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

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

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

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

Q: Calculate the standard enthalpy change for the combustion of methane: $$CH_4(g) + 2O_2(g$$) \rightarrow $$CO_2(g) + 2H_2O(g$$)$$ Given: $$\Delta H_f^\circ($$CH_4) = -74.6$$\ \text{kJ/mol}$$ $$\Delta H_f^\circ($$CO_2) = -393.5$$\ \text{kJ/mol}$$ $$\Delta H_f^\circ(H_2O) = -241.8\ \text{kJ/mol}$$ Use $$\Delta H_{rxn} = \sum n_p \Delta H_f^\circ( ext{products}) - \sum n_r \Delta H_f^\circ( ext{reactants})$$.

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

Question 1

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

Accepted Answer

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

Question 2

Given the reaction: $$2A + 4B \rightarrow 2C$$ and $$\Delta H_1$ for A + 2B$$ \rightarrow C$$, what is $$\Delta $$H_2$$ for the doubled reaction?

Accepted Answer

$$\Delta H_2$ = 2 \Delta H_1$

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

Accepted Answer

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

Question 5

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

Accepted Answer

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

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 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:
$$CH_4(g) + 2O_2(g$$) ightarrow $$CO_2(g) + 2H_2O(g$$)$$
Given:
$$\Delta H_f^\circ($$CH_4) = -74.6$$\ 	ext{kJ/mol}$$
$$\Delta H_f^\circ($$CO_2) = -393.5$$\ 	ext{kJ/mol}$$
$$\Delta H_f^\circ(H_2O) = -241.8\ 	ext{kJ/mol}$$
Use $$\Delta H_{rxn} = \sum n_p \Delta H_f^\circ(	ext{products}) - \sum n_r \Delta H_f^\circ(	ext{reactants})$$.

Accepted Answer

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

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

It is the sum of the enthalpy changes of the steps.

Question 10

Which of the following best describes why $$\Delta H is $$considered a state function?

Accepted Answer

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

Question 11

Which of the following is TRUE about the enthalpy change for the reverse of a reaction?

Accepted Answer

It is equal in magnitude and opposite in sign to the forward reaction.

Question 12

A city of 100,000 people uses $$1 	imes 10$$^{13}$$ kJ of energy per year. If all energy comes from the combustion of octane (C_8H$$_{18}$$), and $$\Delta H_{comb} = -5074.1$$ kJ/mol, how many kilograms of octane are needed? (Molar mass of octane = 114.22 g/mol)

Accepted Answer

2.25 $$	imes$$ $$10^8 kg$$

Question 13

Which of the following reactions is an example of a formation reaction for $$H_2O(l)$$?

Accepted Answer

$$H_2(g$$) + \frac{1}{2}$$O_2(g$$) \rightarrow H_2O(l)$$

Question 14

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

Accepted Answer

A liquid at 2 atm and 50 $$^\circC$$

Question 15

If the enthalpy of formation of $$CO_2(g$$)$$ is -393.5$ kJ/mol and that of H_2O(l$$)$$ is -241.8$ kJ/mol, what is the enthalpy change for the reaction CH_4$(g) + 2O_2(g) \rightarrow CO_2$(g) + 2H_2O(l)$$, given $$\Delta H_f^\circ(CH_4$) = -74.6 kJ/$$mol?

Accepted Answer

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

Question 16

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

Accepted Answer

The enthalpy change for a reaction can be calculated by summing the enthalpy changes of individual steps that add up to the overall reaction.

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

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