Question 1

Which of the following best describes the standard enthalpy of formation $$\Delta H_f^\circ$$ for a compound?

Accepted Answer

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.

Question 2

Given the following bond dissociation enthalpies: $$H_2 = 434 kJ/$$mol, $$Cl_2 = 242 kJ/$$mol, and $$HCl = 431 kJ/$$mol, what is the enthalpy change for the reaction $$\frac{1}{2} H_2$(g) + \frac{1}{2} Cl_2$(g) \rightarrow HCl(g)$$?

Accepted Answer

$$-93 kJ/$$mol

Question 3

Which of the following reactions represents the standard enthalpy of formation $$\Delta H_f^\circ of $$liquid water?

Accepted Answer

$$H_2(g$$) + \frac{1}{2} $$O_2(g$$) \rightarrow H_2O(l)$$

Question 4

If the enthalpy change for $$H_2O(l) \rightarrow H_2O(g) is$$ $$+44.0 kJ/$$mol, what is the enthalpy change for the reverse reaction $$H_2O(g) \rightarrow H_2O(l)$$?

Accepted Answer

$$-44.0 kJ/$$mol

Question 5

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

Accepted Answer

The enthalpy change for a reaction is the same, regardless of the pathway taken, because enthalpy is a state function.

Question 6

Given the following standard enthalpies of formation: $$\Delta H_f^\circ (CO_2$(g)) = -393.5 kJ/$$mol, $$\Delta H_f^\circ (H_2O(l)) = -285.8 kJ/$$mol, and $$\Delta H_f^\circ (C_6H_{12}O_6$(s)) = -1273.3 kJ/$$mol, calculate the standard enthalpy change for the photosynthesis reaction:

$$6CO_2(g) + 6H_2O(l) \rightarrow C_6H_{12}O_6$(s) + 6O_2(g)$$

Accepted Answer

$$+2801 kJ/$$mol

Question 7

Which of the following is the correct chemical equation for the standard enthalpy of formation of ethanol ($$C_2H_5OH(l)$$)?

Accepted Answer

$$2C(graphite) + 3H_2(g) + \frac{1}{2}O_2$(g) \rightarrow C_2H_5OH(l)$$

Question 8

A reaction has $$\Delta H = -890.7 kJ/$$mol. What does the negative sign indicate about the reaction?

Accepted Answer

The reaction is exothermic and releases heat to the surroundings.

Question 9

Which of the following is true about the standard enthalpy of formation $$\Delta H_f^\circ$$ for elements in their standard states?

Accepted Answer

$$\Delta H_f^\circ is $$zero for all elements in their standard states.

Question 10

You are given the following reactions and their enthalpy changes:

$$CH_4(g$$) \rightarrow C(s) + 2H_2(g) \quad \Delta H = +74.8$$ kJ/mol

C(s$$) + $$O_2(g$$) \rightarrow $$CO_2(g$$) \quad \Delta H = -393.5$$ kJ/mol

2H_2(g$$) + $$O_2(g$$) \rightarrow 2H_2O(l) \quad \Delta H = -571.6$$ kJ/mol

Calculate the enthalpy change for the combustion of methane:

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

Accepted Answer

$$-890.3 kJ/$$mol

Question 11

Which of the following best describes a standard state for a substance?

Accepted Answer

The most stable form of the substance at 1 bar pressure and the temperature of interest.

Question 12

If the standard enthalpy of formation of $$NaCl(s) is$$ $$-411.1 kJ/$$mol, what is the enthalpy change for the reaction:

$$Na(s) + \frac{1}{2}Cl_2$(g) \rightarrow NaCl(s)$$?

Accepted Answer

$$-411.1 kJ/$$mol

Question 13

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

Accepted Answer

Adding the enthalpy changes of multiple reactions to find the enthalpy change of a target reaction.

Question 14

A student is asked to calculate the enthalpy change for the reaction $$2A + B \rightarrow C + D$$ using the standard enthalpies of formation. Which formula should they use?

Accepted Answer

$$\Delta H^\circ = [\Delta H_f^\circ(C) + \Delta H_f^\circ(D)] - [2\Delta H_f^\circ(A) + \Delta H_f^\circ(B)]$$

Question 15

Which of the following best explains why most compounds have a negative standard enthalpy of formation?

Accepted Answer

Most compounds are more stable than the elements from which they are formed.

Question 16

A chemical reaction is carried out at constant pressure and releases heat. Which of the following statements is true?

Accepted Answer

The enthalpy change $$\Delta H is $$negative.

Question 17

Which of the following is NOT a standard state as defined in thermochemistry?

Accepted Answer

A gas at 2 bar pressure.

Question 18

If the enthalpy change for the reaction $$A \rightarrow B is$$ $$+50 kJ/$$mol, what is the enthalpy change for the reverse reaction $$B \rightarrow A$$?

Accepted Answer

$$-50 kJ/$$mol

Thermochemistry6.5: Determining Enthalpies of Reaction from Standard Enthalpies of Formation

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