Name
Date
CHAPTER 16 REVIEW
Reaction Energy
Class
SECTION 2
SHORT ANSWER Answer the following questions in the space provided.
1. For the following examples, state whether the change in entropy favors the forward or reverse
reaction:
2.
forward reaction
reverse reaction
forward reaction
reverse reaction
AG AH - TAS
=
negative
a. HCl(1) HCl(g)
b. C6H12O6(aq) → C6H12O6(s)
c. 2NH3(g) N2(g) + 3H2(g)
d. 3C2H4(g) → C6H12(7)
a. Write an equation that shows the relationship between enthalpy,
AH, entropy, AS, and free energy, AG.
b. For a reaction to occur spontaneously, the sign of AG should
be
3. Consider the following equation: NH3(g) + H2O(l) → NH4(aq) + OH¯(aq) + energy
True
reverse reaction
a. The enthalpy factor favors the forward reaction. True or False?
b. The sign of TAS" is negative. This means the entropy factor
favors the
c. Given that AGº for the above reaction in the forward direction is positive, which term is greater
in magnitude and therefore predominates, TAS or AH?
TAS > AH; AH is negative, but AG is positive, indicating the reaction is not
spontaneous. The randomness of the reactants and the temperature of the reaction
overshadow the exothermic factor.
4. Consider the following equation for the vaporization of water:
H₂O(l) → H2O(g)
endothermic
reverse reaction
forward reaction
ΔΗ
=
+40.65 kJ/mol at 100°C
a. Is the forward reaction exothermic or endothermic?
b. Does the enthalpy factor favor the forward or reverse
reaction?
c. Does the entropy factor favor the forward or reverse
reaction?
MODERN CHEMISTRY
Copyright by Holt, Rinehart and Winston. All rights reserved.
REACTION ENERGY
133
Name
Date
Class
SECTION 2 continued
PROBLEMS Write the answer on the line to the left. Show all your work in the space
provided.
5. Halogens can combine with other halogens to form several unstable compounds.
Consider the following equation: 12(s) + Cl2(g) → 2ICl(g)
AH for the formation of ICI = +18.0 kJ/mol and AG =-5.4 kJ/mol.
reverse reaction
forward reaction
a. Is the forward or reverse reaction favored by the
enthalpy factor?
b. Will the forward or reverse reaction occur
spontaneously at standard conditions?
c. Is the forward or reverse reaction favored by the
entropy factor?
forward reaction
+23.4 kJ/(mol·K)
d. Calculate the value of TAS for this system.
0.0785 kJ/(mol·K)
e. Calculate the value of AS for this system at 25°C.
6. Calculate the free-energy change for the reactions described by the equations below. Determine
whether each reaction will be spontaneous or nonspontaneous.
-51.0 kJ/mol; spontaneous
195.8 kJ/mol; nonspontaneous
a. C(s) + 2H2(g) → CH4(8)
ASº
-80.7 J/(mol⚫K), AH°
ΔΗ
T = 298 K
=
-75.0 kJ/mol,
b. 3Fe2O3(s) → 2Fe3O4(s) + 102(8)
ASº = 134.2 J/(mol•K), AHº = 235.8 kJ/mol,
T = 298 K
134 REACTION ENERGY
MODERN CHEMISTRY
Copyright by Holt, Rinehart and Winston. All rights reserved.
Name
Date
CHAPTER 16 REVIEW
Reaction Energy
Class
MIXED REVIEW
SHORT ANSWER Answer the following questions in the space provided.
1. Describe Hess's law.
The overall enthalpy change in a reaction is equal to the sum of enthalpy changes
for the individual steps in the process.
2. What determines the amount of energy absorbed by a material when it is heated?
Each material has its own unique specific heat value, which is the amount of energy
it takes to raise the temperature of 1 g of the substance by 1°C, or 1 K. This value
is dependent on the nature of the material, the mass of the sample, and the change
in temperature
3. Describe what is meant by enthalpy of combustion and how a combustion calorimeter measures this
enthalpy.
The enthalpy of combustion is the enthalpy change that occurs during the complete
combustion of 1 mol of substance and is measured using a combustion calorimeter.
The sample is placed in the calorimeter and is ignited by an electric spark and
burned in an atmosphere of pure oxygen. The energy generated by the combustion
reaction warms the steel bomb and the water surrounding it. A thermometer
measures the temperature change of the water and is used to calculated the energy
that came from the reaction as heat.
MODERN CHEMISTRY
Copyright by Holt, Rinehart and Winston. All rights reserved.
REACTION ENERGY 135
Name
MIXED REVIEW continued
Date
Class
4. The following equation represents a reaction that is strongly favored in the forward direction:
2C7H5(NO2)3(!) + 1202(g) → 14CO2(g) + 5H₂O(g) + 3N2O(g) + energy
a. Why would AG be negative in the above reaction?
Both energy and entropy factors favor the forward spontaneous reaction. The
reaction is exothermic, and there are more gas molecules in the products than in the
reactants.
PROBLEMS Write the answer on the line to the left. Show all your work in the space
provided.
5. Consider the following equation and data: 2NO2(g) → N2O4(8)
AH of N2O4 = +9.2 kJ/mol
AH of NO2
==
+33.2 kJ/mol
ΔΗ
AG°-4.7 kJ/mol N2O4
AH° = −57.2 kJ/mol Use Hess's law to calculate AH° for the above reaction.
6.
2.26 × 104 J
-233.5 kJ/mol
7. a.
Calculate the energy needed to raise the temperature of 180.0 g of water
from 10.0°C to 40.0°C. The specific heat of water is 4.18 J/(K · g).
Calculate the change in Gibbs free energy for the following equation
at 25°C.
2H2O2(1)→ 2H2O(l) + O2(g)
Given AH = -196.0 kJ/mol
AS = +125.9 J/mol
b.
yes
Is this reaction spontaneous?
136 REACTION ENERGY
MODERN CHEMISTRY
Copyright by Holt, Rinehart and Winston. All rights reserved.
POTENTIAL ENERGY DIAGRAM
Potential
Energy
A+B
Ja
Name
e
d
C
b
Reaction Coordinate →
C + D + energy
A + B
C+D
Answer the questions using the graph above.
1. Is the above reaction endothermic or exothermic?
ехо
2. What letter represents the potential energy of the reactants?
3. What letter represents the potential energy of the products?
4. What letter represents the heat of reaction (AH)?
В
F
D
5. What letter represents the activation energy of the forward reaction?
6. What letter represents the activation energy of the reverse reaction?
7. What letter represents the potential energy of the activated complex?
8. Is the reverse reaction endothermic or exothermic?
END O
9. If a catalyst were added, what letter(s) would change? A, C, E
Chemistry IF8766
76
A
五
С
@instructional Fair, In
ENTROPY
Name
tropy is the degree of randomness in a substance. The symbol for change in
aropy is AS.
as are very ordered and have low entropy. Liquids and aqueous ions have more
entropy because they move about more freely, and gases have an even larger amount
entropy. According to the Second Law of Thermodynamics, nature is always
Boceeding to a state of higher entropy.
etermine whether the following reactions show an increase or decrease in entropy.
repse
2KCIO,(s) 2KCI(s) + 30₂(g)
H₂O(1)
H₂O(s)
3. N₂(g) + 3H₂(g) 2NH,(9)
4. NaCl(s) Na*(aq) + Cl(aq)
5. KCI(s) → KCK(1)
6. CO₂(s) CO2(g)
1
1
7. H(aq) + C,H,O,(aq)
->
HC,H,O,(
8. C(s) + O2(g) CO₂(g)
9. H₂(g) + Cl₂(g) → 2HCl(g)
10. Ag+(aq) + Cl(aq) AgCl(s)
A
11. 2N₂O(g)
4NO₂(g) + O2(g)
12. 2Al(s) + 31½(s)
2All,(s)
13. H+(aq) + OH(aq)
-
H₂O(1)
14. 2NO(g) N₂(g) + O2(g)
15. H₂O(g)
H₂O(1)
Chemistry IF8766
↓
77
No
change
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GIBBS FREE ENERGY
Key
Name
For a reaction to be spontaneous, the sign of AG (Gibbs Free Energy) must be negative.
The mathematical formula for this value is:
AG = AH TAS
-
where AH = change in enthalpy or heat of reaction
T = temperature in Kelvin
AS =
change in entropy or randomness
Complete the table for the sign of AG; +,- or
undetermined. When conditions allow for an
undetermined sign of AG, temperature will
decide spontaneity.
Answer the questions below.
1. The conditions in which AG is always
negative is when AH is
and AS is
+
ΔΗ
AS
AG
-
+
+
2. The conditions in which AG is always positive is when AH is
+
-
+
UND
+
- AT LOW Temp
AT HIGH TEMP
UND
+
and
E
V
AS is
3. When the situation is indeterminate, a low temperature favors the ( entropy/
enthalpy) factor, and a high temperature favors the entropy/ enthalpy ) factor.
Answer Problems 4-6 with always, sometimes or never.
4. The reaction: Na(OH),
be spontaneous.
←
Na+(aq) + OH-(aq) + energy will
ALWAYS
Never
be
5. The reaction: energy + 2H2(g) + O2(g) → 2H₂O (1) will
spontaneous.
Sometimes be spontaneous.
6. The reaction: energy + H₂O(s) → H₂O(1) will Sometimes
7. What is the value of AG if AH = -32.0 KJ, AS = +25.0 kJ/K and T = 293 K? 7360 kJ
8. Is the reaction in Problem 7 spontaneous? Yes
9. What is the value of AG if AH = +12.0 kJ, AS = -5.00 kJ/K and T = 290. K?
10. Is the reaction in Problem 9 spontaneous?
Chemistry IF8766
No
78
+
1460
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