Q: Given the following standard enthalpies of formation: $$\Delta H_f^\circ [A] = 433\ \text{kJ/mol}$$, $$\Delta H_f^\circ [B] = -256\ \text{kJ/mol}$$, $$\Delta H_f^\circ [C] = 523\ \text{kJ/mol}$$, calculate the standard enthalpy change for the reaction $$A + B \rightarrow C.$$

$$\Delta H_{rxn}^\circ = 346\ \text{kJ}$$

Q: Which equation correctly expresses the standard enthalpy change of reaction in terms of standard enthalpies of formation?

$$\Delta H_{rxn}^\circ = \sum n_p \Delta H_f^\circ (\text{products}) - \sum n_r \Delta H_f^\circ (\text{reactants})$$

Question 1

Which of the following statements best describes Hess's Law?

Accepted Answer

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

Question 2

Given the following standard enthalpies of formation: $$\Delta H_f^\circ [A] = 433\ 	ext{kJ/mol}$$, $$\Delta H_f^\circ [B] = -256\ 	ext{kJ/mol}$$, $$\Delta H_f^\circ [C] = 523\ 	ext{kJ/mol}$$, calculate the standard enthalpy change for the reaction $$A + B ightarrow C.$$

Accepted Answer

$$\Delta H_{rxn}^\circ = 346\ 	ext{kJ}$$

Question 3

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

Accepted Answer

It is zero.

Question 4

Consider the reaction: $$N_2(g) + 2O_2(g$$) ightarrow 2NO_2(g)$$ with $$\Delta H = 68\ 	ext{kJ}$$. If this reaction is carried out in two steps: 
1) N_2$(g) + O_2$(g) ightarrow 2NO(g)$$, $$\Delta H_1$ = 180\ 	ext{kJ}
2$$) $$2NO(g) + O_2$(g) ightarrow 2NO_2(g)$$, $$\Delta H_2$ = -112\ 	ext{kJ}$$
What is the sum $$\Delta H_1$ + \Delta H_2$?

Accepted Answer

$$68\ 	ext{kJ}$$

Question 5

If you reverse a chemical equation, what happens to the sign of $$\Delta H$$?

Accepted Answer

It changes sign.

Question 6

Which equation correctly expresses the standard enthalpy change of reaction in terms of standard enthalpies of formation?

Accepted Answer

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

Question 7

A reaction is carried out in two different ways: a one-step process and a two-step process with intermediates. According to Hess's Law, how does the total enthalpy change compare?

Accepted Answer

The total enthalpy change is the same for both processes.

Question 8

Given the following reactions:
1) $$C(s,	ext{graphite}) + O_2$(g) ightarrow CO_2$(g)$$, $$\Delta H^\circ = -393.5\ 	ext{kJ}
2$$) $$C(s,	ext{diamond}) + O_2$(g) ightarrow CO_2$(g)$$, $$\Delta H^\circ = -395.4\ 	ext{kJ}$$
What is $$\Delta H^\circ$$ for $$C(s,	ext{graphite}) ightarrow C(s,	ext{diamond})$$?

Accepted Answer

$$+1.9\ 	ext{kJ}$$

Question 9

If the coefficients in a balanced chemical equation are multiplied by a constant, what happens to $$\Delta H$$ for the reaction?

Accepted Answer

$$\Delta H is $$multiplied by the constant.

Question 10

Which of the following best explains why enthalpy is a state function?

Accepted Answer

Because its value depends only on the initial and final states, not on the path taken.

Question 11

Calculate the standard enthalpy change for the reaction:
$$2NH_3(g) + \frac{7}{2}O_2$(g) ightarrow 2NO_2(g) + 3H_2O(g)$$
Given:
$$\Delta H_f^\circ (NH_3$) = -46\ 	ext{kJ/mol}$$
$$\Delta H_f^\circ (NO_2$) = +34\ 	ext{kJ/mol}$$
$$\Delta H_f^\circ (H_2O) = -242\ 	ext{kJ/mol}$$
$$\Delta H_f^\circ (O_2$) = 0$$

Accepted Answer

$$-283\ 	ext{kJ}$$

Question 12

Which of the following is the correct definition of the standard enthalpy of formation ($$\Delta H_f^\circ$$)?

Accepted Answer

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states.

Question 13

Which of the following substances has the highest standard enthalpy of combustion per mole, based on the provided table?

Accepted Answer

$$C_{14}H$$_{30}(l)$$, avg. diesel compound

Question 14

A chemist wants to determine the enthalpy change for the reaction $$3C(s,	ext{graphite}) + 4H_2(g) ightarrow C_3H_8(g). $$Which strategy should they use with Hess's Law?

Accepted Answer

Combine reactions that form $$CO_2$$ and $$H_2O$$ from $$C$$ and $$H_2$$, and reverse the combustion of $$C_3H_8.$$

Question 15

Which of the following best describes a formation reaction?

Accepted Answer

A reaction that forms 1 mole of a compound from its elements in their standard states.

Question 16

If you are given three enthalpy changes for three reactions and need to find the fourth for a related reaction, which law or principle allows you to do this?

Accepted Answer

Hess's Law

Question 17

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

Accepted Answer

Calculating the energy content of fuels used in transportation.

Standard Enthalpy, Hess's Law, and Enthalpy Calculations

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