1. Chapter 17
Thermochemistry

2. Energy Transformations
When fuel is burned in a car engine, chemical energy is released and is used to do work.

3. Measuring and Expressing Enthalpy Changes
A burning match releases heat to its surroundings in all directions.
Chemical energy → …
Quick-write : Describe the energy conversions for this scenario.
How much heat does this exothermic rxn release???

4. Energy Transformations
Thermochemistry:
study of heat changes (ΔH; change in heat content) that occur during
chem rxns and
changes in physical states.
chemical potential energy (chemical energy)
energy stored in the chem bonds of a sub

5. Calorimetry Calorimetry
the precise measurement of the heat flow into or out of a system for chemical (e.g. burning) and physical processes (e.g. boiling, melting—phase changes).

6. Calorimetry In calorimetry, heat released by the system =
heat absorbed by surroundings
Conversely,
heat absorbed by a system = heat released by surroundings

7. Calorimetry Calorimeter
insulated device used to measure the absorption or release of heat in chem or physical processes.

8. Calorimetry Enthalpy (H) of the system. The heat content of a system
cannot be measured directly with instruments.
can measured enthalpy change (ΔH);

9. Calorimetry
Calorimetry expts can be performed at a constant volume using a bomb calorimeter.

11. Thermochemical Equations
A thermochemical eqn
chem eqn that includes the enthalpy change (ΔH).
exothermic
In a chem eqn, the ΔH for the rxn can be written as either a reactant or a product.

12. Thermochemical Equations
The heat of rxn is the ∆H.
Exothermic
Enthalpy change

13. Thermochemical Equations
Exothermic Rxn

14. Thermochemical Equations
Endothermic Rxn

15. Thermochemical Equations

16. Thermochemical Equations
The (molar) heat of combustion is the heat of rxn for the complete burning of 1 mole of a sub.

17. Energy Transformations
In what direction does heat flow?

18. Exothermic and Endothermic Process
In an endothermic process,
system gains heat as the surroundings cool down.(+ve)
In an exothermic process,
system loses heat as the surroundings heat up. (-ve)

19. Exothermic and Endothermic Process
A system
part of the universe on which you focus your attention.
The surroundings
include everything else in the universe.
Law of conservation of energy
states that in any chem or physical process, energy is neither created nor destroyed.

20. Exothermic and Endothermic Processes
An endothermic process is one that absorbs heat from the surroundings.

21. Exothermic and Endothermic Processes
An exothermic process is one that releases heat to its surroundings.

22. Units for Measuring Heat Flow
2 common units for heat flow
calorie ; Calorie (food)
joule.

23. Specific Heat Capacity
The specific heat of a sub (pure)
is the amt of heat it takes to raise the temp of 1 g of the sub 1°C.
J/(g∙°C)
q: qty of heat (J)
m: mass of sub (g)
∆T: change in temperature (°C);
∆T = Tfinal – Tinitial

24. Heat Capacity and Specific Heat
Water releases a lot of heat as it cools.
During freezing weather, farmers protect citrus crops by spraying them with water.

25. Specific Heat

26. Heat Capacity and Specific Heat
Because it is mostly water, the filling of a hot apple pie is much more likely to burn your tongue than the crust.

27. Calculate specific heat of a metal
Problem 1: The temp of a 95.4-g piece of Cu increases from 25°C to 48.0°C when the Cu absorbs 849 J of heat. What is the specific heat of Cu ?

28. Calculate Amt of heat to …
Problem 2: How much heat is required to raise the temp of 250.0g of Hg 52°C? (CHg = 0.14J/(g·°C).

29. Latent Heat Latent heat for phase change
melting, freezing, condensation, vaporization, involved heat changes.
the temperature of the sub remain constant;
only phase change.
Heat of fusion (ΔHfus), heat of solidification (ΔHsolid), heat of vaporization (ΔHvap), heat of condensation (Δhcond)

30. Latent Heats Latent heat (for phase change) abbreviation Sign of ΔH
Heat of fusion
ΔHfus
+ve
Heat of solidification
ΔHsolid
-ve
Heat of vaporization
ΔHvap
Heat of condensation
ΔHcond

31. Latent Heat Heat of fusion (ΔHfus) ΔH for melting (+ve; gain heat);
e.g. ΔHfus of ice = 6.01 kJ/mol
temp of sub remains constant
e.g. ice at 0°C remains 0°C while melting though it keeps on absorbing heat
refer to graph in the next slide.

32. Molar Heat of Fusion ΔHfus of ice = 6.01 kJ/mol Heat absorbed (+ve)
Heat change (to melt 1 mole of ice)
qty of Heat (kJ) =
# moles of sub
Heat absorbed (+ve)

33. Heats of Fusion and Solidification
Molar heat of fusion (∆Hfus)
amt of heat absorbed by 1 mole of a solid sub as it melts to a (l) at a constant temp.
e.g. ΔHfus of ice = 6.01 kJ/mol
Molar heat of solidification (∆Hsolid)
amt heat lost when 1 mole of a (l) solidifies at a constant temp.
e.g. ΔHsolid of water (l) = kJ/mol

34. Heats of Vaporization and Condensation

36. liquid water temp increasing
vaporizing
(e)
liquid water temp increasing
(d)
(100°C)
Temp increasing
All ice 0°C
Boiling water 100°C
melting
(c)
(Liquid H2O + steam)
All liquid water 0°C
(b)
(0°C)
(Ice+water) 0°C
(a)
(Ice) temp increasing

37. Quick-write
Why f.p. of water = 0°C and m.p. of ice is also 0°C?

38. Latent Heat Heat of vaporization (ΔHvap) ΔH is +ve
Heat of fusion (ΔHfus)
melting
ΔH is +ve
Heat of solidification (ΔHsolid)
freezing
ΔH is -ve
Heat of vaporization (ΔHvap)
ΔH is +ve
Heat of condensation (ΔHcond)
ΔH is -ve

39. Heats of Fusion and Solidification
The qty of heat absorbed by a melting (s) is exactly the same as the qty of heat released when the (l) solidifies
∆Hfus = –∆Hsolid

40. Heats of Vaporization and Condensation
How does the qty of heat absorbed by a vaporizing (l) compare to the qty of heat released when the vapor condenses?

41. Heats of Vaporization and Condensation
Molar heat of vaporization (∆Hvap).
amt of heat necessary to vaporize 1 mole of a given (l) at constant temp.
Molar heat of condensation (∆Hcond).
amt of heat released when 1 mol of vapor condenses at the normal b.p.

42. Heats of Vaporization and Condensation
The qty of heat absorbed by a vaporizing (l) is exactly the same as the qty of heat released when the vapor condenses;
∆Hvap = –∆Hcond

43. Heats of Vaporization and Condensation

44. Heats of Vaporization and Condensation
ΔH accompany changes in state.

45. Heat of Soln Heat of Solution
During the formation of a soln, heat is either released or absorbed.
Molar heat of soln (∆Hsoln)
The enthalpy change caused by dissolution of 1 mole of sub.

46. Heat of Solution
When NH4NO3 crystals and water mix inside the cold pack, heat is absorbed as the crystals dissolve.

47. Problem 3: The graph below shows a pure sub which is heated by a constant source of heat supplying j/min. Identify the area described in the questions below and complete the necessary calculations.
UV = 0.36 min, VW = 3.6 min, WX = 3.6 min, XY = 19.4 min, YZ = 0.6 min

48. a. being warmed as a solid ___________ b
a. being warmed as a solid ___________ b. being warmed as a liquid __________ c. being warmed as a gas ____________ d. changing from a solid to a liquid _____ e. changing from a liquid to a gas ______ f. What is its boiling temp? _________________ g. What is its melting temp? _________________ h. How many joules were needed to change the
liquid to a gas? ____________ i. Where on the curve do the molecules have the
highest KE? ______ j. If the sample weighs 10.0 g, what is its heat of
vaporization in J/g? ______

49. Example 4
How many g of ice at 0°C could be melted by the addition of 5.75 kJ of heat? (ΔHfus of ice = 6.01 kJ/mol)

50. CST problem 1
The specific heat of Cu is about 0.4 J/(g·°C). How much heat is needed to change the temperature of a 30-g sample of Cu from 20.0°C to 60.0°C?
A 1000 J
B 720 J
C 480 J
D 240 J

51. CST problem 2
Which of these is an example of an exothermic chemical process?
A evaporation of water
B melting ice
C photosynthesis of glucose
D combustion of gasoline

52. The End

53. Example 5
How much energy is required to convert g of water at 60°C completely to steam at °C? (Given: ΔHvap of water = 0.9 kJ/mol;
Cwater = J/g· °C))

54. Calculating the Enthalpy Change in Solution Formation
How much heat (in kJ) is released when mol NaOH(s) is dissolved in water?

57. How many moles of NH4NO3(s) must be dissolved in water so that 88
How many moles of NH4NO3(s) must be dissolved in water so that 88.0 kJ of heat is absorbed from the water?

58. 1. The molar heat of condensation of a substance is the same, in magnitude, as its molar heat of
A. formation.
B. fusion.
C. solidification.
D. vaporization.

59. 2. The heat of condensation of ethanol (C2H5OH) is 43. 5 kJ/mol
2. The heat of condensation of ethanol (C2H5OH) is 43.5 kJ/mol. As C2H5OH condenses, the temperature of the surroundings
A. stays the same.
B. may increase or decrease.
C. increases.
D. decreases.

60. 3. Calculate the amt of heat absorbed to liquefy 15
3. Calculate the amt of heat absorbed to liquefy 15.0 g of methanol (CH3OH) at its m.p. The molar ∆Hfus for methanol is 3.16 kJ/mol.
A kJ
B kJ
C  103 kJ
D kJ

61. 4. How much heat (in kJ) is released when 50 g of NH4NO3(s), 0
4. How much heat (in kJ) is released when 50 g of NH4NO3(s), moles, are dissolved in water? Hsoln = 25.7 kJ/mol
A kJ
B kJ
C kJ
D kJ