Enthalpy & Thermochemistry practice questions

Question 1

A student dissolves a sample of solid calcium chloride, $\text{CaCl}_2\text{(s)}$, in water contained in an insulated calorimeter. The student observes that the temperature of the water increases from $22.0^\circ\text{C}$ to $31.0^\circ\text{C}$. Which of the following claims and justifications correctly identifies the thermochemical nature of the dissolution process?

Accepted Answer

The process is exothermic because thermal energy flows from the dissolving $\text{CaCl}_2$ (system) to the water (surroundings).

Answer

The process is exothermic because thermal energy flows from the water (surroundings) to the dissolving $\text{CaCl}_2$ (system).

Answer

The process is endothermic because thermal energy flows from the water (surroundings) to the dissolving $\text{CaCl}_2$ (system).

Answer

The process is endothermic because thermal energy flows from the dissolving $\text{CaCl}_2$ (system) to the water (surroundings).

Question 2

A student adds a sample of solid ammonium nitrate, $\text{NH}_4\text{NO}_3\text{(s)}$, to a beaker of water at $25^\circ\text{C}$. The dissolution process is represented by the equation below:

$$\text{NH}_4\text{NO}_3\text{(s)} \rightarrow \text{NH}_4^+\text{(aq)} + \text{NO}_3^-\text{(aq)} \quad \Delta H^\circ > 0$$

Which of the following best predicts the temperature change of the solution as the salt dissolves, and provides the correct justification?

Accepted Answer

The temperature of the solution decreases because the endothermic dissolution process absorbs thermal energy from the solution.

Answer

The temperature of the solution increases because the dissolution process is exothermic and releases thermal energy to the solution.

Answer

The temperature of the solution increases because thermal energy is absorbed by the salt from the surrounding air.

Answer

The temperature of the solution decreases because the dissolution process releases thermal energy to the surroundings.

Question 3

A student performs an experiment by adding solid sodium hydrogen carbonate, $\text{NaHCO}_3(s)$, to an aqueous solution of acetic acid, $\text{CH}_3\text{COOH}(aq)$. The student observes rapid formation of gas bubbles, and the temperature of the mixture decreases. Which of the following correctly classifies this process and identifies the primary particulate-level interactions involved?

Accepted Answer

| Classification | Primary Particulate-Level Interactions ||---|---|| Chemical change | Intramolecular chemical bonds broken and formed |

Answer

| Classification | Primary Particulate-Level Interactions ||---|---|| Physical change | Intermolecular forces overcome |

Answer

| Classification | Primary Particulate-Level Interactions ||---|---|| Physical change | Intramolecular chemical bonds broken and formed |

Answer

| Classification | Primary Particulate-Level Interactions ||---|---|| Chemical change | Intermolecular forces overcome |

Question 4

Propane, $\text{C}_3\text{H}_8(g)$, undergoes complete combustion according to the following thermochemical equation:

$$\text{C}_3\text{H}_8(g) + 5\text{O}_2(g) \rightarrow 3\text{CO}_2(g) + 4\text{H}_2\text{O}(l) \quad \Delta H^\circ_{rxn} = -2200 \text{ kJ/mol}_{rxn}$$

What amount of heat is released when a $4.40 \text{ g}$ sample of $\text{C}_3\text{H}_8(g)$ (molar mass $44.0 \text{ g/mol}$) is completely burned in excess oxygen?

Accepted Answer

$220 \text{ kJ}$

Answer

$22.0 \text{ kJ}$

Answer

$110 \text{ kJ}$

Answer

$9680 \text{ kJ}$

Question 5

A student analyzes the reaction energy profile for the gas-phase reaction 
$\text{NO}_2\text{(g)} + \text{CO(g)} \rightarrow \text{NO(g)} + \text{CO}_2\text{(g)}$, as shown in the graph below. Based on the profile, what are the activation energy ($E_a$) and the enthalpy change ($\Delta H$) for the forward reaction?

Accepted Answer

$E_a = 50 \text{ kJ/mol}$ and $\Delta H = -30 \text{ kJ/mol}$

Answer

$E_a = 90 \text{ kJ/mol}$ and $\Delta H = -30 \text{ kJ/mol}$

Answer

$E_a = 50 \text{ kJ/mol}$ and $\Delta H = +30 \text{ kJ/mol}$

Answer

$E_a = 80 \text{ kJ/mol}$ and $\Delta H = +30 \text{ kJ/mol}$

Question 6

A student is compiling thermochemical data for a renewable-fuel study at standard conditions. Which of the following equations represents the standard enthalpy of formation, $\Delta H_f^\circ$, of liquid ethanol, $\text{C}_2\text{H}_5\text{OH(l)}$?

Accepted Answer

$2\text{C(s, graphite)}+3\text{H}_2\text{(g)}+\dfrac{1}{2}\text{O}_2\text{(g)}\rightarrow\text{C}_2\text{H}_5\text{OH(l)}$

Answer

$2\text{C(s, graphite)}+3\text{H}_2\text{(g)}+\dfrac{1}{2}\text{O}_2\text{(g)}\rightarrow\text{C}_2\text{H}_5\text{OH(g)}$

Answer

$\text{C}_2\text{H}_5\text{OH(l)}+3\text{O}_2\text{(g)}\rightarrow2\text{CO}_2\text{(g)}+3\text{H}_2\text{O(l)}$

Answer

$4\text{C(s, graphite)}+6\text{H}_2\text{(g)}+\text{O}_2\text{(g)}\rightarrow2\text{C}_2\text{H}_5\text{OH(l)}$

Question 7

A student performs an experiment to determine the molar enthalpy of combustion, $\Delta H_{comb}$, of ethanol, $\text{C}_2\text{H}_5\text{OH}(l)$. A known mass of ethanol is burned in a spirit burner to heat a measured mass of water in an open metal calorimeter. The student assumes that all heat released by the combustion is absorbed by the water and uses $q = m c \Delta T$ to calculate the experimental molar enthalpy of combustion. However, a portion of the heat released during combustion is transferred to the surrounding air and metal calorimeter cup rather than to the water. Which of the following claims correctly predicts the effect of this heat loss on the calculated magnitude of $\Delta H_{comb}$ and provides the correct justification?

Accepted Answer

The calculated magnitude of $\Delta H_{comb}$ is lower than the actual value because the measured $\Delta T$ of the water is smaller than if all heat were transferred to the water.

Answer

The calculated magnitude of $\Delta H_{comb}$ is higher than the actual value because the heat lost to the surroundings increases the measured final temperature of the water.

Answer

The calculated magnitude of $\Delta H_{comb}$ is lower than the actual value because the heat loss causes the calculated mass of ethanol burned to be greater than the actual mass consumed.

Answer

The calculated magnitude of $\Delta H_{comb}$ is higher than the actual value because the unmeasured heat absorbed by the cup is added directly to the calculated heat absorbed by the water.

Question 8

A student dissolves a sample of solid calcium chloride, $\text{CaCl}_2\text{(s)}$, in an insulated calorimeter containing water at room temperature. The student claims that the dissolution of $\text{CaCl}_2\text{(s)}$ is an exothermic process. Which of the following experimental observations provides the best evidence to support the student's claim?

Accepted Answer

The temperature of the aqueous solution increases as the solid dissolves, demonstrating that thermal energy is transferred from the chemical system to the surroundings.

Answer

The temperature of the aqueous solution decreases as the solid dissolves, demonstrating that thermal energy is absorbed by the chemical system from the surroundings.

Answer

The solubility of the solid in water increases when the temperature is raised, demonstrating that thermal energy acts as a reactant in the dissolution process.

Answer

The electrical conductivity of the solution increases as the solid dissolves, demonstrating that thermal energy is produced alongside mobile hydrated ions.

Question 9

A student attempts to determine the standard enthalpy change, $\Delta H^\circ_{rxn}$, for the industrial dehydrogenation of propane to propene:

$$\text{C}_3\text{H}_8\text{(g)} \rightarrow \text{C}_3\text{H}_6\text{(g)} + \text{H}_2\text{(g)}$$

The student looks up the standard enthalpies of combustion for the relevant species, as summarized in the table below.

| Reaction | Chemical Equation | Enthalpy Change |
|---|---|---|
| (1) | $\text{C}_3\text{H}_8\text{(g)} + 5\,\text{O}_2\text{(g)} \rightarrow 3\,\text{CO}_2\text{(g)} + 4\,\text{H}_2\text{O(l)}$ | $\Delta H_1$ |
| (2) | $\text{C}_3\text{H}_6\text{(g)} + \dfrac{9}{2}\,\text{O}_2\text{(g)} \rightarrow 3\,\text{CO}_2\text{(g)} + 3\,\text{H}_2\text{O(l)}$ | $\Delta H_2$ |
| (3) | $\text{H}_2\text{(g)} + \dfrac{1}{2}\,\text{O}_2\text{(g)} \rightarrow \text{H}_2\text{O(l)}$ | $\Delta H_3$ |

Which of the following mathematical expressions correctly determines $\Delta H^\circ_{rxn}$ for the dehydrogenation reaction?

Accepted Answer

$\Delta H^\circ_{rxn} = \Delta H_1 - \Delta H_2 - \Delta H_3$

Answer

$\Delta H^\circ_{rxn} = \Delta H_1 + \Delta H_2 + \Delta H_3$

Answer

$\Delta H^\circ_{rxn} = \Delta H_1 - \Delta H_2 + \Delta H_3$

Answer

$\Delta H^\circ_{rxn} = -\Delta H_1 + \Delta H_2 + \Delta H_3$

Question 10

A student investigates the reversible decomposition of phosgene gas, $\text{COCl}_2\text{(g)}$, in a sealed, rigid $1.0 \text{ L}$ flask according to the following equation: $\text{COCl}_2\text{(g)} \rightleftharpoons \text{CO(g)} + \text{Cl}_2\text{(g)}$ with $\Delta H^\circ = +108 \text{ kJ/mol}_{rxn}$. The system is allowed to reach equilibrium at $500 \text{ K}$. Which modification to the experimental procedure will increase the equilibrium yield of $\text{Cl}_2\text{(g)}$?

Accepted Answer

Increasing the temperature of the reaction vessel from $500 \text{ K}$ to $600 \text{ K}$

Answer

Adding a solid catalyst to the flask at $500 \text{ K}$

Answer

Decreasing the volume of the reaction container from $1.0 \text{ L}$ to $0.5 \text{ L}$ at $500 \text{ K}$

Answer

Injecting $0.10 \text{ mol}$ of $\text{Ar(g)}$ into the rigid flask at $500 \text{ K}$

Question 11

A student investigates the reduction of iron(III) oxide by carbon monoxide gas, as represented by the following balanced chemical equation:

$\text{Fe}_2\text{O}_3\text{(s)} + 3\,\text{CO(g)} \rightarrow 2\,\text{Fe(s)} + 3\,\text{CO}_2\text{(g)}$

The student collects the thermodynamic data shown in the table below.

| Substance | $\Delta H^\circ_f\text{ (kJ/mol)}$ |
|---|---|
| $\text{Fe}_2\text{O}_3\text{(s)}$ | $-824$ |
| $\text{CO(g)}$ | $-110$ |
| $\text{Fe(s)}$ | $0$ |
| $\text{CO}_2\text{(g)}$ | $-394$ |

Based on the data in the table, what is the standard enthalpy change, $\Delta H^\circ_{rxn}$, for the reaction?

Accepted Answer

$-28 \text{ kJ/mol}$

Answer

$+28 \text{ kJ/mol}$

Answer

$+540 \text{ kJ/mol}$

Answer

$+2336 \text{ kJ/mol}$

Question 12

A student evaluates the industrial extraction of iron metal from iron(III) oxide using carbon monoxide gas at $298 \text{ K}$. The thermochemical equations for the individual processes are shown below:

$\text{Fe}_2\text{O}_3(s) \rightarrow 2\,\text{Fe}(s) + \frac{3}{2}\,\text{O}_2(g) \quad \Delta G^\circ = +740 \text{ kJ/mol}_{rxn}$

$\text{CO}(g) + \frac{1}{2}\,\text{O}_2(g) \rightarrow \text{CO}_2(g) \quad \Delta G^\circ = -280 \text{ kJ/mol}_{rxn}$

Based on the information above, what is the standard Gibbs free energy change, $\Delta G^\circ$, for the net coupled reaction shown below?

$\text{Fe}_2\text{O}_3(s) + 3\,\text{CO}(g) \rightarrow 2\,\text{Fe}(s) + 3\,\text{CO}_2(g)$

Accepted Answer

$-100 \text{ kJ/mol}_{rxn}$

Answer

$+100 \text{ kJ/mol}_{rxn}$

Answer

$+460 \text{ kJ/mol}_{rxn}$

Answer

$+1020 \text{ kJ/mol}_{rxn}$

Question 13

A student investigates the hydrogenation of ethene to form ethane, represented by the balanced equation below:

$\text{C}_2\text{H}_4\text{(g)} + \text{H}_2\text{(g)} \rightarrow \text{C}_2\text{H}_6\text{(g)}$

| Bond | Average Bond Enthalpy (kJ/mol) |
|---|---|
| $\text{C–C}$ | 348 |
| $\text{C=C}$ | 614 |
| $\text{C–H}$ | 413 |
| $\text{H–H}$ | 436 |

Based on the average bond enthalpies given in the table, what is the estimated standard enthalpy change, $\Delta H^\circ_{rxn}$, for the reaction?

Accepted Answer

$-124 \text{ kJ/mol}$

Answer

$-560 \text{ kJ/mol}$

Answer

$-390 \text{ kJ/mol}$

Answer

$+124 \text{ kJ/mol}$

Question 14

A student determines the standard enthalpy change, $\Delta H^\circ$, for the conversion of graphite to diamond:
$$\text{C(s, graphite)} \rightarrow \text{C(s, diamond)}$$
The student is provided with the following standard enthalpies of combustion:
$$\text{C(s, graphite)} + \text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} \quad \Delta H^\circ = -393.5 \text{ kJ/mol}_{rxn}$$
$$\text{C(s, diamond)} + \text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} \quad \Delta H^\circ = -395.4 \text{ kJ/mol}_{rxn}$$
Based on these data, what is the value of $\Delta H^\circ$ for the conversion of graphite to diamond?

Accepted Answer

$+1.9 \text{ kJ/mol}_{rxn}$

Answer

$+393.5 \text{ kJ/mol}_{rxn}$

Answer

$+395.4 \text{ kJ/mol}_{rxn}$

Answer

$+788.9 \text{ kJ/mol}_{rxn}$

Question 15

A student uses a coffee-cup calorimeter to determine the molar enthalpy of dissolution of solid potassium nitrate, represented by the equation $\text{KNO}_3\text{(s)} \rightarrow \text{K}^+\text{(aq)} + \text{NO}_3^-\text{(aq)}$. The student obtains the following data.

| Measurement | Value |
|---|---:|
| Mass of $\text{KNO}_3\text{(s)}$ | $10.1\text{ g}$ |
| Mass of water | $89.9\text{ g}$ |
| Initial temperature | $25.00^\circ\text{C}$ |
| Final temperature | $16.25^\circ\text{C}$ |

Assume that the solution has a specific heat capacity of $4.00\text{ J/(g}\cdot^\circ\text{C)}$, the calorimeter absorbs negligible heat, and the molar mass of $\text{KNO}_3$ is $101\text{ g/mol}$. What is the molar enthalpy of dissolution of $\text{KNO}_3\text{(s)}$?

Accepted Answer

$+35.0\text{ kJ/mol}$

Answer

$-35.0\text{ kJ/mol}$

Answer

$+350\text{ kJ/mol}$

Answer

$+35{,}000\text{ kJ/mol}$

Question 16

A student evaluates a reaction proposed for use in a reusable heat pack. The molar enthalpy diagram for the reaction is shown.

Which set of values is consistent with the diagram?

Accepted Answer

$H_{\text{reactants}}=120\ \text{kJ/mol}$, $H_{\text{products}}=40\ \text{kJ/mol}$, and $\Delta H=-80\ \text{kJ/mol}$

Answer

$H_{\text{reactants}}=40\ \text{kJ/mol}$, $H_{\text{products}}=120\ \text{kJ/mol}$, and $\Delta H=+80\ \text{kJ/mol}$

Answer

$H_{\text{reactants}}=120\ \text{kJ/mol}$, $H_{\text{products}}=40\ \text{kJ/mol}$, and $\Delta H=+80\ \text{kJ/mol}$

Answer

$H_{\text{reactants}}=120\ \text{kJ/mol}$, $H_{\text{products}}=40\ \text{kJ/mol}$, and $\Delta H=+160\ \text{kJ/mol}$

Question 17

A student passes steam through a metal coil immersed in a cooler water bath. Inside the coil, the steam changes from $\text{H}_2\text{O(g)}$ at $100^\circ\text{C}$ to $\text{H}_2\text{O(l)}$ at $100^\circ\text{C}$, and the temperature of the water bath increases.

Which statement correctly classifies the condensation and explains the associated energy transfer?

Accepted Answer

The condensation is exothermic because stronger intermolecular attractions lower the potential energy of the $\text{H}_2\text{O}$ molecules, transferring energy to the surroundings.

Answer

The condensation is exothermic because the average kinetic energy of the $\text{H}_2\text{O}$ molecules decreases during the phase change.

Answer

The condensation is endothermic because the $\text{H}_2\text{O}$ molecules must absorb energy to overcome intermolecular attractions as they enter the liquid phase.

Answer

The condensation is endothermic because stronger intermolecular attractions lower the potential energy of the $\text{H}_2\text{O}$ molecules, transferring energy to the surroundings.

Question 18

A student compiling thermochemical reference data for a materials study considers the following processes at $298\text{ K}$ and $1\text{ bar}$. Which equation represents a standard formation process for which $\Delta H_f^\circ$ is zero by definition?

Accepted Answer

$\text{Br}_2\text{(l)} \rightarrow \text{Br}_2\text{(l)}$

Answer

$\text{Br}_2\text{(g)} \rightarrow \text{Br}_2\text{(l)}$

Answer

$\text{C(s, graphite)} \rightarrow \text{C(s, diamond)}$

Answer

$\dfrac{1}{2}\text{N}_2\text{(g)}+\dfrac{3}{2}\text{H}_2\text{(g)} \rightarrow \text{NH}_3\text{(g)}$

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