Question 1

Which of the following best describes an isolated system in thermochemistry?

Accepted Answer

A system that cannot exchange either energy or matter with its surroundings.

Question 2

A chemical reaction occurs in a closed container. The system absorbs 250 J of heat from the surroundings and does 100 J of work on the surroundings. What is the change in internal energy ($$\Delta U) of $$the system?

Accepted Answer

$$\Delta U = 150\ \mathrm{J}$$

Question 3

Which of the following statements about state functions is correct?

Accepted Answer

Enthalpy is a state function because it depends only on the initial and final states of the system.

Question 4

A piston contains a gas at 2.00 atm. The gas expands from 1.00 L to 3.00 L against a constant external pressure. Calculate the work done by the system in joules. ($$1\ \mathrm{L\cdot atm} = 101.325\ \mathrm{J})$$

Accepted Answer

$$w = -202.65\ \mathrm{J}$$

Question 5

Which of the following processes is exothermic?

Accepted Answer

Combustion of butane in oxygen

Question 6

The enthalpy change for the reaction $$\mathrm{C(s) + O_2$(g) \rightarrow CO_2$(g)} is$$ $$\Delta H = -393.51\ \mathrm{kJ}. $$How much heat is released when 44 g of $$\mathrm{CO_2$} is $$produced? (Molar mass of $$\mathrm{CO_2$} = 44\ \mathrm{g/mol})$$

Accepted Answer

$$-393.51\ \mathrm{kJ}$$

Question 7

Which equation correctly relates the change in enthalpy ($$\Delta H) to $$the heat exchanged at constant pressure?

Accepted Answer

$$\Delta H = q_p$$

Question 8

A reaction is carried out in a bomb calorimeter and releases 500 J of heat. Which of the following is true?

Accepted Answer

The reaction is exothermic and $$\Delta U \u003c 0.$$

Question 9

Which of the following best describes the difference between specific heat capacity and molar heat capacity?

Accepted Answer

Specific heat capacity is the heat required to raise the temperature of one gram of a substance by 1 K, while molar heat capacity is for one mole.

Question 10

A 100 g sample of water is heated from 25°C to 75°C. How much heat is absorbed? (Specific heat of water = $$4.18\ \mathrm{J/g\cdot K})$$

Accepted Answer

$$q = 20,900\ \mathrm{J}$$

Question 11

Which of the following is a correct application of Hess’s Law?

Accepted Answer

The enthalpy change for a reaction is the sum of the enthalpy changes for each step in a series of reactions.

Question 12

Given the following reactions:
(i) $$\mathrm{P_4$(s) + 5O_2(g) \rightarrow P_4O_{10}(s)}$$, $$\Delta H = -2940.1\ \mathrm{kJ}
(ii$$) $$\mathrm{P_4$(s) + 3O_2(g) \rightarrow P_4O_6(s)}$$, $$\Delta H = -1640.1\ \mathrm{kJ}$$
Calculate $$\Delta H$$ for $$\mathrm{P_4O_6(s) + 2O_2(g) \rightarrow P_4O_{10}(s)}.$$

Accepted Answer

$$\Delta H = -1300.0\ \mathrm{kJ}$$

Question 13

Which of the following statements about bond enthalpy is correct?

Accepted Answer

Bond enthalpy is always positive because energy is required to break bonds.

Question 14

Calculate the enthalpy change ($$\Delta H_{rxn}) $$for the reaction:
$$\mathrm{CH_4$(g) + 2O_2(g) \rightarrow CO_2$(g) + 2H_2O(l)}$$
Given:
$$\Delta H_f^\circ [\mathrm{CO_2$(g)}] = -393.5\ \mathrm{kJ/mol}$$
$$\Delta H_f^\circ [\mathrm{H_2O(l)}] = -285.8\ \mathrm{kJ/mol}$$
$$\Delta H_f^\circ [\mathrm{CH_4$(g)}] = -103.85\ \mathrm{kJ/mol}$$
$$\Delta H_f^\circ [\mathrm{O_2$(g)}] = 0\ \mathrm{kJ/mol}$$

Accepted Answer

$$\Delta H_{rxn} = -2219.9\ \mathrm{kJ}$$

Question 15

Which of the following best explains why the combustion of gasoline in a car engine is considered an exothermic process?

Accepted Answer

The system releases heat to the surroundings, increasing the temperature of the engine.

Question 16

Which of the following is the correct formula for calculating the heat absorbed or released by a substance?

Accepted Answer

$$q = mC_p\Delta T$$

Question 17

A reaction has $$\Delta H = +75\ \mathrm{kJ}. $$Which of the following statements is true?

Accepted Answer

The reaction is endothermic and heat is absorbed.

Question 18

Which of the following is a real-world application of calorimetry?

Accepted Answer

Measuring the amount of heat released when a food sample is burned to determine its caloric content.

Thermochemistry: Energy, Systems, and Enthalpy in Chemical Reactions

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