Question 1

Which of the following best describes the enthalpy change ($$\Delta H) $$measured in a constant-pressure calorimeter?

Accepted Answer

It equals the heat flow into or out of the system at constant pressure.

Question 2

A student dissolves ammonium nitrate ($$\mathrm{NH_4NO_3}) in $$water in a coffee-cup calorimeter and observes the temperature of the solution decreases. Which statement is correct?

Accepted Answer

The process is endothermic because $$\Delta H \u003e 0.$$

Question 3

Which equation correctly represents the relationship between enthalpy change ($$\Delta H$$), internal energy change ($$\Delta U$$), pressure ($$P$$), and change in volume ($$\Delta V) $$for a reaction at constant pressure?

Accepted Answer

$$\Delta H = \Delta U + P\Delta V$$

Question 4

Which of the following statements about exothermic and endothermic reactions is correct?

Accepted Answer

An exothermic reaction has $$\Delta H \u003c 0$$ and releases heat to the surroundings.

Question 5

A reaction is carried out in a coffee-cup calorimeter. The temperature of the solution increases by $$2.5\ ^\circ\mathrm{C}. $$What can you conclude about the sign of $$\Delta H$$ for the reaction?

Accepted Answer

$$\Delta H is $$negative; the reaction is exothermic.

Question 6

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

Accepted Answer

The enthalpy change for a reaction can be calculated by summing the enthalpy changes of a series of steps that add up to the overall reaction.

Question 7

Given the following reactions and their enthalpy changes:

$$\mathrm{C(s) + O_2$(g) \rightarrow CO_2$(g)} \quad \Delta H_1$

$$\mathrm{$$CO_2(g$$) \rightarrow CO(g) + \frac{1}{2}$$O_2(g$$)} \quad \Delta $$H_2$$

What is the enthalpy change for the reaction $$\mathrm{C(s) + \frac{1}{2}O_2$(g) \rightarrow CO(g)}$$?

Accepted Answer

$$\Delta H = \Delta H_1$ + \Delta H_2$

Question 8

Which of the following is the correct definition of the standard enthalpy of formation ($$\Delta H_f^\circ) $$for a compound?

Accepted Answer

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

Question 9

Which of the following is NOT a naturally occurring diatomic molecule in its standard state?

Accepted Answer

$$\mathrm{CO_2$(g)}$$

Question 10

Calculate the enthalpy change ($$\Delta H_{rxn}) $$for the following reaction using standard enthalpies of formation:

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

Given: $$\Delta H_f^\circ [\mathrm{H_2O(l)}] = -286\ \mathrm{kJ/mol}$$, $$\Delta H_f^\circ [\mathrm{H_2$(g)}] = 0$$, $$\Delta H_f^\circ [\mathrm{O_2$(g)}] = 0$$

Accepted Answer

$$\Delta H_{rxn} = -572\ \mathrm{kJ}$$

Question 11

Which of the following best explains why the enthalpy of reaction can be calculated using enthalpies of formation?

Accepted Answer

Because enthalpy is a state function and depends only on the initial and final states, not the path taken.

Question 12

A chemical reaction is carried out in a constant-pressure calorimeter. The temperature of the solution decreases. Which of the following statements is true?

Accepted Answer

The reaction is endothermic and $$\Delta H is $$positive.

Question 13

Which of the following equations correctly calculates the enthalpy change of a reaction using standard enthalpies of formation?

Accepted Answer

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

Question 14

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

Accepted Answer

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

Question 15

A chemical reaction is multiplied by a factor of 3. What happens to the enthalpy change ($$\Delta H) $$for the reaction?

Accepted Answer

$$\Delta H is $$multiplied by 3.

Question 16

Which of the following best describes the standard state of an element?

Accepted Answer

The most stable physical form of the element at 1 bar pressure and a specified temperature, usually 25$$^\circC.$$

Question 17

A medical chemist is studying the combustion of glucose ($$\mathrm{C_6H_{12}O_6$}) in $$the body. Which of the following best describes the enthalpy change for this reaction?

Accepted Answer

It is exothermic; $$\Delta H is $$negative because energy is released as heat.

Question 18

Which of the following statements about constant-pressure and constant-volume calorimetry is correct?

Accepted Answer

Constant-pressure calorimetry measures $$q_p ($$heat at constant pressure), while constant-volume calorimetry measures $$q_v ($$heat at constant volume).

Question 19

A student calculates the enthalpy change for a reaction using Hess's Law and obtains a positive value. What does this indicate about the reaction?

Accepted Answer

The reaction is endothermic and absorbs heat from the surroundings.

Chem - Chapter 9.5 study guide

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