Q: Which equation correctly represents the calculation of the standard enthalpy change for a reaction using enthalpies of formation?

$$\Delta H^\circ_{rxn} = \sum n \Delta H^\circ_f (\text{products}) - \sum m \Delta H^\circ_f (\text{reactants})$$

Question 1

Which principle allows you to calculate the enthalpy change for a reaction by summing the enthalpy changes of individual steps?

Accepted Answer

Hess's Law

Question 2

Given the following reactions:

$$\mathrm{$$CaCO_3$$ (s) \rightarrow CaO (s) + CO_2$ (g)}$$  $$\Delta H = 178.1\ \mathrm{kJ}$$

$$\mathrm{C (s, graphite) + O_2$ (g) \rightarrow CO_2$ (g)}$$  $$\Delta H = -393.5\ \mathrm{kJ}$$

What is the enthalpy change for the reaction:

$$\mathrm{$$CaCO_3$$ (s) \rightarrow CaO (s) + C (s, graphite) + O_2$ (g)}$$?

Accepted Answer

$$215.4\ \mathrm{kJ}$$

Question 3

Which equation correctly represents the calculation of the standard enthalpy change for a reaction using enthalpies of formation?

Accepted Answer

$$\Delta H^\circ_{rxn} = \sum n \Delta H^\circ_f (	ext{products}) - \sum m \Delta H^\circ_f (	ext{reactants})$$

Question 4

Given the following reactions:

(1) $$2\mathrm{NO} \rightarrow \mathrm{N_2$} + \mathrm{O_2$}$$  $$\Delta H = -180\ \mathrm{kJ}

(2$$) $$2\mathrm{NO} + \mathrm{O_2$} \rightarrow 2\mathrm{NO_2$}$$  $$\Delta H = -112\ \mathrm{kJ}$$

What is the enthalpy change for the reaction:

$$\mathrm{N_2$} + 2\mathrm{O_2$} \rightarrow 2\mathrm{NO_2$}$$?

Accepted Answer

$$-292\ \mathrm{kJ}$$

Question 5

Which of the following statements about standard enthalpy of formation ($$\Delta H^\circ_f) is $$correct?

Accepted Answer

It is the enthalpy change for the formation of one mole of a compound from its elements in their standard states.

Question 6

Using the data from the table, what is the $$\Delta H^\circ_{rxn}$$ for the reaction:

$$4\mathrm{NH_3$ (g)} + 5\mathrm{O_2$ (g)} \rightarrow 4\mathrm{NO (g)} + 6\mathrm{H_2O (l)}$$?

Given:
$$\Delta H^\circ_f (\mathrm{NH_3$ (g)}) = -46\ \mathrm{kJ/mol}$$
$$\Delta H^\circ_f (\mathrm{NO (g)}) = 90\ \mathrm{kJ/mol}$$
$$\Delta H^\circ_f (\mathrm{H_2O (l)}) = -286\ \mathrm{kJ/mol}$$

Accepted Answer

$$-1172\ \mathrm{kJ}$$

Question 7

What is the $$\Delta H^\circ_{rxn}$$ for the combustion of one mole of methane ($$\mathrm{CH_4$}) to $$form $$\mathrm{CO_2$ (g)}$$ and $$2\mathrm{H_2O (l)}$$?

$$\mathrm{CH_4$ (g)} + 2\mathrm{O_2$ (g)} \rightarrow \mathrm{CO_2$ (g)} + 2\mathrm{H_2O (l)}$$

Given:
$$\Delta H^\circ_f (\mathrm{CH_4$ (g)}) = -74.8\ \mathrm{kJ/mol}$$
$$\Delta H^\circ_f (\mathrm{CO_2$ (g)}) = -393.5\ \mathrm{kJ/mol}$$
$$\Delta H^\circ_f (\mathrm{H_2O (l)}) = -285.8\ \mathrm{kJ/mol}$$

Accepted Answer

$$-890\ \mathrm{kJ}$$

Question 8

If the $$\Delta H^\circ$$ for the reaction $$\mathrm{$$CaCO_3$$ (s) \rightarrow CaO (s) + CO_2$ (g)} is$$ $$177.8\ \mathrm{kJ}$$, and $$\Delta H^\circ_f (\mathrm{$$CaCO_3$$ (s)}) = -1207.0\ \mathrm{kJ/mol}$$, $$\Delta H^\circ_f (\mathrm{CO_2$ (g)}) = -393.7\ \mathrm{kJ/mol}$$, what is $$\Delta H^\circ_f (\mathrm{CaO (s)})$$?

Accepted Answer

$$813.4\ \mathrm{kJ/mol}$$

Question 9

Which of the following best describes why the enthalpy of formation for $$\mathrm{O_2$ (g)} is $$zero?

Accepted Answer

Because $$\mathrm{O_2$ (g)} is $$the standard state of oxygen.

Question 10

A chemical engineer is designing a process to synthesize ammonia ($$\mathrm{NH_3$}) $$from nitrogen and hydrogen gases. Using the standard enthalpy of formation values, what is the enthalpy change for the reaction:

$$\mathrm{N_2$ (g)} + 3\mathrm{H_2$ (g)} \rightarrow 2\mathrm{NH_3$ (g)}$$

Given:
$$\Delta H^\circ_f (\mathrm{NH_3$ (g)}) = -46\ \mathrm{kJ/mol}$$

Accepted Answer

$$-92\ \mathrm{kJ}$$

Question 11

Which of the following reactions is an example of a combustion reaction?

Accepted Answer

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

Question 12

A student calculates the enthalpy change for the reaction $$\mathrm{CO (g) + \frac{1}{2} O_2$ (g) \rightarrow CO_2$ (g)}$$ using the following values:
$$\Delta H^\circ_f (\mathrm{CO (g)}) = -110.5\ \mathrm{kJ/mol}$$
$$\Delta H^\circ_f (\mathrm{CO_2$ (g)}) = -393.7\ \mathrm{kJ/mol}$$
What is the enthalpy change for the reaction?

Accepted Answer

$$-283.2\ \mathrm{kJ}$$

Question 13

Which of the following is NOT a correct application of Hess's Law?

Accepted Answer

Calculating the enthalpy change for a reaction by multiplying the enthalpy change of a single step by the number of steps.

Question 14

A medical researcher is studying the combustion of ethanol ($$\mathrm{C_2H_5OH}) in $$the body. Using the table, what is the enthalpy change for the reaction:

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

Given:
$$\Delta H^\circ_f (\mathrm{C_2H_5OH (l)}) = -277.0\ \mathrm{kJ/mol}$$
$$\Delta H^\circ_f (\mathrm{CO_2$ (g)}) = -393.5\ \mathrm{kJ/mol}$$
$$\Delta H^\circ_f (\mathrm{H_2O (l)}) = -285.8\ \mathrm{kJ/mol}$$

Accepted Answer

$$-1367.4\ \mathrm{kJ}$$

Question 15

Which of the following best explains why the enthalpy change for the formation of water vapor ($$\mathrm{H_2O (g)}) is $$different from that of liquid water ($$\mathrm{H_2O (l)}$$)?

Accepted Answer

Because the phase change from liquid to gas requires energy (endothermic).

Question 16

A student wants to calculate the enthalpy change for the reaction $$\mathrm{2 NO_2$ (g) \rightarrow 2 NO (g) + O_2$ (g)}$$ using the following values:
$$\Delta H^\circ_f (\mathrm{NO_2$ (g)}) = 34\ \mathrm{kJ/mol}$$
$$\Delta H^\circ_f (\mathrm{NO (g)}) = 90\ \mathrm{kJ/mol}$$
What is the enthalpy change for the reaction?

Accepted Answer

$$112\ \mathrm{kJ}$$

Question 17

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

Accepted Answer

Determining the energy released in the combustion of fuels for power generation.

Thermochemistry: Hess's Law and Standard Enthalpies of Formation

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