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AP Chemistry Cram Sheet #5
Big idea 5: The laws of thermodynamics describe the essential role of energy and explain and predict the direction of
changes in matter
Thermochemistry: 1st Law of Thermodynamics
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Energy is conserved; can be neither created nor destroyed, just changed
o Heat is a transfer of energy because of a difference in temperature. Temperature is a measure
of the average kinetic energy of a substance.
Exothermic: System release heat energy to the surrounding.
Endothermic: System absorbs heat energy from the surrounding.
http://lechemistryblog.blogspot.com/2013_11_01_archive.html
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Enthalpy (H) is heat constant pressure, where PV work is conducted. When a gas is heated in a
piston, the volume of gas expands and the system does work on the surroundings.
W=-PΔV
If ΔH is (-), the process is exothermic. If the ΔH is (+), the process is endothermic.
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Enthalpies of Reactions
Symbol
Types of Process
ΔHrxn
Heat of Reaction
Ag(NO3)3(aq) + HCl(aq) →AgCl(s) + HNO3(aq)
ΔHcomb
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
ΔHfus
H2O(s)→H2O(l)
ΔHvap
H2O(l)→H2O(g)
ΔHbond
H2(g) → H(g) + H(g)
ΔHf
H2(g) + ½ O2(g) → H2O(l)
ΔHsln
Heat of Combustion
Heat of Fusion
Heat of Vaporization
Bond Dissociation
Energy
Heat of Formation
Heat of solution
LiCl(s) → Li+(aq) + Cl-(aq)
1.
Definition of Process with Examples
Enthalpy change for any chemical
reaction
ΔHrxn = -68 kJ
Enthalpy change for any
combustion reaction
ΔHcomb = -802 kJ
Heat change when a solid melts
ΔHfus +6.0 kJ
Heat change when a liquid
vaporizes
ΔHvap = 41 kJ
Heat required to break a chemical
bond
ΔHbond =435 kJ
Heat change when one mole of a
substance is formed from its
elements ΔHf = -285.8 kJ
Heat change when solute
dissolves in a solvent
ΔHsln = -37.4 kJ
The heat of combustion of gaseous butane is -2658 kJ/mol and the heat of combustion of liquid butane is -2635
kJ/mol when , in both cases, all products are gases.
a. Write a balanced chemical equation for the combustion of gaseous butane.
b. How many grams of gaseous butane combust when 1550 kJ of heat are produced?
c. What is the magnitude and sign of the molar heat of vaporization of butane? Explain your reasoning
using Hess’s law. Is your sign for the heat of vaporization realistic? Explain.
2. Answer the following
a. Show the complete equation for the combustion of butane, C4H8.
b. Given the following table of bond energies, estimate the enthalpy change for the reaction noted in 1(a).
Average Bond Energies (kJ/mol)
C-H
413
C-O
358
C-C
347
C=O
799
C=C
614
H-O
467
C=C
839
H-H
432
O=O
495
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c. Often the heat of reaction (enthalpy) calculated from bond energies differs by 10-20% or more form the
laboratory determined values. Suggest why this is so?
d. Is this process thermodynamically favored at all temperature?
3. A 1.00 g sample of magnesium metal is added to a calorimeter containing 100. g of HCl (specific heat capacity
4.184 J/g°C) initially at 25.0 °C. The solution temperature increases to 29.5 °C. What is the heat of reaction, in
kJ/mol?
4. The molar heats of fusion and vaporization of benzene are 10.9 kJ/mol and 31.0 kJ/mol, respectively. The
melting temperature of benzene is 5.5 °C and it boils at 80.1 °C.
a. Calculate the entropy changes for solid → liquids, and for liquid → vapor for benzene.
b. Would you expect the ΔS for these two changes to be about the same? Comment on the physical
significance of the difference in these two values.
c. Why are the values for heat of vaporization usually so much greater than the heats of fusion?
Chemical Thermodynamics: 2nd Law of Thermodynamics
 All spontaneous reactions bust increase the entropy of the universe.
 Suniv=Ssys+Ssurr
 Ssurr=ΔH/T
For questions 5, 6, and 7, consider the ΔG of four different combination of ΔH and ΔS. Assume that both H and S are
temperature independent.
5. Which processes are spontaneous at all temperatures?
a. ΔH = + and ΔS = +
b. ΔH = + and ΔS = c. ΔH = - and ΔS = d. ΔH = - and ΔS = +
6. The process which is nonspontaneous at all values of temperature is
a. ΔH = + and ΔS = +
b. ΔH = + and ΔS = c. ΔH = - and ΔS = d. ΔH = - and ΔS = +
7. Which of these four process is improbable at a low temperature but becomes more probable as the
temperature rises?
a. ΔH = + and ΔS = +
b. ΔH = + and ΔS = c. ΔH = - and ΔS = d. ΔH = - and ΔS = +
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PCl5(g)  PCl3(g) + Cl2(g)
8. For the reaction above, H = +22.1 kilocalories per mole at 25C
a. Does the tendency of reactions to proceed to a state of minimum energy favor the formation of the
products of this reaction? Explain
b. Does the tendency of reactions to proceed to a state of maximum entropy favor the formation of
the products of this reaction? Explain.
c. State whether an increase in temperature drives this reaction to the right, to the left, or has no
effect. Explain.
d. State whether a decrease in the volume of the system at constant temperature drives this reaction
to the right, to the left or has no effect. Explain?
AgNO3(s)  Ag+(aq) + NO3–(aq)
9. The dissolving of AgNO3(s) in pure water is represented by the equation above..
a. Is ∆G for the dissolving of AgNO3(s) positive, negative, or zero? Justify your answer.
b. Is ∆S for the dissolving of AgNO3(s) positive, negative, or zero? Justify your answer.
c. The solubility of AgNO3(s) increases with increasing temperature.
i. What is the sign of ∆H for the dissolving process? Justify your answer.
ii. Is the answer you gave in part (a) consistent with your answers to parts (b) and (c) (i)?
Explain.
The compound NaI dissolves in pure water according to the equation NaI(s)  Na+(aq) + I–(aq). Some of
the information in the table of standard reduction potentials given below may be useful in answering the
questions that follow.
Half-reaction
E˚ (V)
O2(g) + 4 H+ + 4 e-  2 H2O(l)
1.23
I2(s) + 2 e-  2 I–
0.53
2 H2O(l) + 2 e-  H2(g) + 2 OH– -0.83
Na+ + e-  Na(s)
-2.71
d. An electric current is applied to a 1.0 M NaI solution.
i. Write the balanced oxidation half reaction for the reaction that takes place.
ii. Write the balanced reduction half-reaction for the reaction that takes place.
iii. Which reaction takes place at the anode, the oxidation reaction or the reduction reaction?
iv. All electrolysis reactions have the same sign for ∆G˚. Is the sign positive or negative?
Justify your answer.