Q: Which of the following is the correct equation for calculating the enthalpy change of a reaction using standard heats of formation?

$$\Delta H^\circ = \sum \Delta H_f^\circ[\text{Products}] - \sum \Delta H_f^\circ[\text{Reactants}]$$

Question 1

Which of the following best describes an exothermic reaction?

Accepted Answer

A reaction that releases heat to the surroundings.

Question 2

What is the specific heat ($$s) of a $$substance defined as?

Accepted Answer

The amount of heat required to raise the temperature of 1 g of a substance by $$1^\circ \mathrm{C}.$$

Question 3

Which of the following is the correct formula for calculating the heat ($$q) $$absorbed or released by a substance?

Accepted Answer

$$q = s \cdot m \cdot \Delta T$$

Question 4

According to Hess's Law, how is the enthalpy change ($$\Delta H) of an $$overall process determined?

Accepted Answer

By adding the enthalpy changes of the individual steps.

Question 5

Which of the following statements about standard state conditions is correct?

Accepted Answer

Standard state conditions are 1 atm pressure, 25 $$^\circC$$, and 1 M concentration.

Question 6

What is the standard heat of formation ($$\Delta H_f^\circ) of an $$element in its most stable form at 1 atm and 25 $$^\circC$$?

Accepted Answer

$$\Delta H_f^\circ = 0 kJ/$$mol

Question 7

Given the following reaction and standard heats of formation, calculate $$\Delta H^\circ$$ for the reaction:

$$\mathrm{C$$_{12}$$H_{22}O$$_{11}(s) + 3O_2(g) \rightarrow 9C(s, graphite) + 3CO_2(g) + 11H_2O(l)}$$

$$\Delta H_f^\circ[\mathrm{$$C_{12}H$$_{22}$$O_{11}(s)}] = -2221.2 kJ/$$mol
$$\Delta H_f^\circ[\mathrm{O_2$(g)}] = 0 kJ/$$mol
$$\Delta H_f^\circ[\mathrm{C(s, graphite)}] = 0 kJ/$$mol
$$\Delta H_f^\circ[\mathrm{CO_2$(g)}] = -393.5 kJ/$$mol
$$\Delta H_f^\circ[\mathrm{H_2O(l)}] = -285.8 kJ/$$mol

Accepted Answer

$$-2103.1 kJ$$

Question 8

Which of the following best explains why bond dissociation energies are always positive?

Accepted Answer

Bond breaking always requires an energy input.

Question 9

Use average bond dissociation energies to estimate the standard heat of reaction for the following:

$$\mathrm{$$PCl_3$$(g) + Cl_2$(g) \rightarrow $$PCl_5$$(g)}$$

Given:
$$D_{P-Cl} = 331 kJ/$$mol
$$D_{Cl-Cl} = 243 kJ/$$mol

Accepted Answer

$$-662 kJ$$

Question 10

Which of the following is the correct equation for calculating the enthalpy change of a reaction using standard heats of formation?

Accepted Answer

$$\Delta H^\circ = \sum \Delta H_f^\circ[	ext{Products}] - \sum \Delta H_f^\circ[	ext{Reactants}]$$

Question 11

A chemical reaction is exothermic if:

Accepted Answer

The bonds formed in the products are stronger than those broken in the reactants.

Question 12

Given the reaction:

$$4NH_3(g) + 5O_2(g) \rightarrow 4NO(g) + 6H_2O(g)$$

$$\Delta H^\circ = -901.2 kJ$$
$$\Delta H_f^\circ[O_2$(g)] = 0 kJ/$$mol
$$\Delta H_f^\circ[NO(g)] = 91.3 kJ/$$mol
$$\Delta H_f^\circ[H_2O(g)] = -241.8 kJ/$$mol

What is the standard heat of formation for $$NH_3(g$$)$$?

Accepted Answer

$$-184.4 kJ/$$mol

Question 13

Which of the following best describes the molecular-level source of energy released in an exothermic chemical reaction?

Accepted Answer

The energy comes from the difference in relative bond strengths between reactants and products.

Question 14

Use average bond dissociation energies to estimate the standard heat of reaction for:

$$SiCl_4(g) + 2H_2O(g$$) \rightarrow $$SiO_2(g) + 4$$HCl(g)$$

Given:
D$$_{Si-Cl} = 381$$ kJ/mol
D$$_{Si-O} = 368$$ kJ/mol
D$$_{H-O} = 460$$ kJ/mol
D$$_{O-H} = 498$$ kJ/mol
D$$_{Cl-Cl} = 243$$ kJ/mol
D$$_{H-Cl} = 432$$ kJ/mol

Accepted Answer

$$-528 kJ$$

Question 15

Which of the following is NOT a correct statement about standard heats of formation ($$\Delta H_f^\circ$$)?

Accepted Answer

They are always negative for all compounds.

Question 16

A student is asked to calculate the enthalpy change for a reaction but does not have all the standard heats of formation. Which alternative method can they use?

Accepted Answer

Use the average bond dissociation energies for the reactant and product bonds.

Question 17

Which of the following reactions is a formation reaction as defined by standard heats of formation?

Accepted Answer

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

Question 18

If a reaction has a positive $$\Delta H^\circ$$, which of the following is true?

Accepted Answer

The reaction is endothermic and absorbs heat.

Standard States, Heats of Formation, and Bond Dissociation Energies in Thermochemistry

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