AP Chemistry
Unit Three - Thermochemistry
Define each term and
explain the difference
between the words in
each pair.
potential energy and kinetic energy
heat and temperature
system and surroundings
endothermic and exothermic
heat capacity and specific heat
Write down the equation
containing specific heat.
Define each variable.
Use your understanding of 1. You have 100g samples of two different metals (A and B) at 20 oC. The specific heat of A
specific heat to answer this is larger than that of B. Under the same heating conditions, which metal would take longer
to reach a temperature of 25oC? Explain your answer.
question.
Complete the following
calculations.
2. When 3601 joules of heat are added to 140.0 g of iron, what is the increase in
temperature? The specific heat of iron is 0.450 J / g . oC.
3. Calculate the amount of heat liberated from 37 g of mercury when it cools from 77 oC to
12oC. The specific heat of mercury is 0.140 J/ g . oC.
Sketch the heating curve
for water and label each
segment.
For each segment, write
down the equation used
to calculate the
associated heat change.
Write down the symbol
and value for the constant
associated with each
segment.
(Use 2.135 J / g . oC as
the specific heat of solid
H2O.)
Practice Problems
4. Calculate the amount of heat absorbed by 1 cup of water (237g) when it is heated from
-12.0oC to 110.oC.
5. Calculate the amount of heat needed to melt 35.0 g of ice at 0°C. Express your answer
in kilojoules.
6. You have a sample of H2O with a mass of 23.0 g at a temperature of 112°C. How much
heat is released when that sample is cooled to room temperature (23°C)?
Define calorimetry.
Describe how to set up
and perform a calorimetry
experiment.
Define enthalpy. What is
the symbol used for
enthalpy?
Enthalpy is a state function.
What is the definition of
“state function”?
Draw potential energy
curves for exothermic and
endothermic reactions.
Label which one is which.
On each PE Curve, label
the location of the
reactants and the
products. Label the
enthalpy of reaction.
Define thermochemical
equation.
Complete the following
calculation.
7. The first step in the industrial recovery of zinc from zinc sulfide ore is “roasting”, that
is, the conversion of ZnS ot ZnO by heating:
2ZnS(s) + 3O2(g) → 2ZnO(s) + 2SO2(g) △ Hrxn = -879 kJ
Calculate the heat evolved in kJ for every 100. g of ZnS roasted.
Because enthalpy is a state
function, one method for
calculating the enthalpy of a
reaction is Hess’s Law.
Explain how Hess’s Law
works.
Complete the following
calculation.
8. From the following heats of combustion reactions
CH3OH(l) + ½ O2(g) → CO2(g) + 2H2O(l)
C(graphite) + O2(g) → CO2(g)
H2(g) + ½ O2(g) → H2O(l)
△ Horxn = -726.4 kJ
△ Horxn = -393.5 kJ
△ Horxn = -285.8 kJ
Calculate the enthalpy of formation of methanol (CH3OH) from its elements:
C(graphite) + 2H2(g) + ½ O2(g) → CH3OH(l)
Fill in the blanks in the
definition of standard
enthalpy of formation.
The standard enthalpy of formation (symbolized by ___________) is the change in
enthalpy that accompanies the formation of _______________ of a compound from
________________________ in their ___________________________.
How are standard states
defined in thermochemistry?
(Hint: see p. 260 of your
textbook for help)
Answer the following
question.
9. For which of the following reactions is △ H equal to the standard enthalpy of formation
of NH3?
(a) 3H2(g) + N2(g) → 2NH3(g)
(b) 2NH3(g) → 3H2(g) + N2(g)
(c) 3/2H2(g) + 1/2N2(g) → NH3(g)
(d) NH3(g) → 3/2H2(g) + 1/2N2(g)
(e) 3H(g) + N(g) → NH3(g)
What is the standard
enthalpy of formation for
an element in its standard
state?
How do you use standard
enthalpy of formation
values to calculate the
heat of reaction?
Use the △Hf values found
in Appendix 4 of your
textbook to complete the
following calculation.
10. 2 C2H6(g) + 7 O2(g) → 4 CO2(g) + 6H2O(l)
Choose the correct word
for each blank:
When bonds are broken, energy is ______________ (added/released) to the system.
Breaking bonds is an _______________ (endothermic/exothermic) process. When
bonds are formed, energy is ______________ (added/released) to the system. Bond
formation is an _______________ (endothermic/exothermic) process.
Given that bond energy
values are always reported
as positive values and the
statements above, how do
you calculate the heat of
reaction give bond
energies?
Use the bond energy
values found on p.372 of
your textbook to complete
the following calculation.
11. C2H4(g) + F2(g) → C2H4F2
Lewis Structures for each compound are shown below. Unshared pairs of electrons have
been omitted.
Explain what is meant by a
spontaneous process.
Use your understanding of 12. Which of the following are spontaneous processes?
spontaneity to answer the
(a) dissolving table salt in hot soup
following question.
(b) climbing Mt. Everest
(c) spreading perfume in a room by removing the cap from the bottle
(d) a ball rolling down a hill
(e) separating helium and neon from a mixture of the gases
(f) making up your bed
Define entropy.
What are the units of
entropy?
Use your understanding
of entropy to answer the
following question.
13. Predict whether the entropy change is positive or negative for each of the following
processes:
(a) a solid melts
(b) H2O(g) → H2O(l)
(c) a solid sublimes
(d) calcium chloride is dissolved in water
(e) 2KClO4(s) → 2KClO3(s) + O2(g)
(f) U(s) + 3F2(g) → UF6(s)
(g) a metal drill bit is cooled after heavy use
Use the S values found in 14. 2Al(s) + 3ZnO(s) → Al2O3(s) + 3Zn(s)
Appendix 4 of your textbook
to complete the following
calculation.
How do enthalpy and
entropy relate to reaction
spontaneity?
Define Gibbs Free Energy.
How does Gibbs Free
Energy relate to reaction
spontaneity?
Use the △G values found
in Appendix 4 of your
textbook to complete the
following calculation.
15. 3K + AlCl3 → 3KCl + Al
Write down the equation for
calculating Gibb’s Free
Energy using △H and △S
values.
16. Calculate △ G for a reaction with the following thermodynamic values at 25 oC: △ H =
10.5 kJ and △ S = 30 J/K
Answer the following
questions:
Is this reaction spontaneous at 25oC?
17. At what temperatures would reactions with the following thermodynamic values
become spontaneous:
(a) △ H = 126 kJ and △ S = -84 J/K
(b) △ H = 31.7 kJ and △ S = 105 J/K
ANSWERS (maybe):
1. Metal A will take longer to reach 25 oC. A larger specific heat value means the metal can absorb more heat with only 1 oC
temp increase. More heat must be added to this metal to get the same amount of temperature change.
2. 57.2oC
3. 340 J
4. 724 kJ
5. 11.7 kJ
6. -59.9 kJ
7. 451 kJ
8. -238.7 kJ
9. c
10. -3121 kJ
11. -549 kJ
12. a, c, d
13. a) +
14. -11 J/K
b) -
c) +
d) +
e) +
f) -
g) -
15. -595 kJ
17. a) spontaneous at NO TEMPS (always nonspontaneous)
b) spon at temps above 302 K
16. 1.56 kJ, not spontaneous