Download 6 Thermodynamics

South Pasadena • AP Chemistry
Name
6 ▪ Thermodynamics
Period
UNIT TEST
Date
–
PRACTICE
Part 1 – Multiple Choice
You should allocate 25 minutes to finish this portion of the test. No calculator should be used. A periodic table
and data table will be provided. Select the answer that best responds to each question.
1. Naphthalene (C10H8), a key ingredient in
traditional mothballs, undergoes sublimation at
25°C:
C10H8 (s) → C10H8 (g)
If a mothball is placed in an insulated container
with an inert gas sample at 40°C, what may be
observed?
(A) The temperature of the gas decreases
slightly.
(B) The temperature of the gas increases slightly.
(C) The temperature of the gas stays the same.
(D) The temperature of the mothball increases
slightly.
2. A sample of H2O (g) is placed in a cylinder with a
piston against a constant external pressure of 2.0
atm. How is the internal energy of the system
changing when the sample condenses?
(A) The change in internal energy of the system
cannot be determined from the information
given.
(B) The internal energy of the system is
decreasing.
(C) The internal energy of the system is
increasing.
(D) The internal energy of the system remains
constant.
3. A cup with water at room temperature is placed in
a tub of ice at 0°C. Which of the following takes
place?
(A) The attractions between molecules in the ice
are weakened, increasing the motion of water
molecules in the cup.
(B) The attractions between molecules in the ice
are weakened, decreasing the motion of
water molecules in the cup.
(C) The attractions between molecules in the ice
are strengthened, increasing the motion of
water molecules in the cup.
(D) The attractions between molecules in the ice
are strengthened, decreasing the motion of
water molecules in the cup.
4. When 0.100 g benzoic acid (HC7H5O2, molar mass
122) and excess oxygen is ignited in a bomb
calorimeter, the temperature of the water changes
from 25.000°C to 25.225°C. The heat capacity of
the calorimeter is 603 J/°C. What is the energy of
this sample of benzoic acid?
(A) 13.5 J
(B) 135 J
(C) 1350 J
(D) 15,100 J
5. Using the enthalpies of formation given below,
calculate the ∆Hcombustion for propane, C3H8.
∆H°f (kJ/mol)
H2O(l)
−286
CO2(g)
−394
C3H8(g)
−104
(A) +576 kJ/mol
(B) −576 kJ/mol
(C) −2222 kJ/mol
(D) −2330 kJ/mol
6. As ammonium nitrate is dissolved in water, the
temperature of the system decreases. This process
can be thought of in the following steps:
I. Break ionic bonds between NH4+ and
NO3− ions.
II. Break hydrogen bonds between water
molecules.
III. Form ion-dipole attractions between water
and NH4+ and NO3− ions.
Which of the following explains the change in
temperature observed?
(A) The sum of the energies required in steps I
and II is greater than the energy released in
step III.
(B) The sum of the energies required in steps I and
II is less than the energy released in step III.
(C) All three steps release energy to the
surroundings.
(D) All three steps require energy to be absorbed.
7. Consider the following potential energy graph for
the C−C bond:
Which of these describes two interacting C atoms
that are separated by 0.100 nm?
(A) The attractions between the atoms are stronger
than the repulsions between the atoms, and the
internuclear distance will decrease.
(B) The repulsions between the atoms are
stronger than the attractions between the
atoms, and the internuclear distance will
increase.
(C) The attractions are maximized and repulsions
are minimized, so the internuclear distance
will not change.
(D) There are no attractive or repulsive forces
between the atoms because they are separated
from each other.
8. Given the following equations for the enthalpies of
formations, ∆H°f:
P4 (s) + 5 O2 (g) → P4O10 (s)
∆H°f = x
H2 (g) + ½ O2 (g) → H2O (ℓ)
∆H°f = y
3/2 H2 (g) + 1/4 P4 (s) +2 O2 (g) → H3PO4 (aq)
∆H°f = z
Which of the following expressions represent the
enthalpy of reaction, ∆H°rxn, for the following
reaction?
P4O10 (s) + 6 H2O (ℓ) → 4 H3PO4 (aq)
(A) 4z – x – 6y
(B) 4z – x + 6y
(C) x + 6y – 4z
(D) z – x – y
9. Use the following data to find the standard entropy
change for the following reaction,
Cu(s) + ½ O2(g)  CuO(s).
S[Cu(s)] = 33 J/K·mol
S[O2(g)] = 205 J/K·mol
S[CuO(s)] = 42 J/K·mol
(A) −111.5 J/mol·K
(B) −93.5 J/mol·K
(C) +111.5 J/mol·K
(D) +93.5 J/mol·K
10. In which of the following processes is the standard
entropy of reaction, ∆S°, expected to be negative?
(A) (NH4)2CO3 (s) →
2 NH3 (g) + H2O (ℓ) + CO2 (g)
(B) Pb(NO3)2 (aq) + 2 HCl (aq) →
PbCl2 (s) + 2 HNO3 (aq)
(C) P4O10 (s) + 6 H2O (ℓ) →4 H3PO3 (aq)
(D) CO2 (s) → CO2 (g)
11. NH4Cl (s) was placed in a container filled with
water, and the temperature dropped considerably.
The reaction proceeds according to the following
equation:
NH4Cl (s) → NH4+ (aq) + Cl– (aq)
What are the signs for ∆H, ∆S, and ∆G for the
process?
(A) ∆H < 0 ∆S < 0 ∆G > 0
(B) ∆H < 0 ∆S > 0 ∆G < 0
(C) ∆H > 0 ∆S < 0 ∆G > 0
(D) ∆H > 0 ∆S > 0 ∆G < 0
12. For the reaction below, ∆H°rxn = –114 kJ/mol and
∆S° = –146.5 J/mol-K. For which temperatures is
the reaction thermodynamically favorable?
Assume that the values do not change with
temperature.
2 NO (g) + O2 (g) → 2 NO2 (g)
(A) The process is thermodynamically favorable at
all temperatures
(B) The process is only thermodynamically
favorable above 778 K
(C) The process is only thermodynamically
favorable at 778 K
(D) The process is only thermodynamically
favorable below 778 K
13. Given the following information, calculate G
for the reaction below at 25C:
2 H2O2(l)  2 H2O(l) + O2(g)
Compound H(kJ/mol) S(J/K·mol)
H2O2(l)
−190
110
H2O(l)
−290
70
O2(g)
−−
205
(A) −37,500 kJ/mol
(B) −3,330 kJ/mol
(C) −237 kJ/mol
(D) −203 kJ/mol
14. Consider the following reaction:
SnCl4(l) + 2 H2O(l)  SnO2(s) + 4 HCl(g)
At 25°C, H = +133.0 kJ/mol and S = +401.5
J/mol·K. What is true about the reaction at this
temperature?
(A) Q > K
(B) Q = K
(C) Q < K
(D) Nothing can be stated about the equilibrium of
this reaction.
15. The free energy change for a given reaction is −25
kJ. What is the equilibrium constant at 300 K?
(A) e0 = 1.0
(B) e1 = 2.7
(C) e−10 = 4.4 × 10−5
(D) e10 = 2.3 × 104
Part 2 – Free Response
You should allocate 30 minutes to finish this portion of the test. You may use a scientific calculator. A periodic
table and data table will be provided. Respond to each part of the questions completely. Be sure to show your
work clearly for questions that involve calculations.
16. Gaseous acetaldehyde, CH3CHO (molar mass 44.05 g/mol), undergoes combustion according to the following
reaction:
2 CH3CHO (g) + 5 O2 (g) → 4 CO2 (g) + 4 H2O (ℓ)
When 79.62 g of CH3CHO is consumed, 4,309 kJ of energy was released. The process takes place in a
container against an external pressure of 101.3 kPa and temperature of 25°C. The volume of the container
decreases by 66.3 L. (1 kPa·L = 1 J)
(a) What is the value of w and ∆E of the system in J?
q = −4,309 kJ
w = −P·∆V = −(101.3 kPa)(−66.3 L)(1 J/1 kPa·L) = +6720 J
∆E = q + w = −4,309,000 J + 6,720 J = −4,302,000 J
(b) What is the value for ∆H°rxn for this process in kJ/mol? Assume that the work of the system is negligible.
–4309 kJ
q
2 mol CH3CHO = –4,760 kJ/mol
∆H° = =
n 1.81 mol CH3CHO  1 mol reaction 
(c) Using the structure of acetaldehyde and the average bond energies given, determine the approximate bond
energy of the C=O bond.
H
H
O
C
C
H
H
2H
H
O
C
C
H
Bond
Average Bond Energy
C–H
C–C
O–H
O=O
413 kJ/mol
347 kJ/mol
467 kJ/mol
495 kJ/mol
+ 5[ O=O ] → 4 [ O=C=O ] + 4 [H–O–H]
H Register
Please
∆H° = 2[(Break 4 C–H) + (Break 1 C–C) + (Break 1 C=O)] + 5[Break 1 O=O]
+ 4[Form 2 C=O] + 4[Form 2 H–O]
–4,760 = 2[4(413) + (347) + ∆HC=O] + 5(495) + 4[2(–∆HC=O)] + 4[2(–467)]
–4,760 = 2737 – 6∆HC=O
∆HC=O = 1,250 kJ/mol
(d) Acetaldehyde is formed from ethane, C2H4. Use the following reactions to determine the value of ∆S°rxn
in J/mol-K for the combustion of acetaldehyde.
2 C2H4 (g) + O2 (g) → 2 CH3CHO (g)
∆S°1 = –323.86 J/mol-K
C2H4 (g) + 3 O2 (g) → 2 CO2 (g) + 2 H2O (ℓ)
∆S°2 = –267.68 J/mol-K
2 CH3CHO (g) → 2 C2H4 (g) + O2 (g)
∆S°1 = +323.86 J/mol-K
2 C2H4 (g) + 6 O2 (g) → 4 CO2 (g) + 4 H2O (ℓ)
∆S°2 = –535.36 J/mol-K
––––––––––––––––––––––––––––––––––––––––––––––––––––––––––
2 CH3CHO (g) + 5 O2 (g) → 4 CO2 (g) + 4 H2O (ℓ) ∆S° = –211.5 J/mol-K
(e) What is the value of ∆G°rxn for the combustion of acetaldehyde in kJ/mol? Is the process spontaneous?
Why or why not?
∆G° = ∆H° – T∆S° = (–4,760 kJ/mol) – (298 K)(–211.5 J/mol-K)(1 kJ/1000 J) = –4699 kJ/mol
The reaction is spontaneous because ∆G < 0.
(f) If ∆G°f for CO2 (g) and H2O (ℓ) are –394.36 kJ/mol and –237.13 kJ/mol, respectively, what is the value of
∆G°f for CH3CHO (g)?
Use [Products – Reactants]
∆G°rxn = [4∆G° f,CO2 + 4∆G°f,H2O] – [2∆G° f,CH3CHO + 5∆G° f,O2]
–4699 = [4(–394.36) + 4(–237.13)] – [2∆G° f,CH3CHO + 5(0)]
∆G° f,CH3CHO = –1087 kJ/mol
(g) At what temperatures is this process spontaneous?
Process becomes non-spontaneous at ∆G° = 0
∆H° – ∆G°
(–4760 kJ/mol) – 0
∆G° = ∆H° – T∆S°  T =
=
= 22,500 K
∆S°
(–211.5 J/mol-K)(1 kJ/1000 J)
It will be spontaneous at temperatures less than 22,500 K.
17. Consider the catalyzed isomerization of cis-2-butene gas to produce trans-2-butene gas, represented below,
and the thermodynamic data provided below.
cis-2-butene
trans-2-butene
∆H°f (kJ/mol)
S° (J/mol·K)
−7.0
−11.2
+301
+296
Potential Energy
(a) Compare the bonds between cis-2-butene and trans-2-butene. Assume that the corresponding bonds
between the compounds have the same bond energy except for the C=C bond. The potential energy graph
for the C=C bond for each structure is shown below. Identify to which structure each bond correlates.
Explain briefly.
Inter-nuclear
separation
cis-2-butene
trans-2-butene
The cis- structure has a smaller C−C bond energy because its ∆Hf is more positive, and it has a
higher potential energy.
(b) Find the values for ∆G° for this reaction at 298 K.
∆H°rxn = (−11.2 kJ/mol) – (−7.0 kJ/mol) = −4.2 kJ/mol
∆S°rxn = (+296 J/mol·K) – (+301 J/mol·K) = −5 J/mol·K
∆G°rxn = ∆H° − T·∆S° = (−4.2 kJ/mol) – (298 K)(−0.005 kJ/mol·K) = −2.71 kJ/mol
(c) Calculate the value of the equilibrium constant, K, at 298 K.
−∆G°rxn/R·T
Keq = e
=e
−(−2710/(8.314)(298))
= e1.09 = 3.0
South Pasadena • AP Chemistry
Name
6 ▪ Thermodynamics
Period
UNIT TEST
Date
BLUEPRINT
Part 1: Multiple Choice
 Format: 15 questions, four answer choices: (A)-(D)
 Expected time: 25 minutes
 Allowed resources: Periodic Table, Equations and Constants. No calculators.
Q Lesson
Topic
1 6.0
Endo/Exo
2 6.1
Heat vs. Work
3 6.0
Molecular
Descriptions
∆H = q/n
4 6.1
Objective
Describe a process as exothermic vs. endothermic (e.g. value of q, direction of
energy flow, graph, thermochemical equation).
Explain how the internal energy of a system can change with heat flow and
expansion work.
Describe the particulate behavior of a change in temperature or state.
13 6.2
14 6.3
Calculate the heat involved in the change in temperature of an object.
Calculate the heat involved in the change in state of a sample.
Calculate the heat involved in a physical or chemical process using ∆H.
∆Hf
Determine the ∆Hrxn using ∆Hf of substances, and by analyzing energy of
covalent bonds formed and broken.
Bond Energy and
Determine the ∆Hrxn using ∆Hf of substances, and by analyzing energy of
∆H°
covalent bonds formed and broken.
PE of Covalent Bond Explain the bond length and energy of a covalent bond using a potential energy
graph.
Hess’s Law
Calculate the value of ∆Hrxn using Hess’s Law
Entropy
Predict whether a physical or chemical process is increasing or decreasing in
entropy.
Entropy Change
Predict whether a physical or chemical process is increasing or decreasing in
entropy.
Spontaneity
Determine under what conditions a process is spontaneous (thermodynamically
favorable) using the value of ∆G.
Spontaneity
Determine under what conditions a process is spontaneous (thermodynamically
favorable) using the value of ∆G.
∆G = ∆H − T∆S
Calculate the value of ∆Grxn using ∆Gf or Hess’s Law.
∆G vs. Q
Determine the spontaneous direction of a reversible process.
15 6.3
∆G° vs. Keq
5 6.1
6 6.1
7 6.1
8 6.1
9 6.2
10 6.2
11 6.2
12 6.2
Find the value of Keq from thermodynamic data.
Part 2: Free Response




Format:
o 1 long question (5-8 parts)
o 1 short questions (2-4 parts)
Expected time: 30 minutes
Allowed resources: Periodic Table, Equations and Constants, and scientific calculators.
Topics: Any