Unit 3 Part 1 Name:________________________ AP Chemistry Unit 3: Thermochemistry, Atomic Structure, and Periodicity Chapter 6-­‐Thermochemistry The Nature of Energy Vocabulary Terms: Energy: Units – joule (J) 1 J = kg·∙m2/s2 (1 cal = 4.184 J) The Law of Conservation of Energy (1st Law of Thermodynamics): Potential Energy (PE): Kinetic Energy (KE = ½mv2): Heat (q): Enthalpy (H): Work (W = F x d): System: Surroundings: Exothermic: Endothermic: State Function (state property): Standard Conditions: Energy and Work ΔE = Efinal -­‐ Einitial ΔE = q(heat) + w(work) q w ΔE + -­‐ Example Problem 1: Calculate ΔE for a system undergoing an endothermic process in which 15.6 kJ of heat flows and where 1.4 kJ of work is done on the system. (Answer: 17.0 kJ) Pressure-­‐Volume Work (expansion or compression of a gas) w = -­‐PΔV Example Problem 2: Calculate the work associated with the expansion of a gas from 46 L to 64 L at a constant external pressure of 15 atm. (Answer: -­‐270 L·∙atm) Example Problem 3: A balloon is being inflated to its full extent by heating the air inside it. In the final stages of this process, the volume of the balloon changes from 4.00 x 106 L to 4.50 x 106 L by the addition of 1.3 x 108 J of energy as heat. Assuming that the balloon expands against a constant pressure of 1.0 atm, calculate ΔE for the process. (To convert between L·∙atm and J, use 1 L·∙atm = 101.3 J.) (Answer: 8.0 x 107 J) Enthalpy and Calorimetry Enthalpy (H): ΔH = Hproducts – Hreactants (at constant pressure, ΔH = q) Enthalpies of Reaction (heat of reaction): ΔHrxn CH4(g) + 2O2(g) à CO2(g) + 2H2O(g) ΔH = -­‐890 kJ What does it mean when ΔH is negative? What does it mean when ΔH is positive? The enthalpy for the reverse reaction 2H2(g) + O2(g) à 2H2O(g) ΔH= -­‐483.6 kJ/mol 2H2O(g) à 2H2(g) + O2(g) ΔH= ? Enthalpy can be calculated from several sources, including: stoichiometry Hess’s Law calorimetry Bond energies from tables of standard values Example Problem 4: (Stoichiometrically) When 1 mole of methane is burned at constant pressure, 890 kJ of energy is released as heat. Calculate ΔH for a process in which a 5.8 g sample of methane is burned at constant pressure. (Answer: -­‐320 kJ) Example Problem 5: (Stoichiometrically) Upon adding solid potassium hydroxide pellets to water the following reaction takes place: KOH(s) à KOH(aq) + 43 kJ/mol Answer the following questions regarding the addition of 14.0 g of KOH to water: (a) Does the beaker get warmer or colder? (b) Is the reaction endothermic or exothermic? (c) What is the enthalpy change for the dissolution of the 14.0 g of KOH? Answers: (a) warmer (b) exothermic (c) -­‐10.7 kJ/mol Calorimetry: Types of Calorimetry (1) Coffee-­‐cup calorimetry (2) Bomb calorimetry Vocabulary Terms Heat Capacity (C): Specific Heat Capacity (Cp or Cs): q = Cs x m x ΔT Cs(H2O) = 4.18 J/g·∙°C or 1.00 cal/g·∙°C Molar Heat Capacity (Cm): Heat lost by substance = Heat gained by water Example Problem 6: In a coffee cup calorimeter, 100.0 mL of 1.0 M NaOH and 100.0 mL of 1.0 M HCl are mixed. Both solutions were originally at 24.6°C. After the reaction, the final temperature is 31.3°C. Assuming that all solutions have a density of 1.0 g/cm3 and a specific heat capacity of 4.184 J/g°C, calculate the change in enthalpy for the neutralization of HCl by NaOH. Assume that no heat is lost to the surroundings or the calorimeter. (Answer: -­‐5.6 kJ/mol) Example Problem 7: When 1.00 L of 1.00 M barium nitrate solution at 25.0°C is mixed with 1.00 L of 1.00 M sodium sulfate solution at 25°C in a calorimeter, a white solid precipitate forms and the temperature of the mixture increases to 28.1°C. Assuming the calorimeter absorbs only a negligible quantity of heat, that the specific heat of the solution is 4.18 J/g°C, and the density of the final solution is 1.0 g/mL, calculate the enthalpy change per mole of precipitate formed. (Answer: -­‐26 kJ/mol) Bomb Calorimetry Example Problem 8: A 0.5865 g sample of lactic acid (HC3H5O3) is burned in a bomb calorimeter whose heat capacity is 4.812 kJ/ºC. The temperature increases from 23.10ºC to 24.95ºC. Calculate the heat of combustion of lactic acid (a) per gram and (b) per mole. (Answers: (a) -­‐15.18 kJ/g (b) -­‐1367 kJ/mol) Example Problem 9: It has been suggested that hydrogen gas obtained by the decomposition of water might be a substitute for natural gas (principally methane). To compare the energies of combustion of these fuels, the following experiment was carried out using a bomb calorimeter with a heat capacity of 11.3 kJ/°C. When a 1.50-­‐g sample of methane gas was burned with excess oxygen in the calorimeter, the temperature increased by 7.3°C. When a 1.15-­‐g sample of hydrogen gas was burned with excess oxygen, the temperature increase was 14.3°C. Calculate the energy of combustion per gram (ΔHcomb) for hydrogen and methane. (Answer: methane=55 kJ/g; hydrogen=141 kJ/g) Vocabulary Terms Enthalpy of Fusion (ΔHfus) Enthalpy of Vaporization (ΔHvap) Hess’s Law Hess’s Law: Enthalpy of formation (ΔH°f) ΔHrxn = ΣΔHf(products) – ΣΔHf(reactants) ΔH°f for elements in their standard states = N2(g) + 2O2(g) à 2NO2(g) ΔH = 68 kJ, so ΔH°f is 34 kJ/mol (N + O2 à NO2) * elements must be in standard states and one mole of product formed C(s) + 2H2(g) + ½O2(g) à CH3OH(l) ΔHf° = -­‐239 kJ/mol Table 6.2 (p. 266) and Appendix 4 (pp. A19) Example Problem 10: Calculate ΔH°rxn for the following: 3 Al(s) + 3 NH4ClO4(s) à Al2O3(s) + AlCl3(s) + 3 NO(g) + 6 H2O(g) Given the following values: Substance NH4ClO4(s) Al2O3(s) AlCl3(s) NO(g) H2O(g) ΔH°f (kJ/mol) -­‐295 -­‐1676 -­‐704 90.0 -­‐242 Answer: -­‐2,677 kJ/mol (exothermic) Example Problem 11: Sometimes all values are not found in the table of thermodynamic data. For most substances it is impossible to go into lab and directly synthesize a compound from its free elements. The heat of formation of the substance must be found by working backwards from its heat of combustion. Find the ΔHf of C6H12O6(s) from the following information: C6H12O6(s) + 6 O2(g) à 6 CO2(g) + 6H2O(l) + 2800 kJ Substance CO2(g) H2O(l) ΔH°f (kJ/mol) -­‐393.5 -­‐285.8 Answer: ΔH°f=-­‐1276 kJ/mol for glucose Example Problem 12: Calculate the standard enthalpy change for the thermite reaction: 2Al(s) + Fe2O3(s) à Al2O3(s) + 2Fe(s) Answer: -­‐850. kJ/mol Another form of Hess’s Law “Since enthalpy is a state function, the change in enthalpy in going from some initial state to some final state is independent of the pathway.” What does this mean? Example Problem 13: Given the following equations H3BO3(aq) à HBO2(aq) + H2O(l) ΔHrxn= -­‐0.02 kJ/mol H2B4O7(aq) + H2O(l) à 4 HBO2(aq) ΔHrxn= -­‐11.3 kJ/mol H2B4O7(aq) à 2 B2O3(s) + H2O(l) ΔHrxn= 17.5 kJ/mol find the ΔH for this overall reaction: 2 H3BO3(aq) à B2O3(s) + 3 H2O(l) Answer: 14.4 kJ/mol (endothermic) Example Problem 14: Diborane (B2H6) is a highly reactive substance once considered as a possible rocket fuel for the U.S. space program. Calculate ΔH for the synthesis of diborane from its elements, according to the equation 2B(s) + 3H2(g) à B2H6(g) Use the following data: 2 B(s) + 3/2 O2(g) à B2O3(s) ΔH = -­‐1273 kJ B2H6(g) + 3O2(g) à B2O3(s) + 3H2O(g) ΔH = -­‐2035 kJ H2(g) + ½ O2(g) à H2O(l) ΔH = -­‐286 kJ H2O(l) à H2O(g) ΔH = 44 kJ Answer: +36 kJ/mol Bond energy Bond Energies Energy must be added/absorbed to BREAK bonds (endothermic). Energy is released when bonds are FORMED (exothermic). ΔH = ΣBond Energiesbroken – ΣBond Energiesformed Example Problem 15: Using bond energies, calculate the change in energy that accompanies the following reaction: H2(g) + F2(g) à 2 HF(g) Bond Type Bond Energy H – H 432 kJ/mol F – F 154 kJ/mol H – F 565 kJ/mol Answer: -­‐544 kJ/mol Summary for Enthalpy: What does it really tell us about an equation? If ΔH is positive (+): If H is negative (-­‐): Ch. 6 End of Chapter MC Solutions: C; A; B; B; C; D; B;D; D; D;B;C;C;A;A AP CHEMISTRY UNIT 3 REVIEW (Chapter 6) Part A 1. 2. 3. 4. 5. 6. 7. Calculate the kinetic energy of a 1.0 x 10-­‐5 g object with a velocity of 2.0 x 105 cm/s. Calculate ΔE for each of the following cases: a. q = +51 kJ and w = -­‐15 kJ b. A gas absorbs 45 kJ of heat and does 29 kJ of work. A balloon filled with 39.1 mol He has a volume of 876 L at 0.0ºC and 1.00 atm pressure. The temperature of the balloon is increased to 38.0ºC as it expands to a volume of 998 L with the pressure remaining constant. Calculate q, w, and ΔE for He in the balloon. Molar heat capacity for He is 20.8 J/ºC mol. Are the following processes exothermic or endothermic? a. When solid KBr is dissolved in water, the solution gets colder. b. Natural gas (CH4) is burned in a furnace. c. When concentrated sulfuric acid is added to water, the solution gets very hot. d. Water is boiled in a tea kettle. Consider the reaction: B2H6(g) + 3 O2(g) à B2O3(s) + 3 H2O(g) ΔH = -­‐2035 kJ Calculate the heat released when each of the following amounts of diborane (B2H6) is burned. a. 1.0 grams b. 1.0 mol c. 1.0 x 102 mol d. A mixture of 10.0 g of B2H6 and 10.0 g of O2. The specific heat of aluminum ios 0.900 J/gºC. a. Calculate the energy needed to raise the temperature of a 8.50 x 102 g block of Al from 22.8ºC to 94.6ºC. b. Calculate the molar heat capacity of aluminum. A 30.0 g sample of water at 280 K is mixed with 50.0 g of water at 330 K. Calculate the final temperature of the mixture assuming no heat loss to the surroundings. 8. A 15.0 g sample of nickel metal is heated to 99.8ºC and placed in a coffee cup calorimeter containing 150.0 g of water at 23.5ºC. After the metal cools, the final temperature of the metal and water is 25.0ºC. Calculate the specific heat capacity of nickel, assuming no heat escapes to surroundings or is transferred to the calorimeter. 9. A 15.0 g sample of nickel is heated to 100.0ºC and dropped into 55.0 g of water, initially at 23.0ºC. Assuming all the heat lost by the nickel is absorbed by the water, calculate the final temperature of the nickel and water. Cp for nickel is 0.444 J/gºC. 10. A coffee cup calorimeter initially contains 125 g of water at 24.2ºC. Potassium bromide (10.5 g) at same temperature is added to the water, and after the solid dissolves the final temperature is 21.1ºC. Calculate the enthalpy change for dissolving the salt in water in J/g and kJ/mol. Cp for solution is 4.18 J/gºC and no heat is transferred to surroundings or calorimeter. 11. In a coffee cup calorimeter, 50.0 mL of 0.100 M silver nitrate and 50.0 mL of 0.100 M hydrochloric acid are mixed to yield the following reaction: Ag+ + Cl-­‐ à AgCl(s) The two solutions were initially at 22.60ºC and the final temperature is 23.40ºC. Calculate the heat that accompanies this reaction in kJ/mol of AgCl formed. Assume the combined solution has a mass of 100.0 g and has a specific heat capacity of 4.18 J/gºC. 12. Camphor (C10H16O) has an energy of combustion of -­‐5903 kJ/mol. When a sample of camphor with mass of 0.1204 grams is burned in a bomb calorimeter the temperature increases by 2.28ºC. Calculate the heat capacity of the calorimeter. 13. A 0.1964 g sample of quinine (C6H4O2) is burned in a calorimeter with heat capacity of 1.56 kJ/ºC. The temperature of the calorimeter increases by 3.2ºC. Calculate the energy of combustion of quinine per gram and per mole. Answers: 1.) 0.020 J 2a.) +36 kJ 2b.) +16 kJ 3.) w = -­‐12.4 kJ q = 30.9 kJ ΔE = 18.5 kJ 4a.) endothermic 4b.) exothermic 4c.) exothermic 4d.) endothermic 3
5
5a.) -­‐74 kJ 5b.) -­‐2.0 x 10 kJ 5c.) -­‐2.0 x 10 kJ 5d.) -­‐211 kJ 6a.) 54.9 kJ 6b.) 24.3 J/molºC 7.) 311 K 8.) 0.84 J/gºC 9.) 25.2ºC 1
10.) 170 J/g and 2.0 x 10 kJ/mol 11.) -­‐66 kJ/mol 12.) 2.05 kJ/ºC 13.) 25 kJ/g and 2700 kJ/mol AP CHEMISTRY UNIT 3 REVIEW (Chapter 6) Part B 1. 2. Given the following data: C2H2(g) + 5/2 O2(g) à 2 CO2(g) + H2O(l) C(s) + O2(g) à CO2(g) H2(g) + ½ O2(g) à H2O(l) ΔH = -­‐1300 kJ ΔH = -­‐394 kJ ΔH = -­‐286 kJ Calculate ΔH for the reaction 2 C(s) + H2(g) à C2H2(g) The bombardier beetle uses an explosive discharge as a defense mechanism. The chemical reaction involved is the oxidation of hydroquinone by hydrogen peroxide to produce quinone and water. C6H4(OH)2(aq) + H2O2(aq) à C6H4O2(aq) + 2 H2O(l) Calculate DH for this reaction from the following data: C6H4(OH)2(aq) à C6H4O2(aq) + H2(g) ΔH = +177.4 kJ H2(g) + O2(g) à H2O2(aq) ΔH = -­‐191.2 kJ H2(g) + ½ O2(g) à H2O(g) ΔH = -­‐241.8 kJ H2O(g) à H2O(l) ΔH = -­‐43.8 kJ 3. 4. 5. Calculate the ΔH for the reaction N2H4(l) + O2(g) à N2(g) + 2 H2O(l) given the following data: 2 NH3(g) + 3 N2O(g) à 4 N2(g) + 3 H2O(l) ΔH = -­‐1010. kJ N2O(g) + 3 H2(g) à N2H4(l) + H2O(l) ΔH = -­‐317 kJ 2 NH3(g) + ½ O2(g) à N2H4(l) + H2O(l) ΔH = -­‐143 kJ H2(g) + ½ O2(g) à H2O(l) ΔH = -­‐286 kJ Use values of ΔHfº to calculate ΔHº for the following reaction: a. 2 NH3(g) + 3 O2(g) + 2 CH4(g) à 2 HCN(g) + 6 H2O(g) b. 6500 grams of NH3 will produce how many kilojoules? Use values of ΔHfº to calculate ΔHº for the following reaction: a. C2H5OH(l) + 3 O2(g) à 2 CO2(g) + 3 H2O(g) b. How many molecules of C2H5OH are needed to produce 35,000 kJ? Answers: 1.) ΔH = +226 kJ 2.) ΔH = -­‐202.6 kJ 3.) ΔH = -­‐623 kJ 5
4a.) ΔH = -­‐941 kJ 4b.) 1.8 x 10 kJ 25
5a.) ΔH = -­‐1277 kJ 5b.) 1.7 x 10 mc