Q: A reaction is carried out in a coffee-cup calorimeter. The temperature of the solution increases by $2.5\ ^\circ\mathrm{C}$. What can you conclude about the sign of $\Delta H$ for the reaction?

$\Delta H$ is negative; the reaction is exothermic.

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

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

Q: Given the following reactions and their enthalpy changes: $\mathrm{C(s) + O_2(g) \rightarrow CO_2(g)} \quad \Delta H_1$ $\mathrm{CO_2(g) \rightarrow CO(g) + \frac{1}{2}O_2(g)} \quad \Delta H_2$ What is the enthalpy change for the reaction $\mathrm{C(s) + \frac{1}{2}O_2(g) \rightarrow CO(g)}$?

$\Delta H = \Delta H_1 + \Delta H_2$

Q: Which of the following is the correct definition of the standard enthalpy of formation ($\Delta H_f^\circ$) for a compound?

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

Q: Calculate the enthalpy change ($\Delta H_{rxn}$) for the following reaction using standard enthalpies of formation: $\mathrm{2H_2(g) + O_2(g) \rightarrow 2H_2O(l)}$ Given: $\Delta H_f^\circ [\mathrm{H_2O(l)}] = -286\ \mathrm{kJ/mol}$, $\Delta H_f^\circ [\mathrm{H_2(g)}] = 0$, $\Delta H_f^\circ [\mathrm{O_2(g)}] = 0$

$\Delta H_{rxn} = -572\ \mathrm{kJ}$

Q: Which of the following best explains why the enthalpy of reaction can be calculated using enthalpies of formation?

Because enthalpy is a state function and depends only on the initial and final states, not the path taken.

Question 1

Which of the following best describes the enthalpy change ($\Delta H$) measured in a constant-pressure calorimeter?

Accepted Answer

It equals the heat flow into or out of the system at constant pressure.

Question 2

A student dissolves ammonium nitrate ($\mathrm{NH_4NO_3}$) in water in a coffee-cup calorimeter and observes the temperature of the solution decreases. Which statement is correct?

Accepted Answer

The process is endothermic because $\Delta H > 0$.

Question 3

Which equation correctly represents the relationship between enthalpy change ($\Delta H$), internal energy change ($\Delta U$), pressure ($P$), and change in volume ($\Delta V$) for a reaction at constant pressure?

Accepted Answer

$\Delta H = \Delta U + P\Delta V$

Question 4

Which of the following statements about exothermic and endothermic reactions is correct?

Accepted Answer

An exothermic reaction has $\Delta H < 0$ and releases heat to the surroundings.

Question 5

A reaction is carried out in a coffee-cup calorimeter. The temperature of the solution increases by $2.5\ ^\circ\mathrm{C}$. What can you conclude about the sign of $\Delta H$ for the reaction?

Accepted Answer

$\Delta H$ is negative; the reaction is exothermic.

Question 6

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 a series of steps that add up to the overall reaction.

Question 7

Given the following reactions and their enthalpy changes:

$\mathrm{C(s) + O_2(g) \rightarrow CO_2(g)} \quad \Delta H_1$

$\mathrm{CO_2(g) \rightarrow CO(g) + \frac{1}{2}O_2(g)} \quad \Delta H_2$

What is the enthalpy change for the reaction $\mathrm{C(s) + \frac{1}{2}O_2(g) \rightarrow CO(g)}$?

Accepted Answer

$\Delta H = \Delta H_1 + \Delta H_2$

Question 8

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

Accepted Answer

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

Question 9

Which of the following is NOT a naturally occurring diatomic molecule in its standard state?

Accepted Answer

$\mathrm{CO_2(g)}$

Question 10

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

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

Given: $\Delta H_f^\circ [\mathrm{H_2O(l)}] = -286\ \mathrm{kJ/mol}$, $\Delta H_f^\circ [\mathrm{H_2(g)}] = 0$, $\Delta H_f^\circ [\mathrm{O_2(g)}] = 0$

Accepted Answer

$\Delta H_{rxn} = -572\ \mathrm{kJ}$

Question 11

Which of the following best explains why the enthalpy of reaction can be calculated using enthalpies of formation?

Accepted Answer

Because enthalpy is a state function and depends only on the initial and final states, not the path taken.

Question 12

A chemical reaction is carried out in a constant-pressure calorimeter. The temperature of the solution decreases. Which of the following statements is true?

Accepted Answer

The reaction is endothermic and $\Delta H$ is positive.

Question 13

Which of the following equations correctly calculates the enthalpy change of a reaction using standard enthalpies of formation?

Accepted Answer

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

Question 14

If a reaction is reversed, what happens to the sign of $\Delta H$?

Accepted Answer

The sign of $\Delta H$ is reversed.

Question 15

A chemical reaction is multiplied by a factor of 3. What happens to the enthalpy change ($\Delta H$) for the reaction?

Accepted Answer

$\Delta H$ is multiplied by 3.

Question 16

Which of the following best describes the standard state of an element?

Accepted Answer

The most stable physical form of the element at 1 bar pressure and a specified temperature, usually 25$^\circ$C.

Question 17

A medical chemist is studying the combustion of glucose ($\mathrm{C_6H_{12}O_6}$) in the body. Which of the following best describes the enthalpy change for this reaction?

Accepted Answer

It is exothermic; $\Delta H$ is negative because energy is released as heat.

Question 18

Which of the following statements about constant-pressure and constant-volume calorimetry is correct?

Accepted Answer

Constant-pressure calorimetry measures $q_p$ (heat at constant pressure), while constant-volume calorimetry measures $q_v$ (heat at constant volume).

Question 19

A student calculates the enthalpy change for a reaction using Hess's Law and obtains a positive value. What does this indicate about the reaction?

Accepted Answer

The reaction is endothermic and absorbs heat from the surroundings.

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