Q: Which equation correctly expresses the use of enthalpies of formation to calculate the enthalpy change of a reaction?

$$\Delta H = \sum n \Delta H_f^\circ (\text{products}) - \sum m \Delta H_f^\circ (\text{reactants})$$

Question 1

Which statement best describes Hess's Law?

Accepted Answer

The enthalpy change for a reaction is independent of the pathway taken, and depends only on the initial and final states.

Question 2

Given the following reactions and their enthalpy changes:

$$\mathrm{A \rightarrow B} \quad \Delta H_1$ = +50\ \mathrm{kJ}$$

$$\mathrm{B \rightarrow C} \quad \Delta H_2$ = -30\ \mathrm{kJ}$$

What is $$\Delta H$$ for the overall reaction $$\mathrm{A \rightarrow C}$$?

Accepted Answer

$$\Delta H = +20\ \mathrm{kJ}$$

Question 3

Which of the following is true about the standard enthalpy of formation ($$\Delta H_f^\circ) $$for an element in its most stable form?

Accepted Answer

$$\Delta H_f^\circ is $$always $$0\ \mathrm{kJ/mol}$$

Question 4

Consider the combustion of propane:

$$\mathrm{C_3H_8 (g) + 5\ O_2$ (g) \rightarrow 3\ CO_2$ (g) + 4\ H_2O (l)}$$

Given the following standard enthalpies of formation:
$$\Delta H_f^\circ (\mathrm{CO_2$ (g)}) = -393.5\ \mathrm{kJ/mol}$$
$$\Delta H_f^\circ (\mathrm{H_2O (l)}) = -241.8\ \mathrm{kJ/mol}$$
$$\Delta H_f^\circ (\mathrm{C_3H_8 (g)}) = -103.85\ \mathrm{kJ/mol}$$
$$\Delta H_f^\circ (\mathrm{O_2$ (g)}) = 0\ \mathrm{kJ/mol}$$

Calculate $$\Delta H$$ for the reaction.

Accepted Answer

$$\Delta H = -2043.9\ \mathrm{kJ}$$

Question 5

Which equation correctly expresses the use of enthalpies of formation to calculate the enthalpy change of a reaction?

Accepted Answer

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

Question 6

Why is the number of moles of each substance important when calculating the enthalpy change using enthalpies of formation?

Accepted Answer

Because the enthalpy change is proportional to the stoichiometric coefficients in the balanced equation.

Question 7

Which of the following best explains why some endothermic reactions still proceed spontaneously?

Accepted Answer

The increase in entropy ($$S) $$can make the reaction favorable even if $$\Delta H \u003e 0.$$

Question 8

Which statement about bond enthalpy is correct?

Accepted Answer

Energy is required to break a chemical bond, and energy is released when a bond is formed.

Question 9

A reaction is exothermic if:

Accepted Answer

The products are at a lower energy state than the reactants.

Question 10

Which of the following is a correct application of Hess's Law in a real-world scenario?

Accepted Answer

Calculating the enthalpy change for the combustion of a fuel by summing the enthalpy changes of individual steps.

Question 11

Which of the following reactions has $$\Delta H_f^\circ = 0$$?

Accepted Answer

$$\mathrm{O_2$ (g) \rightarrow O_2$ (g)}$$

Question 12

If a reaction is carried out in three steps with enthalpy changes $$\Delta H_1$, $$\Delta $$H_2$$, and $$\Delta H_3$, what is the enthalpy change for the overall reaction?

Accepted Answer

$$\Delta H_{	ext{overall}} = \Delta H_1$ + \Delta H_2$ + \Delta H_3$

Question 13

Which of the following best describes entropy ($$S$$)?

Accepted Answer

A measure of disorder or chaos in a system.

Question 14

In the reaction $$\mathrm{N_2$ (g) + 3 H_2$ (g) \rightarrow 2 NH_3$ (g)}$$, which statement is true regarding the enthalpy of formation?

Accepted Answer

The enthalpy of formation of $$\mathrm{N_2$ (g)}$$ and $$\mathrm{H_2$ (g)} is$$ $$0\ \mathrm{kJ/mol}.$$

Question 15

Which of the following is NOT a correct statement about enthalpy ($$H) $$and entropy ($$S$$)?

Accepted Answer

A reaction always proceeds if the enthalpy change is negative.

Question 16

Which of the following best explains why the enthalpy of formation for $$\mathrm{O_2$ (g)} is$$ $$0\ \mathrm{kJ/mol}$$?

Accepted Answer

Because $$\mathrm{O_2$ (g)} is $$the most stable form of oxygen under standard conditions.

Question 17

Which of the following is a correct statement about the energy changes in chemical reactions?

Accepted Answer

Energy is absorbed to break bonds and released when new bonds are formed.

Question 18

A chemical reaction proceeds spontaneously when:

Accepted Answer

The overall change in free energy is negative, which can result from favorable enthalpy and/or entropy changes.

Hess's Law, Enthalpies of Formation, and Bond Enthalpies

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