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 only be transferred or transformed.

Question 2

A chemical reaction in a closed container releases 250 J of heat to the surroundings and does 150 J of work on the surroundings. What is the change in internal energy ($$\Delta E) of $$the system?

Accepted Answer

$$\Delta E = -400\ \mathrm{J}$$

Question 3

Which of the following is the correct equation for the standard enthalpy of formation of liquid water?

Accepted Answer

$$\mathrm{H}_2(g) + \frac{1}{2}\mathrm{O}_2(g) \rightarrow \mathrm{H}_2\mathrm{O}(l)$$

Question 4

Which of the following elements has a nonzero standard enthalpy of formation ($$\Delta H_f^\circ) at 25$$ $$^\circC $$and 1 atm?

Accepted Answer

$$\mathrm{C(diamond)}$$

Question 5

A reaction has $$\Delta H \u003c 0$$ and $$\Delta E \u003c 0. $$Which of the following is true about this reaction?

Accepted Answer

The reaction is exothermic and releases energy to the surroundings.

Question 6

Which of the following best describes an extensive property?

Accepted Answer

A property that depends on the amount of substance present.

Question 7

Calculate the standard enthalpy change ($$\Delta H_{rxn}^\circ) $$for the following reaction using the given standard enthalpies of formation:

$$\mathrm{C}_2\mathrm{H}_6(g) + \frac{7}{2}\mathrm{O}_2(g) \rightarrow 2\mathrm{CO}_2(g) + 3\mathrm{H}_2\mathrm{O}(l)$$

Given:
$$\Delta H_f^\circ[\mathrm{C}_2\mathrm{H}_6(g)] = -84\ \mathrm{kJ/mol}$$
$$\Delta H_f^\circ[\mathrm{CO}_2(g)] = -394\ \mathrm{kJ/mol}$$
$$\Delta H_f^\circ[\mathrm{H}_2\mathrm{O}(l)] = -286\ \mathrm{kJ/mol}$$

Accepted Answer

$$-1426\ \mathrm{kJ}$$

Question 8

Which of the following statements about Hess's Law is correct?

Accepted Answer

The enthalpy change for a reaction is independent of the pathway between initial and final states.

Question 9

A sample of gas absorbs 500 J of heat from the surroundings and has 200 J of work done on it by the surroundings. What is the change in internal energy ($$\Delta E$$)?

Accepted Answer

$$\Delta E = 700\ \mathrm{J}$$

Question 10

Which of the following best describes the difference between potential and kinetic energy in a chemical context?

Accepted Answer

Potential energy is due to position or chemical composition, while kinetic energy is due to motion.

Question 11

Which of the following is the correct relationship for the change in internal energy of a system?

Accepted Answer

$$\Delta E = q + w$$

Question 12

If a reaction is carried out in a series of steps, what does Hess's Law allow you to do?

Accepted Answer

Add the enthalpy changes of the individual steps to find the overall enthalpy change.

Question 13

Which of the following is true about the direction of heat flow?

Accepted Answer

Heat always flows from warmer objects to cooler objects.

Question 14

A reaction has $$\Delta H = +150\ \mathrm{kJ}. $$Which of the following best describes this reaction?

Accepted Answer

The reaction is endothermic and absorbs heat.

Question 15

Which of the following is the correct formula for calculating the enthalpy change of a reaction using standard enthalpies of formation?

Accepted Answer

$$\Delta H_{rxn}^\circ = \sum n \Delta H_f^\circ(	ext{products}) - \sum m \Delta H_f^\circ(	ext{reactants})$$

Question 16

Which of the following is NOT a correct statement about standard enthalpy of formation ($$\Delta H_f^\circ$$)?

Accepted Answer

It is always a negative value.

Question 17

A reaction is carried out at constant pressure and releases 300 J of heat while the system does 100 J of work on the surroundings. What is the change in enthalpy ($$\Delta H$$)?

Accepted Answer

$$\Delta H = -300\ \mathrm{J}$$

Question 18

Which of the following real-life scenarios best illustrates the concept of exothermic reaction?

Accepted Answer

Combustion of gasoline in a car engine.

Thermochemistry: Energy, Enthalpy, and Standard Enthalpies of Reaction

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