Q: Which equation correctly expresses the relationship between the standard enthalpy of reaction and the standard enthalpies of formation of reactants and products?

$$\Delta H^\circ_{rxn} = \sum n_p \Delta H_f^\circ (\text{products}) - \sum n_r \Delta H_f^\circ (\text{reactants})$$

Question 1

Which of the following best defines the standard enthalpy of formation, $$\Delta H_f^\circ$$?

Accepted Answer

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

Question 2

What are the thermodynamic standard conditions for enthalpy measurements?

Accepted Answer

Pressure of 1 bar (or 1 atm), temperature of 298 K, and all substances in their standard states.

Question 3

Which of the following substances has a standard enthalpy of formation, $$\Delta H_f^\circ$$, of zero at 298 K?

Accepted Answer

Aluminum (Al, s)

Question 4

Given the following reaction at 298 K:

$$\mathrm{C_2H_2(g) + 2H_2(g) \rightarrow C_2H_6(g)}$$

Using the standard enthalpies of formation from the table, calculate $$\Delta H^\circ_{rxn}.$$

Accepted Answer

$$-311.4\ \mathrm{kJ}$$

Question 5

Which equation correctly expresses the relationship between the standard enthalpy of reaction and the standard enthalpies of formation of reactants and products?

Accepted Answer

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

Question 6

Calculate the standard enthalpy change for the combustion of ethanol ($$\mathrm{C_2H_5OH(l)}) to $$form carbon dioxide and water vapor at 298 K:

$$\mathrm{C_2H_5OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(g)}$$

Use the following data:
$$\Delta H_f^\circ [\mathrm{C_2H_5OH(l)}] = -277.0\ \mathrm{kJ/mol}$$
$$\Delta H_f^\circ [\mathrm{CO_2$(g)}] = -393.5\ \mathrm{kJ/mol}$$
$$\Delta H_f^\circ [\mathrm{H_2O(g)}] = -241.8\ \mathrm{kJ/mol}$$

Accepted Answer

$$-1235.4\ \mathrm{kJ}$$

Question 7

Which of the following statements about Hess's Law is correct?

Accepted Answer

The enthalpy change for a reaction is independent of the pathway between initial and final states.

Question 8

A student wants to estimate the enthalpy change for the reaction:

$$\mathrm{2CO(g) + O_2$(g) \rightarrow 2CO_2(g)}$$

Given $$\Delta H_f^\circ [\mathrm{CO(g)}] = -110.5\ \mathrm{kJ/mol}$$ and $$\Delta H_f^\circ [\mathrm{CO_2$(g)}] = -393.5\ \mathrm{kJ/mol}$$, what is $$\Delta H^\circ_{rxn}$$?

Accepted Answer

$$-566.0\ \mathrm{kJ}$$

Question 9

Which of the following is a real-world application of standard enthalpies of formation?

Accepted Answer

Estimating the energy released by fuels during combustion in engines.

Question 10

Why are the standard enthalpies of formation for all elements in their standard states defined as zero?

Accepted Answer

Because they are chosen as the reference point for measuring enthalpy changes.

Question 11

Which of the following reactions would have a positive standard enthalpy of reaction ($$\Delta H^\circ_{rxn}) $$based on the enthalpies of formation provided?

Accepted Answer

$$\mathrm{C(s) + H_2O(g) \rightarrow CO(g) + H_2$(g)}$$; $$\Delta H^\circ_{rxn} = +131.3\ \mathrm{kJ}$$

Question 12

A medical device uses the combustion of propane ($$\mathrm{C_3H_8}) $$for heat. Calculate the standard enthalpy change for the reaction:

$$\mathrm{C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(l)}$$

Given:
$$\Delta H_f^\circ [\mathrm{C_3H_8(g)}] = -103.8\ \mathrm{kJ/mol}$$
$$\Delta H_f^\circ [\mathrm{CO_2$(g)}] = -393.5\ \mathrm{kJ/mol}$$
$$\Delta H_f^\circ [\mathrm{H_2O(l)}] = -285.8\ \mathrm{kJ/mol}$$

Accepted Answer

$$-2220.0\ \mathrm{kJ}$$

Question 13

Which of the following best explains why Hess's Law is valid?

Accepted Answer

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

Question 14

Which of the following is NOT a correct use of standard enthalpies of formation?

Accepted Answer

Determining the color of a compound.

Question 15

A chemical engineer needs to design a process that minimizes energy consumption. Which data should they use to estimate the energy required for a reaction under standard conditions?

Accepted Answer

Standard enthalpies of formation ($$\Delta H_f^\circ)$$

Standard Enthalpies of Formation and Their Application in Thermochemistry

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