Q: A 44.02 g/mol N$$_2O $$molecule at 25°C has an average speed of 37 m/s. What is its average kinetic energy in joules?

$$KE = \frac{1}{2}mv^2$ = \frac{1}{2} \times 7.31 \times 10$$^{-26}$$ \text{ kg} \times (37 \text{ m/s})^2$ = 5.00 \times 10$$^{-23}$$ \text{ J}$$

Q: A system releases 144.5 kJ of heat to its surroundings and does 65.5 kJ of work on its surroundings. What is the change in internal energy ($$\Delta U) of $$the system?

$$\Delta U = q + w = -144.5 \text{ kJ} + (-65.5 \text{ kJ}) = -210.0 \text{ kJ}$$

Q: How much heat, in kilojoules, must be added to a 275.0 g iron block (specific heat = 0.449 J g$$^{-1}$$ °C$$^{-1}) to $$increase its temperature from 25°C to its melting temperature of 1535°C?

$$q = m \times C_s \times \Delta T = 275.0 \text{ g} \times 0.449 \text{ J g}^{-1} \text{ °C}^{-1} \times (1535 - 25) \text{ °C} = 185,000 \text{ J} = 185 \text{ kJ}$$

Q: A 437.2 g sample of an unknown solid requires 8460 J to raise its temperature from 19.3°C to 68.9°C. What is its specific heat capacity?

$$C_s = \frac{q}{m \Delta T} = \frac{8460 \text{ J}}{437.2 \text{ g} \times (68.9 - 19.3) \text{ °C}} = 0.39 \text{ J g}^{-1} \text{ °C}^{-1}$$

Q: Calculate the standard enthalpy change ($$\Delta H^\circ) $$for the reaction: $$Fe_3O_4(s) + 4CO(g) \rightarrow 3Fe(s) + 4CO_2(g)$$, given $$\Delta H^\circ_f[Fe_3O_4(s)] = -1118.4 kJ/$$mol, $$\Delta H^\circ_f[CO(g)] = -110.5 kJ/$$mol, $$\Delta H^\circ_f[CO_2$(g)] = -393.5 kJ/$$mol, and $$\Delta H^\circ_f[Fe(s)] = 0 kJ/$$mol.

$$\Delta H^\circ = [3 \times 0 + 4 \times (-393.5)] - [1 \times (-1118.4) + 4 \times (-110.5)] = -1574.6 \text{ kJ}$$

Question 1

Which of the following statements best describes the First Law of Thermodynamics?

Accepted Answer

The total energy of the universe is constant; energy can neither be created nor destroyed, but it can be transferred from one object or system to another.

Question 2

A 44.02 g/mol N$$_2O $$molecule at 25°C has an average speed of 37 m/s. What is its average kinetic energy in joules?

Accepted Answer

$$KE = \frac{1}{2}mv^2$ = \frac{1}{2} 	imes 7.31 	imes 10$$^{-26}$$ 	ext{ kg} 	imes (37 	ext{ m/s})^2$ = 5.00 	imes 10$$^{-23}$$ 	ext{ J}$$

Question 3

Which of the following is a state function?

Accepted Answer

Change in altitude

Question 4

A system releases 144.5 kJ of heat to its surroundings and does 65.5 kJ of work on its surroundings. What is the change in internal energy ($$\Delta U) of $$the system?

Accepted Answer

$$\Delta U = q + w = -144.5 	ext{ kJ} + (-65.5 	ext{ kJ}) = -210.0 	ext{ kJ}$$

Question 5

Which of the following would show the largest temperature change upon absorbing 200.0 J of heat, assuming all sample masses are the same?

Accepted Answer

Iron, specific heat capacity = 0.290 J g$$^{-1}$$ °C$$^{-1}$$

Question 6

How much heat, in kilojoules, must be added to a 275.0 g iron block (specific heat = 0.449 J g$$^{-1}$$ °C$$^{-1}) to $$increase its temperature from 25°C to its melting temperature of 1535°C?

Accepted Answer

$$q = m 	imes C_s 	imes \Delta T = 275.0 	ext{ g} 	imes 0.449 	ext{ J g}^{-1} 	ext{ °C}^{-1} 	imes (1535 - 25) 	ext{ °C} = 185,000 	ext{ J} = 185 	ext{ kJ}$$

Question 7

Which of the following sign conventions for $$q$$ and $$w$$ represents a system that is doing work on the surroundings and losing heat to the surroundings?

Accepted Answer

$$q \u003c 0$$, $$w \u003c 0$$

Question 8

A 437.2 g sample of an unknown solid requires 8460 J to raise its temperature from 19.3°C to 68.9°C. What is its specific heat capacity?

Accepted Answer

$$C_s = \frac{q}{m \Delta T} = \frac{8460 	ext{ J}}{437.2 	ext{ g} 	imes (68.9 - 19.3) 	ext{ °C}} = 0.39 	ext{ J g}^{-1} 	ext{ °C}^{-1}$$

Question 9

Which of the following processes is exothermic?

Accepted Answer

Combustion of natural gas

Question 10

A chemical reaction at constant pressure has $$\Delta H = -2044 kJ/$$mol. What does this value indicate?

Accepted Answer

The reaction is exothermic and releases 2044 kJ of heat per mole.

Question 11

Which equation correctly relates enthalpy change ($$\Delta H) to $$internal energy change ($$\Delta U) $$for a reaction at constant pressure?

Accepted Answer

$$\Delta H = \Delta U + P \Delta V$$

Question 12

If a chemical equation is reversed, what happens to the enthalpy change ($$\Delta H$$)?

Accepted Answer

It changes sign.

Question 13

Which of the following is the correct formation equation for N$$_2O$_4(g$$)?

Accepted Answer

$$N_2(g) + 2O_2(g$$) \rightarrow N_2O_4(g)$$

Question 14

Calculate the standard enthalpy change ($$\Delta H^\circ) $$for the reaction: $$Fe_3O_4(s) + 4CO(g) \rightarrow 3Fe(s) + 4CO_2(g)$$, given $$\Delta H^\circ_f[Fe_3O_4(s)] = -1118.4 kJ/$$mol, $$\Delta H^\circ_f[CO(g)] = -110.5 kJ/$$mol, $$\Delta H^\circ_f[CO_2$(g)] = -393.5 kJ/$$mol, and $$\Delta H^\circ_f[Fe(s)] = 0 kJ/$$mol.

Accepted Answer

$$\Delta H^\circ = [3 	imes 0 + 4 	imes (-393.5)] - [1 	imes (-1118.4) + 4 	imes (-110.5)] = -1574.6 	ext{ kJ}$$

Question 15

Which of the following statements about the dissolution of ammonium chloride in water is true?

Accepted Answer

The dissolution of ammonium chloride is an endothermic process.

Question 16

A reaction has $$\Delta H = +90.3 kJ/$$mol. Which statement is correct?

Accepted Answer

The reaction is endothermic and absorbs heat from the surroundings.

Thermochemistry: Energy, Work, and Heat in Chemical Systems

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