = 100 C = 373.15 K R = 8.314 J/(mol K) ___________________________________ 140 ___________________________________ ___________________________________ ___________________________________ Slide 141 ___________________________________ Plugging and chugging time… ___________________________________ Whatever you do, DON’T ROUND! ___________________________________ How do I isolate Pvap2? That’s right ex! ___________________________________ 141 ___________________________________ ___________________________________ ___________________________________ Slide 142 ___________________________________ Plugging and chugging time… ___________________________________ ___________________________________ Does this make sense? It is less than 1 atm and I’m below the boiling point! ___________________________________ 142 ___________________________________ ___________________________________ ___________________________________ Slide 143 ___________________________________ Another little problem What is the boiling point of water at the top of Mt. Everest where the average atmospheric pressure is 0.64 atm? ___________________________________ ___________________________________ ___________________________________ 143 ___________________________________ ___________________________________ ___________________________________ Slide 144 ___________________________________ Vapor Pressure ___________________________________ What do I know? Pvap1 = ? Pvap2 = ? Hvap, water = ? T2 = ? T1 = ? R=? ___________________________________ ___________________________________ 144 ___________________________________ ___________________________________ ___________________________________ Slide 145 ___________________________________ Vapor Pressure ___________________________________ What do I know? Pvap1 = 1 atm Pvap2 = 0.64 atm ___________________________________ Hvap, water = 44.0 kJ/mol at 25 C T2 = ? T1 = 100 C = 373.15 K R = 8.314 J/(mol K) ___________________________________ 145 ___________________________________ ___________________________________ ___________________________________ Slide 146 ___________________________________ Plugging and chugging time… ___________________________________ Whatever you do, DON’T ROUND! ___________________________________ ln 1.5625= -5292.278 [0.0026798 – 1/T2] 0.446287 = -5292.278 [0.0026798 – 1/T2] -0.00008432797 = 0.0026798 – 1/T2 1/T2 = 0.0027642 T2 = 361.77 K = 88.6 C Does this make sense? Lower atmospheric pressure, lower boiling point! ___________________________________ 146 ___________________________________ ___________________________________ ___________________________________
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