Question 1

Which of the following best describes the enthalpy change ($$\Delta H) $$for an exothermic reaction?

Accepted Answer

$$\Delta H is $$negative because energy is released to the surroundings.

Question 2

Which equation correctly represents the standard enthalpy change of combustion for methane ($$\mathrm{CH_4$}$$)?

Accepted Answer

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

Question 3

Which of the following statements about standard enthalpy change of neutralisation is correct?

Accepted Answer

It is the enthalpy change when one mole of water is formed from the reaction of an acid and a base under standard conditions.

Question 4

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

Accepted Answer

The total enthalpy change for a reaction is independent of the pathway taken.

Question 5

Given the following data:

$$\mathrm{C(s) + O_2$(g) \rightarrow CO_2$(g)}$$ $$\Delta H = -393.5\ \mathrm{kJ\ $$mol^{-1}$$}$$

$$\mathrm{H_2$(g) + \frac{1}{2}O_2$(g) \rightarrow H_2O(l)}$$ $$\Delta H = -285.8\ \mathrm{kJ\ $$mol^{-1}$$}$$

Calculate the standard enthalpy change of formation of methanol ($$\mathrm{CH_3OH(l)}) $$using the following equation:

$$\mathrm{C(s) + 2H_2(g) + \frac{1}{2}O_2$(g) \rightarrow CH_3OH(l)}$$

Given $$\mathrm{CH_3OH(l) + \frac{3}{2}O_2$(g) \rightarrow CO_2$(g) + 2H_2O(l)}$$ $$\Delta H = -726.0\ \mathrm{kJ\ $$mol^{-1}$$}$$

Accepted Answer

$$-238.7\ \mathrm{kJ\ $$mol^{-1}$$}$$

Question 6

Which of the following best describes mean bond enthalpy?

Accepted Answer

The average energy required to break one mole of a given type of bond in the molecules of a gaseous compound.

Question 7

A student calculates the enthalpy change for the reaction $$\mathrm{H_2$(g) + Cl_2$(g) \rightarrow 2HCl(g)}$$ using mean bond enthalpies:

$$\mathrm{H-H}$$: $$436\ \mathrm{kJ\ $$mol^{-1}$$}$$
$$\mathrm{Cl-Cl}$$: $$242\ \mathrm{kJ\ $$mol^{-1}$$}$$
$$\mathrm{H-Cl}$$: $$431\ \mathrm{kJ\ $$mol^{-1}$$}$$

What is the enthalpy change for the reaction?

Accepted Answer

$$-184\ \mathrm{kJ\ $$mol^{-1}$$}$$

Question 8

Which of the following is a correct statement about standard enthalpy change of formation?

Accepted Answer

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

Question 9

Which of the following best explains why mean bond enthalpy values are only approximate?

Accepted Answer

Bond enthalpy values are averages taken from a range of compounds.

Question 10

A chemical company wants to design a fuel with a high enthalpy change of combustion. Which property should the fuel have?

Accepted Answer

It should have a large number of strong bonds in the products compared to the reactants.

Question 11

Which of the following is the correct enthalpy level diagram for an endothermic reaction?

Accepted Answer

The products are at a higher enthalpy than the reactants.

Question 12

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

Accepted Answer

Calculating the enthalpy change for reactions that cannot be measured directly, such as the formation of ozone in the atmosphere.

Question 13

Which of the following equations represents the standard enthalpy change of neutralisation for hydrochloric acid and sodium hydroxide?

Accepted Answer

$$\mathrm{HCl}(aq) + \mathrm{NaOH}(aq) \rightarrow \mathrm{NaCl}(aq) + \mathrm{H_2O}(l)$$

Question 14

Which of the following is a correct definition of standard conditions in thermochemistry?

Accepted Answer

A temperature of $$298\ \mathrm{K}$$, a pressure of $$1\ \mathrm{atm}$$, and all substances in their standard states.

Question 15

Which of the following best describes why the enthalpy change of combustion is always negative?

Accepted Answer

Combustion reactions release energy to the surroundings.

Question 16

A student uses mean bond enthalpies to estimate the enthalpy change for the reaction $$\mathrm{C_2H_4(g) + H_2$(g) \rightarrow C_2H_6(g)}. $$Given:

$$\mathrm{C=C}$$: $$612\ \mathrm{kJ\ $$mol^{-1}$$}$$
$$\mathrm{C-C}$$: $$348\ \mathrm{kJ\ $$mol^{-1}$$}$$
$$\mathrm{C-H}$$: $$412\ \mathrm{kJ\ $$mol^{-1}$$}$$
$$\mathrm{H-H}$$: $$436\ \mathrm{kJ\ $$mol^{-1}$$}$$

What is the estimated enthalpy change?

Accepted Answer

$$-124\ \mathrm{kJ\ $$mol^{-1}$$}$$

Energetics: Enthalpy, Combustion, Neutralisation, and Bond Enthalpies

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