Heat of Solution Lesson
pink: questions to ask class
purple: ideal student answers to questions
Introduction:
We have talked about heat of vaporization (or how much heat is needed to change a specific
liquid to a gas). Molar heat of vaporization of water was the number of kJ of energy needed to
change 1 mole of water molecules from liquid to vapor state.
What about during the formation of a solution? When something dissolves, it involves breaking
and making bonds.
What do we know about breaking and making bonds, what does it require? energy
So we know this process involves energy.
Recall:
breaking bonds requires or absorbs energy
Forming bonds releases energy
What if the overall reaction releases energy, what is that reaction classified as? Exothermic
What if the overall reaction absorbs energy? Endothermic
Dissolution (overall) can be either of these and depends on the overall energy transfer
What about a practical example, like when detergent dissolves in water.
We will find out if this process is exothermic or endothermic...
Short Activity
Materials: (Tide laundry detergent)
Worksheet for this “short activity:”
Procedure:
Pair up
Have one partner wet both of their hands
Have the other partner put a little detergent into just one of the wet hands
Close both hands and compare what happens to the hand with the detergent and the hand with
just water in it.
Observations:
The water and detergent “solution” got warm after the detergent dissolved in the water. While
the hand with water remained the same temperature
Knowing endothermic reactions absorb energy and exothermic reactions release energy, what
do you think the dissolving of detergent is, an exothermic or endothermic reaction? Justify your
answer using your observations
When the detergent dissolves in water the solution gets warm. The solution getting warm shows
it is releasing energy into the surroundings­ therefore it is exothermic.
Discussion (back to seats)
What happened to the hand (bag) that had the detergent in it? The hand got warmer
Knowing it got warmer (gave off heat) what does that mean about the dissolving of detergent?
The release of heat means it is exothermic
We know when the detergent dissolves it is exothermic ­ so we know the delta H of solution is
negative.
What about a hot pack. What happens when you use a hot pack? It gives off heat
Hot packs are usually made up of Calcium Chloride and water. So when you initiate this process
, you are mixing these together and the Calcium chloride dissolves. This produces heat –
therefore exothermic
What about a cold pack. The cold pack usually contains ammonium nitrate and water.
How come the “instant” cold packs aren’t cold until you “start” them? They need to mix and
dissolve to absorb the heat and become “cold”
What happens when you “start” the cold pack (use what you know about hot packs)? The
dissolving of the ammonium nitrate in water makes the pack cold ­ endothermic and overall
absorbing energy
Short Worksheet/Assignment:
Draw a picture of the cold pack and the heat flow to/from the surroundings. Define/label what the
system is and what the surroundings are.
Example Calculation (Model/Coach):
So we know that heat is absorbed for a cold pack and the reaction is as follows:
NH4NO3 (s) ­ H2O (l) ­­> NH4+ (aq) +NO3­ (aq)
Given the Delta H of solution is 25.7 kJ/mol,
Calculate how much heat is released when 1.233 moles of NH4NO3 is dissolved in water?
How do we set this up?
1.233 moles NH4NO3
25.7 kJ
= 31.6881 kJ
mol
What if the reaction was exothermic? How does it change the delta H? the sign would be
negative
What about when 3.40 gram sample of CaCl2 dissolves in 40 grams of water. The temperature
inside the calorimeter drops from 34.0 to 20.2. Calculate the delta H for the process
CaCl2 (s) ­­H20(l)­­> Ca2+(aq) + 2Cl­ (aq)
What equation are we going to use?? The Q calculation
3.4 gram CaCl2
1 mol CaCl2
= .0306 moles CaCl2
110.984 g
(I will finish this calculation later ­ I might change the numbers)
Homework (Time to work) (Fade):
Now work on problems 29­33 in your homework packet
29. Calculate the total energy needed to take 10 grams of water from 15 degrees C to steam at
100 degrees C
30. How much heat (kJ) is released when 0.677 mol NaOH is dissolved in water? AHsol =
­455.1 kJ/mol
31. When a 12.8 gram sample of KCl dissolves in 75.0g of water in a calorimeter, the
temperature drops from 31.0 degrees to 21.6 degrees. Calculate the delta H for the process
KCl(s) → K+ (aq) + Cl­ (aq)
32. When a 25.7 gram sample of NaI dissolves in 80.0 grams of water in a calorimeter, the
temperature rises from 20.5 to 24.4. Calculate the delta H for the process
NaI (s) → Na+ + I­
33. When a 16.9 gram sample of NaOH dissolves in 70.0g of water in a calorimeter, the temp
rises from 22.4 to 86.5. Calculate the delta H for the process:
NaOH (s) → Na+ + OH­