Question 1

Which statement best describes Hess's Law?

Accepted Answer

The enthalpy change for an overall reaction equals the sum of the enthalpy changes for the individual steps.

Question 2

What is the definition of 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 3

Given the reaction $$\mathrm{C_3H_8(g) + 5 O_2$(g) \rightarrow 3 CO_2$(g) + 4 H_2O(l)}$$, which equation correctly calculates $$\Delta H$$ using standard enthalpies of formation?

Accepted Answer

$$\Delta H = [3\Delta H_f^\circ(\mathrm{CO_2$}) + 4\Delta H_f^\circ(\mathrm{H_2O})] - [\Delta H_f^\circ(\mathrm{C_3H_8}) + 5\Delta H_f^\circ(\mathrm{O_2$})]$$

Question 4

Why is the bond enthalpy always a positive value?

Accepted Answer

Because energy is required to break chemical bonds.

Question 5

Which of the following is true about the enthalpy change for a reaction that occurs in multiple steps?

Accepted Answer

It is equal to the sum of the enthalpy changes for each individual step.

Question 6

A student wants to calculate the enthalpy change for the combustion of methane, $$\mathrm{CH_4$(g)}. $$Which data should they use?

Accepted Answer

Standard enthalpies of formation for $$\mathrm{CH_4$(g)}$$, $$\mathrm{CO_2$(g)}$$, and $$\mathrm{H_2O(l)}$$

Question 7

If the enthalpy of formation of $$\mathrm{CO_2$(g)} is$$ $$-393.5 kJ/$$mol and that of $$\mathrm{H_2O(l)} is$$ $$-285.8 kJ/$$mol, what is the enthalpy change for the reaction $$\mathrm{C_3H_8(g) + 5 O_2$(g) \rightarrow 3 CO_2$(g) + 4 H_2O(l)} if$$ $$\Delta H_f^\circ(\mathrm{C_3H_8(g)}) = -103.85 kJ/$$mol$?

Accepted Answer

$$\Delta H = [3(-393.5) + 4(-285.8)] - [-103.85 + 0] = -2219.9 kJ$$

Question 8

Which of the following best explains why $$\Delta H is a $$state function?

Accepted Answer

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

Question 9

Which process would require the use of average bond enthalpies rather than standard enthalpies of formation?

Accepted Answer

Estimating the enthalpy change for a gas-phase reaction where formation enthalpies are not available.

Question 10

A reaction is carried out in three steps with enthalpy changes $$\Delta H_1$ = +103.85 kJ$$, $$\Delta H_2$ = -1180.5 kJ$$, and $$\Delta H_3$ = -1143.2 kJ. $$What is the overall enthalpy change?

Accepted Answer

$$\Delta H = +103.85 - 1180.5 - 1143.2 = -2219.85 kJ$$

Question 11

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

Accepted Answer

Combining multiple reactions and their enthalpy changes to find the enthalpy change of a target reaction.

Question 12

Which of the following reactions represents a standard enthalpy of formation?

Accepted Answer

$$\mathrm{C(graphite) + O_2$(g) \rightarrow CO_2$(g)}$$

Question 13

Which statement about bond enthalpy is correct?

Accepted Answer

The greater the bond enthalpy, the stronger the bond.

Question 14

A chemist wants to estimate the enthalpy change for the reaction $$\mathrm{N_2$(g) + 3 H_2$(g) \rightarrow 2 NH_3$(g)}$$ using bond enthalpies. Which calculation should they perform?

Accepted Answer

Subtract the total bond enthalpies of bonds formed from the total bond enthalpies of bonds broken.

Question 15

Which of the following is NOT a standard condition for measuring standard enthalpy of formation?

Accepted Answer

Temperature of 100 K

Question 16

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

Accepted Answer

The sign of $$\Delta H is $$reversed.

Question 17

Which real-world scenario best illustrates the use of Hess's Law?

Accepted Answer

Calculating the enthalpy change for the combustion of a fuel using known enthalpy changes of related reactions.

Thermochemistry: Hess's Law, Enthalpies of Formation, and Bond Enthalpies

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