Q: Which of the following is the correct expression for the enthalpy change of a reaction using enthalpies of formation?

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

Question 1

Which of the following best describes the standard enthalpy of formation ($\Delta H_f^\circ$) for a compound?

Accepted Answer

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states at 1 atm and 25°C.

Question 2

A 12 g piece of aluminum ($c = 0.215 \ \mathrm{cal}/\mathrm{g}^\circ\mathrm{C}$) at 70°C is placed in a beaker containing 35 g of water ($c = 1.0 \ \mathrm{cal}/\mathrm{g}^\circ\mathrm{C}$) at 15°C. What is the final temperature ($T_f$) when thermal equilibrium is reached? (Assume no heat loss to surroundings.)

Accepted Answer

$T_f = 18.5^\circ\mathrm{C}$

Question 3

Which equation correctly represents the total heat ($q_{total}$) required to heat a substance from temperature $T_1$ to $T_3$, passing through a phase change at $T_2$?

Accepted Answer

$q_{total} = q_1 + q_{phase} + q_2$

Question 4

In a calorimetry experiment, 5.62 g of graphite is burned in a calorimeter at 25°C and 1 atm. The temperature rises to 25.89°C and the heat capacity of the calorimeter is $C = 20.7 \ \mathrm{kJ}/^\circ\mathrm{C}$. What is the molar enthalpy of reaction ($\Delta H_{rxn}$)?

Accepted Answer

$\Delta H_{rxn} = -393 \ \mathrm{kJ}/\mathrm{mol}$

Question 5

Which statement about Hess's Law is correct?

Accepted Answer

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

Question 6

Given the following reactions and their enthalpy changes:

$\mathrm{N}_2(g) + \mathrm{O}_2(g) \rightarrow 2\mathrm{NO}(g) \quad \Delta H = 180 \ \mathrm{kJ}$

$2\mathrm{NO}(g) + \mathrm{O}_2(g) \rightarrow 2\mathrm{NO}_2(g) \quad \Delta H = -112 \ \mathrm{kJ}$

What is $\Delta H$ for the reaction $\mathrm{N}_2(g) + 2\mathrm{O}_2(g) \rightarrow 2\mathrm{NO}_2(g)$?

Accepted Answer

$\Delta H = 68 \ \mathrm{kJ}$

Question 7

Which of the following is NOT a method to determine $\Delta H_{rxn}$ for a chemical reaction?

Accepted Answer

Using the ideal gas law.

Question 8

If the coefficients in a chemical equation are multiplied by a constant, how does this affect the enthalpy change ($\Delta H$) for the reaction?

Accepted Answer

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

Question 9

Which of the following best explains why enthalpy ($H$) is considered a state function?

Accepted Answer

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

Question 10

A bomb calorimeter is used to measure the heat of a combustion reaction. Which equation correctly relates the heat released ($q_{rxn}$) to the heat absorbed by the calorimeter ($q_{cal}$)?

Accepted Answer

$q_{rxn} = -q_{cal}$

Question 11

Which of the following is the correct expression for the enthalpy change of a reaction using enthalpies of formation?

Accepted Answer

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

Question 12

When two substances of different temperatures are mixed and reach thermal equilibrium, which equation correctly describes the heat exchange?

Accepted Answer

$q_1 + q_2 = 0$

Question 13

Which of the following statements about the standard state of a substance is correct?

Accepted Answer

It is the most stable form of the substance at 1 atm pressure and 25°C.

Question 14

Which of the following is a direct consequence of Hess's Law?

Accepted Answer

The enthalpy change for a reaction can be calculated by summing the enthalpy changes of individual steps.

Question 15

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

Accepted Answer

The sign of $\Delta H$ is reversed.

Question 16

Which of the following is true about calorimetry experiments?

Accepted Answer

They can be used to measure the heat of reaction by monitoring temperature changes.

Question 17

Which of the following reactions has a $\Delta H$ value that is directly proportional to its stoichiometric coefficients?

Accepted Answer

$2\mathrm{H}_2(g) + \mathrm{O}_2(g) \rightarrow 2\mathrm{H}_2\mathrm{O}(g)$

Question 18

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

Accepted Answer

Calculating the enthalpy change for the formation of water vapor from hydrogen and oxygen using known enthalpy changes for intermediate reactions.

Enthalpy Changes, Thermal Equilibrium, Standard Enthalpy, Calorimetry, and Hess’s Law

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