Chemistry Practice Test: Thermochemistry
- Thermochemistry
- 10 questions
- 15 minutes
This test covers heat, specific heat and enthalpy changes in reactions. Use c = 4.18 J/(g·°C) for water. Tip: a negative ΔH means the reaction releases heat to its surroundings.
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Question 1
How much heat is needed to warm 50.0 g of water from 20.0 °C to 30.0 °C?
Correct answer: C
q = mcΔT = 50.0 g × 4.18 J/(g·°C) × 10.0 °C = 2,090 J. 6,270 J uses the final temperature of 30 °C instead of the temperature change.
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Question 2
A 100. g piece of metal absorbs 900. J of heat, and its temperature rises by 10.0 °C. What is the specific heat of the metal?
Correct answer: A
c = q ÷ (mΔT) = 900. ÷ (100. × 10.0) = 0.900 J/(g·°C), which is about the value for aluminum. 9.00 forgets to divide by the temperature change.
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Question 3
A reaction takes place in a beaker, and the beaker feels warm to the touch. Which statement is true?
Correct answer: B
The beaker warms because the reaction releases heat to its surroundings, which makes it exothermic. Heat leaving the system means the system's enthalpy goes down, so ΔH is negative.
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Question 4
Use these reactions:
C(s) + O2(g) → CO2(g), ΔH = −394 kJ
CO(g) + ½O2(g) → CO2(g), ΔH = −283 kJ
What is ΔH for C(s) + ½O2(g) → CO(g)?Correct answer: D
Keep the first reaction and reverse the second (+283 kJ); adding them gives C + ½O2 → CO with ΔH = −394 + 283 = −111 kJ. −677 kJ adds the two values without reversing the second reaction.
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Question 5
Bond enthalpies (kJ/mol): H-H 436, Cl-Cl 243, H-Cl 432. What is ΔH for H2(g) + Cl2(g) → 2HCl(g)?
Correct answer: A
ΔH = bonds broken − bonds formed = (436 + 243) − 2 × 432 = 679 − 864 = −185 kJ. +247 kJ counts only one H-Cl bond formed, even though two form.
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Question 6
Methane burns: CH4 + 2O2 → CO2 + 2H2O, ΔH = −890 kJ. Atomic masses: C = 12, H = 1. How much heat is released when 8.0 g of methane burns?
Correct answer: C
8.0 g ÷ 16 g/mol = 0.50 mol, and 0.50 × 890 kJ = 445 kJ. 890 kJ is the heat for a full mole of methane.
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Question 7
The heat of fusion of ice is 6.01 kJ/mol, and the molar mass of water is 18 g/mol. How much heat is needed to melt 36 g of ice at 0 °C?
Correct answer: D
36 g ÷ 18 g/mol = 2.0 mol, and 2.0 × 6.01 kJ = 12.0 kJ. Multiplying 6.01 by the grams instead of the moles gives 216 kJ.
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Question 8
100 g of water at 80 °C is mixed with 100 g of water at 20 °C in an insulated cup. What is the final temperature?
Correct answer: B
Heat lost by the hot water equals heat gained by the cold water, and with equal masses the final temperature is the average: (80 + 20) ÷ 2 = 50 °C. Temperatures do not add, so 100 °C is impossible.
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Question 9
Which of these processes is endothermic?
Correct answer: C
Melting needs heat to pull water molecules away from their fixed positions, so it absorbs energy. Freezing and condensing are the reverse changes and release heat, and burning releases heat as well.
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Question 10
Which substance has a standard enthalpy of formation equal to zero?
Correct answer: A
By definition, an element in its most stable form at standard conditions has ΔHf° = 0, and for oxygen that is O2 gas. Ozone, O3, is a less stable form of oxygen, so its value is not zero.
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