Question 1

Which of the following statements about enthalpy ($\Delta H$) is TRUE?

Accepted Answer

If a chemical equation is reversed, the sign of $\Delta H$ is also reversed.

Question 2

Given the reaction: $\mathrm{C(s) + \frac{1}{2} O_2(g) \rightarrow CaO(s)}$ with $\Delta H = -635.1\ \mathrm{kJ}$, what is the enthalpy change for the formation of 3 moles of CaO(s)?

Accepted Answer

$\Delta H = -1905.3\ \mathrm{kJ}$

Question 3

Which law allows you to calculate the enthalpy change of a reaction by adding the enthalpy changes of individual steps?

Accepted Answer

Hess's Law

Question 4

If $\Delta H_1 = 50\ \mathrm{kJ}$ and $\Delta H_2 = -125\ \mathrm{kJ}$, what is $\Delta H_3$ for the overall reaction according to Hess's Law?

Accepted Answer

$\Delta H_3 = -75\ \mathrm{kJ}$

Question 5

For the reaction $\mathrm{C(s) + H_2O(g) \rightarrow CO(g) + H_2(g)}$, given the following data:
$\mathrm{C(s) + O_2(g) \rightarrow CO_2(g)}\ \Delta H = -393.5\ \mathrm{kJ}$
$\mathrm{2CO(g) + O_2(g) \rightarrow 2CO_2(g)}\ \Delta H = -566.0\ \mathrm{kJ}$
$\mathrm{2H_2(g) + O_2(g) \rightarrow 2H_2O(g)}\ \Delta H = -483.6\ \mathrm{kJ}$
What is $\Delta H_{rxn}$ for the target reaction?

Accepted Answer

$\Delta H_{rxn} = 131.3\ \mathrm{kJ}$

Question 6

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

Accepted Answer

The enthalpy change when one mole of a compound forms from its constituent elements in their standard states.

Question 7

What is the standard enthalpy of formation ($\Delta H_f^\circ$) for a pure element in its standard state?

Accepted Answer

$\Delta H_f^\circ = 0$

Question 8

Calculate the standard enthalpy change for the combustion of methane:
$\mathrm{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)}$
Given:
$\Delta H_f^\circ(\mathrm{CO_2(g)}) = -393.5\ \mathrm{kJ/mol}$
$\Delta H_f^\circ(\mathrm{H_2O(g)}) = -241.8\ \mathrm{kJ/mol}$
$\Delta H_f^\circ(\mathrm{CH_4(g)}) = -74.6\ \mathrm{kJ/mol}$

Accepted Answer

$\Delta H_{rxn} = -802.5\ \mathrm{kJ}$

Question 9

A chemical reaction is expressed as the sum of a series of steps. What is the enthalpy change for the overall reaction?

Accepted Answer

The sum of the enthalpy changes for each step.

Question 10

Which of the following best describes a state function?

Accepted Answer

A property that depends only on the initial and final states, not on the path taken.

Question 11

If the enthalpy change for the reaction $\mathrm{A + 2B \rightarrow C}$ is $\Delta H_1$, what is the enthalpy change for the reaction $\mathrm{2A + 4B \rightarrow 2C}$?

Accepted Answer

$2 	imes \Delta H_1$

Question 12

Which of the following is NOT a standard state for a substance?

Accepted Answer

A solution at 0.1 M concentration

Question 13

A city of 100,000 people uses $1 	imes 10^{13}$ kJ of energy per year. If all energy comes from the combustion of octane ($\mathrm{C_8H_{18}}$), and $\Delta H_{comb}$ for octane is $-5074.1$ kJ/mol, how many moles of octane are needed?

Accepted Answer

$1.97 	imes 10^7$ moles

Question 14

Which of the following reactions would have $\Delta H = 0$ under standard conditions?

Accepted Answer

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

Question 15

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

Accepted Answer

The enthalpy change for a reaction can be calculated by adding the enthalpy changes of reactions that sum to the overall reaction.

Question 16

If the enthalpy of formation of water is $-241.8\ \mathrm{kJ/mol}$, what is the enthalpy change for the formation of 2 moles of water?

Accepted Answer

$-483.6\ \mathrm{kJ}$

Question 17

Which of the following best explains why most compounds have a negative enthalpy of formation?

Accepted Answer

Formation of compounds from elements usually releases energy.

Thermochemistry: Enthalpy Changes and Hess's Law (General Chemistry, Chapter 7)

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