Question 1

Which of the following statements best describes the concept of a 'state function' in thermochemistry?

Accepted Answer

A property that depends only on the initial and final states of a system, not on the path taken.

Question 2

If the chemical equation $$\mathrm{C(s) + O_2$(g) \rightarrow CO_2$(g)}$$ has $$\Delta H = -393.5\ \mathrm{kJ}$$, what is $$\Delta H$$ for the reverse reaction $$\mathrm{CO_2$(g) \rightarrow C(s) + O_2$(g)}$$?

Accepted Answer

$$\Delta H = +393.5\ \mathrm{kJ}$$

Question 3

According to Hess's Law, what is the enthalpy change for a reaction that occurs in multiple steps?

Accepted Answer

The sum of the enthalpy changes for each individual step.

Question 4

Which of the following is true about the standard enthalpy of formation $$\Delta H_f^\circ$$ for a pure element in its standard state?

Accepted Answer

$$\Delta H_f^\circ = 0$$

Question 5

A chemical equation is multiplied by a factor of 2. How does this affect the enthalpy change $$\Delta H_{rxn}$$?

Accepted Answer

$$\Delta H_{rxn} is $$multiplied by 2

Question 6

Given the reaction $$2A + B \rightarrow C$$ with $$\Delta H_{rxn} = 122\ \mathrm{J}$$, what is $$\Delta H_{rxn}$$ for the reaction $$2C \rightarrow 4A + 2B$$?

Accepted Answer

$$\Delta H_{rxn} = -244\ \mathrm{J}$$

Question 7

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

Accepted Answer

The most stable form of the substance at 1 atm pressure and a specified temperature, usually 25°C.

Question 8

Which method is NOT a way to determine the enthalpy change $$\Delta H_{rxn}$$ for a chemical reaction?

Accepted Answer

By measuring the pressure of the reactants.

Question 9

The enthalpy change for a chemical reaction is defined as:

Accepted Answer

The difference in enthalpy between the products and reactants.

Question 10

For the combustion of methane: $$\mathrm{CH_4$(g) + 2O_2(g) \rightarrow CO_2$(g) + 2H_2O(l)}$$, given $$\Delta H_f^\circ$$ values: $$\mathrm{CH_4$(g)} = -74.87\ \mathrm{kJ/mol}$$, $$\mathrm{CO_2$(g)} = -393.5\ \mathrm{kJ/mol}$$, $$\mathrm{H_2O(l)} = -285.8\ \mathrm{kJ/mol}$$, $$\mathrm{O_2$(g)} = 0$$, calculate $$\Delta H_{rxn}^\circ.$$

Accepted Answer

$$\Delta H_{rxn}^\circ = -890.1\ \mathrm{kJ}$$

Question 11

Which of the following is an example of an extensive property?

Accepted Answer

Enthalpy change $$\Delta H$$

Question 12

Why is the standard enthalpy of formation for $$\mathrm{O_2$(g)}$$ defined as zero?

Accepted Answer

Because it is a pure element in its standard state.

Question 13

A student wants to calculate the enthalpy change for a reaction using standard enthalpies of formation. Which formula should they use?

Accepted Answer

$$\Delta H_{rxn}^\circ = \sum n_p \Delta H_f^\circ(	ext{products}) - \sum n_r \Delta H_f^\circ(	ext{reactants})$$

Question 14

Which of the following best illustrates the application of Hess's Law in a real-world scenario?

Accepted Answer

Calculating the enthalpy change for the combustion of natural gas by combining known enthalpy changes for related reactions.

Question 15

If a reaction is carried out in a solution with a concentration of 1 M, what does this indicate about the conditions?

Accepted Answer

The reaction is occurring under standard conditions.

Question 16

Which statement is true regarding the sign of $$\Delta H_{rxn}$$ when a chemical equation is reversed?

Accepted Answer

The sign of $$\Delta H_{rxn} is $$reversed.

Question 17

Which of the following is NOT a standard condition for thermochemical measurements?

Accepted Answer

A pressure of 2 atm.

Question 18

A reaction has $$\Delta H_{rxn} = -500\ \mathrm{kJ}$$ when 2 moles of reactant are used. What is $$\Delta H_{rxn} if $$only 1 mole of reactant is used?

Accepted Answer

$$\Delta H_{rxn} = -250\ \mathrm{kJ}$$

Question 19

Which of the following best explains why enthalpy is considered an extensive property?

Accepted Answer

It depends on the quantity of reactants undergoing the reaction.

Thermochemistry - Enthalpy, Hess's Law, and Standard Enthalpies of Reaction

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