South Pasadena • Honors Chemistry
Name
6 · Thermochemistry
Period
Date
UNIT TEST
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PRACTICE
You may use a pencil, eraser, and scientific calculator to complete the test.
You will be given a periodic table. No other resources are allowed.
Please transfer your answers for Multiple Choice and Matching questions onto the Answer Document. Work
for these questions will not be graded. However, sufficient and appropriate work must be shown for the Free
Response questions in order to receive full credit.
SECTION 1: A, B, or C. Determine whether each description refers to an endothermic process, exothermic
process, or either. (1 point each)
(A)
(B)
(C)
1. CaCl2 (s), when dissolved in water, warms up.
Endothermic Exothermic
Either
Endothermic
Exothermic
Either
3. 2 KClO3 (s) + energy  2 KCl (s) + 3 O2 (g)
Endothermic
Exothermic
Either
4. The following potential energy
graph:
Endothermic
Exothermic
Either
5. The reaction mixture has a temperature of 200°C.
Endothermic
Exothermic
Either
6. q = ‒ 250 J
Endothermic
Exothermic
Either
Energy
2. Energy is absorbed in a reaction process.
Reactants
Products
SECTION 2: TRUE/FALSE. (1 point each)
Evaluate the statement in column 1. If it’s TRUE, fill in bubble A. If it’s FALSE, fill in bubble B.
Evaluate the statement in column 2. If it’s TRUE, fill in bubble C. If it’s FALSE, fill in bubble D.
If column 2 is the correct explanation for column 1, then bubble E.
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7.
8.
9.
If both columns are TRUE, but column 2 is the correct explanation, bubble ACE.
If both columns are TRUE, but column 2 is not the correct explanation, bubble AC.
If column 1 is TRUE and column 2 is FALSE, bubble AD.
If column 1 is FALSE and column 2 is TRUE, bubble BC.
If column 1 is FALSE and column 2 is FALSE, bubble BD.
Column 1
An exothermic process has a high
temperature
BC
When ice at ‒15°C is heated, initially the
ice will start to melt
BC
In a reaction A  B + heat, the value of
∆Hrxn is negative
ACE
BECAUSE
Column 2
exothermic processes release energy.
BECAUSE
the heat added increases the average kinetic
energy of the sample of ice.
BECAUSE
∆Hf of B is less than ∆Hf of A.
SECTION 3: MULTIPLE CHOICE. Select the best answer choice for each question. (1 point each)
10. Which of the following is true about the freezing
of water?
I. H2O (ℓ)  H2O (s) + heat
II. q > 0
III. Molecular attractions are weakened.
(A) I only.
(B) I and II only.
(C) II and III only.
(D) I, II and III.
15. 30.0 g samples of liquid ethanol and water are
heated at their respective boiling points. Which
sample will take longer to vaporize completely?
(∆Hvap: ethanol = 200 cal/g, water = 540 cal/g)
(A) Ethanol
(B) Water
(C) Both will take the same amount of time.
(D) It cannot be determined from the information
given.
11. The units for specific heat are:
(A) J
(B) J/°C
(C) J/mol
(D) J/g·°C
16. Which of the following is the correct equation for
the condensation of water?
(A) heat + H2O (ℓ)  H2O (g)
(B) H2O (ℓ)  H2O (g) + heat
(C) heat + H2O (g)  H2O (ℓ)
(D) H2O (g)  H2O (ℓ) + heat
13. A substance with high specific heat:
(A) Is a conductor.
(B) Is exothermic
(C) Metals are typical examples.
(D) Requires more energy to raise the
temperature than a substance with lower
specific heat.
14. Which of the following is an example of an
endothermic process?
(A) The combustion of methane, CH4, releasing
heat.
(B) The dissolving of CaCl2, which feels warmer.
(C) The freezing of water at 0°C.
(D) The vaporization of ethanol, C2H5OH, when
heated.
17. Which of the following takes place initially when
heat is added to a block of ice at 0°C?
I. The temperature increases.
II. The ice melts.
III. Molecular attractions strengthen.
(A) I only.
(B) II only.
(C) I and II only.
(D) II and III only.
18. In which of the following portion(s) of the heating
curve is the substance changing states?
4
Temp
12. Which of the following is true for an endothermic
process?
(A) Heat is a product.
(B) q < 0
(C) The reaction mixture may increase in
temperature.
(D) The reaction releases heat.
3
1
2
(A) Segment 2 only
(B) Segment 3 only
(C) Segments 1 and 3 only
(D) Segments 2 and 4 only
19. Blocks A and B are made from different metals
and have the same mass. When they are heated
with 500 J of energy, block A increases 15°C,
while block B increases 90°C. Which has a larger
specific heat?
(A) Block A
(B) Block B
(C) They have the same specific heat.
(D) There is not enough information to compare.
20. When a 20-g of a metal is heated with 140 J of
energy, the temperature increases from 20.0°C to
50.8°C. Which of the following could be the
metal?
(A) Aluminum
C = 0.897 J/g·°C
(B) Tin
C = 0.227 J/g·°C
(C) Titanium
C = 0.523 J/g·°C
(D) Tungsten
C = 0.134 J/g·°C
21. Consider the following reaction:
C2H2 (g) + 2 H2 (g)  C2H6 (g) + 310 kJ
What volume of H2 at STP reacts if 155 kJ is
released?
(A) 5.75 L
(B) 11.2 L
(C) 22.4 L
(D) 44.8 L
22. Consider the following reaction:
2 H‒O‒H  2 H‒H + O=O
If ∆Hrxn = +572 kJ/mol, find the bond energy of
the O=O bond.
Bond
Bond Energy
(A) 70 kJ/mol
O‒H
467 kJ/mol
(B) 432 kJ/mol
H‒H
432 kJ/mol
(C) 502 kJ/mol
(D) 1570 kJ/mol
23. What is the value of ∆Hrxn for the following
reaction? 2 N2O5 (s)  2 N2 (g) + 5 O2 (g)
Substance
∆Hf
(A) ‒84 kJ/mol
N
O
(s)
‒42
kJ/mol
2
5
(B) +42 kJ/mol
(C) +84 kJ/mol
(D) Not enough information to determine.
24. Consider the following reaction:
PCl5 (g) + 4 H2O (ℓ)  H3PO4 (aq) + 5 HCl (aq)
If 65.0 g PCl5 reacts (MM: 208 g/mol), 72 kJ of
energy is released. What is the value of ∆Hrxn?
(A) +230 kJ/mol
(B) +23 kJ/mol
(C) ‒23 kJ/mol
(D) ‒230 kJ/mol
25. When 1.0 mol AlCl3 is decomposed, 290 kJ of
energy is absorbed. Which of the following
represents this process?
(A) 2 AlCl3 (s)  2 Al (s) + 3 Cl2 (g) + 290 kJ
(B) 2 AlCl3 (s)  2 Al (s) + 3 Cl2 (g) + 580 kJ
(C) 290 kJ + 2 AlCl3 (s)  2 Al (s) + 3 Cl2 (g)
(D) 580 kJ + 2 AlCl3 (s)  2 Al (s) + 3 Cl2 (g)
SECTION 4: FREE RESPONSE. Show all your work to receive full credit.
26. Explain why it feels cold when NH4Cl is dissolved in water. Is this process endothermic or exothermic?
Use complete sentences. (4 points)
The process is endothermic. The reaction feels cold because the reaction is absorbing energy from the
surroundings.
27. What is the final temperature when a 32.0 g piece of diamond at 33.5°C is heated with 360 J of energy?
(C = 0.509 J/g·°C)
I E S
q = 360 J
q = m · C · ∆T
q
360 J
m = 32.0 g
∆T =
=
m · C (32.0 g)(0.509 J/g·°C)
C = 0.509 J/g·°C
Tf ‒ 33.5°C = 22.1
∆T = Tf ‒ 33.5°C
Tf = 55.6°C
A
28. What is the value of ∆Hvap of methanol, CH3OH, if 27,050 J is released when 24.5 g of methanol condenses?
q = ‒27,050 J = ‒27.05 kJ
q = n · ∆H
1 mol CH3OH 
q ‒27.05 kJ
n = 24.5 g CH3OH 
∆H = =
= ‒35.4 kJ/mol
n 0.765 mol
32.042 g CH3OH = 0.765 mol CH3OH
∆H = ‒∆Hvap
∆Hvap = 35.4 kJ/mol
Temp
29. A 40.0 g block of ice at ‒5°C is heated to water at 25°C. For H2O, ∆Hfus = 6.01 kJ/mol, ∆Hvap = 40.68 kJ/mol,
Cice = 2.10 J/ g·°C, Cwater = 4.18 J/ g·°C, Csteam = 2.08 J/g·°C.
(a) Darken the portion(s) of the heating curve that corresponds to this process. (2 points)
(iii)
(i) (ii)
(b) For each segment that takes place, describe what is happening in terms of (1) states and (2) the
temperature. Use complete sentences. (4 points)
In segment (i), the sample stays as a solid, but the temperature is increasing from ‒5°C to 0°C.
In segment (ii), the sample is melting, turning from solid to liquid, but temperature remains at 0°C.
In segment (iii), the sample stays as a liquid, but the temperature is increasing from 0°C to 25°C.
(c) Calculate the heat required for this process. (6 points)
q(i) = m·C·∆T = (40.0 g)(2.10 J/g·°C)(0°C ‒ (‒5°C))
1 kJ 
1000 J = 0.420 kJ
1 mol  6.01 kJ
q(ii) = n · ∆H = (40.0 g)
18.016 g  1 mol  = 13.3 kJ
1 kJ 
q(iii) = m·C·∆T = (40.0 g)(4.18 J/g·°C)(25°C ‒ 0°C)
1000 J = 4.18 kJ
q = q(i) + q(ii) + q(iii) = 0.420 kJ + 13.3 kJ + 4.18 kJ = 17.9 kJ
30. Consider the reaction:
2 NO (g) + O2 (g)  2 NO2 (g) + 112 kJ
(a) How many grams of NO (g) is used up when 428 kJ of energy is released?
1 mol rxn 2 mol NO 30.01 g NO
‒428 kJ 
= 229 g NO
 ‒112 kJ  1 mol rxn  1 mol NO 
(b) If ∆Hf of NO2 is +34 kJ/mol, find the value of ∆Hf of NO.
∆Hrxn = [2(∆Hf,NO2)] ‒ [2(∆Hf,NO) + 1(∆Hf,O2)]
‒112 kJ/mol = [2(34)] ‒ [2(∆Hf,NO) + 1(0)]
∆Hf,NO = +90 kJ/mol
(c) The N-O bond energy (in the compound NO) is 607 kJ/mol, and the O=O bond energy (in O2) is 495
kJ/mol. Find the bond energy of the N- -O bond in NO2 (O- -N- -O).
∆Hrxn = Break 2 (N-O) + Break 1 (O=O) + Form 4 (N- -O)
‒112 kJ/mol = 2(+607 kJ/mol) + 1(495 kJ/mol) + 4(‒BEN- -O)
BEN- -O = 455 kJ/mol