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

$$\Delta H_r^\circ = \sum n_{\text{products}} \Delta H_f^\circ(\text{products}) - \sum n_{\text{reactants}} \Delta H_f^\circ(\text{reactants})$$

Question 1

Which of the following elements is shown in its standard state at SATP (25°C, 100 kPa) in the first image?

Accepted Answer

Mercury, Hg(l)

Question 2

A chemical plant emits sulfur dioxide, SO2(g), as shown in the third image. Which process is most directly responsible for the formation of acid precipitation from these emissions?

Accepted Answer

SO2(g) reacts with O2(g) to form SO3(g)

Question 3

Which of the following statements about standard enthalpy of formation, $$\Delta H_f^\circ$$, is correct?

Accepted Answer

It is the enthalpy change when 1 mol of a compound is formed from its elements in their standard states.

Question 4

Which of the following substances has a standard enthalpy of formation of zero at SATP?

Accepted Answer

O2(g)

Question 5

Given the following data: $$\Delta H_f^\circ(Fe$_2O$_3(s)) = -824.2$ kJ/mol, $$\Delta H_f^\circ$$(Al_2$O_3$(s)) = -1675.7 kJ/$$mol, calculate the standard enthalpy change for the thermite reaction: $$2	ext{Al}(s) + 	ext{Fe}_2	ext{O}_3(s) ightarrow 	ext{Al}_2	ext{O}_3(s) + 2	ext{Fe}(s).$$

Accepted Answer

$$\Delta H_r^\circ = -2851.5 kJ$$

Question 6

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

Accepted Answer

$$\Delta H_r^\circ = \sum n_{	ext{products}} \Delta H_f^\circ(	ext{products}) - \sum n_{	ext{reactants}} \Delta H_f^\circ(	ext{reactants})$$

Question 7

The combustion of methane is represented by: $$	ext{CH}_4(g) + 2	ext{O}_2(g) ightarrow 	ext{CO}_2(g) + 2	ext{H}_2	ext{O}(l). $$Using the following values: $$\Delta H_f^\circ(	ext{CH}_4(g)) = -74.4 kJ/$$mol, $$\Delta H_f^\circ(	ext{CO}_2(g)) = -393.5 kJ/$$mol, $$\Delta H_f^\circ(	ext{H}_2	ext{O}(l)) = -285.8 kJ/$$mol$$, calculate $$\Delta H_r^\circ$ for the reaction.

Accepted Answer

$$\Delta H_r^\circ = -890.7 kJ$$

Question 8

Which of the following best explains why the standard enthalpy of formation for O$$_2(g) is $$zero?

Accepted Answer

O$$_2(g) is $$the most stable form of oxygen at standard conditions.

Question 9

A student calculates the enthalpy change for the reaction $$2	ext{H}_2(g) + 	ext{O}_2(g) ightarrow 2	ext{H}_2	ext{O}(l)$$ using $$\Delta H_f^\circ(	ext{H}_2	ext{O}(l)) = -285.8 kJ/$$mol. What is the correct value for $$\Delta H_r^\circ$$?

Accepted Answer

$$\Delta H_r^\circ = -571.6 kJ$$

Question 10

Which of the following is a correct statement about the use of standard enthalpies of formation in calculating reaction enthalpy?

Accepted Answer

The enthalpy change for a reaction is the sum of the enthalpies of formation of the products minus the sum for the reactants, each multiplied by their coefficients.

Question 11

Which of the following best describes the physical state of bromine and mercury in their standard states at SATP?

Accepted Answer

Bromine is a liquid, mercury is a liquid.

Question 12

A power plant emits sulfur dioxide, SO$$_2(g$$), as shown in the third image. Which environmental problem is most directly associated with this emission?

Accepted Answer

Acid precipitation

Question 13

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

Accepted Answer

$$\frac{1}{2}N_2$(g) + O_2$(g) \rightarrow NO_2$(g)$$

Question 14

If the standard enthalpy of combustion of ethene, C$$_2H$_4(g$$), is $$-1411.1 kJ/$$mol, and the standard enthalpies of formation are $$\Delta H_f^\circ(CO$_2(g$$)) = $$-393.5 kJ/$$mol and $$\Delta H_f^\circ(H$_2O(l$$)) = $$-285.8 kJ/$$mol, what is $$\Delta H_f^\circ(C$_2H$_4(g$$))?

Accepted Answer

$$+52.3 kJ/$$mol

Question 15

Which of the following best explains why the enthalpy of formation of solid and liquid gallium at SATP are not equal?

Accepted Answer

Energy is required to convert solid gallium to liquid gallium (melting is endothermic).

Question 16

Which of the following is the correct thermochemical equation for the melting of gallium, given that the enthalpy of fusion is $$5.59 kJ/$$mol?

Accepted Answer

$$	ext{Ga}(s) ightarrow 	ext{Ga}(l) \quad \Delta H = +5.59 kJ/$$mol

Lesson 5.5: Standard Enthalpies of Formation and Their Application in Thermochemistry

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