Q: Which of the following is the correct formula for calculating the standard enthalpy change of a reaction ($$\Delta H_{rxn}^\circ) $$from standard enthalpies of formation?

$$\Delta H_{rxn}^\circ = \sum n \Delta H_f^\circ (\text{products}) - \sum m \Delta H_f^\circ (\text{reactants})$$

Question 1

Which of the following statements best explains why increasing the temperature generally increases the rate of a chemical reaction?

Accepted Answer

Because higher temperatures increase the number of collisions with sufficient energy to overcome the activation energy barrier.

Question 2

A reaction mechanism consists of the following steps:

1. $$A + B \xrightarrow{k_1$} C ($$slow)
2. $$C + D \xrightarrow{k_2$} E ($$fast)

Which of the following is the correct rate law for the overall reaction?

Accepted Answer

$$	ext{Rate} = k[A][B]$$

Question 3

Which of the following best describes the role of a catalyst in a chemical reaction?

Accepted Answer

It provides an alternative reaction pathway with a lower activation energy.

Question 4

A sample of gas occupies 2.50 L at 1.20 atm and 300 K. What will be its volume at 2.40 atm and 300 K?

Accepted Answer

1.25 L

Question 5

Which of the following is the correct integrated rate law for a first-order reaction?

Accepted Answer

$$\ln [A]_t = -kt + \ln [A]_0$$

Question 6

A reaction has a rate law $$	ext{Rate} = k[A]^0[B]^2. If $$the concentration of $$B is $$doubled while $$A is $$held constant, what happens to the reaction rate?

Accepted Answer

The rate increases by a factor of 4.

Question 7

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

Accepted Answer

$$\Delta H_f^\circ is $$zero for any element in its standard state.

Question 8

A 50.0 g sample of water is heated from 20.0°C to 70.0°C. How much heat is absorbed? ($$c_{water} = 4.18\ \mathrm{J/g\cdot ^\circ C})$$

Accepted Answer

10,450 J

Question 9

Which of the following best describes Dalton's Law of Partial Pressures?

Accepted Answer

The total pressure of a mixture of gases is equal to the sum of the partial pressures of each individual gas.

Question 10

A reaction has an activation energy ($$E_a) of 50 kJ/$$mol. If the temperature increases from 300 K to 310 K, what happens to the rate constant $$k$$ according to the Arrhenius equation?

Accepted Answer

$$k$$ increases because the exponential factor $$e^{-E_a/RT}$$ increases.

Question 11

A 1.00 L container holds a mixture of 0.50 mol N$$_2$$, 0.30 mol O$$_2$$, and 0.20 mol Ar at a total pressure of 1.00 atm. What is the partial pressure of O$$_2$$?

Accepted Answer

0.30 atm

Question 12

Which of the following is the correct expression for the density of a gas in terms of its molar mass ($$M$$), pressure ($$P$$), and temperature ($$T$$)?

Accepted Answer

$$d = \dfrac{MP}{RT}$$

Question 13

Which of the following is NOT a property of a catalyst?

Accepted Answer

It changes the overall enthalpy change ($$\Delta H) of $$the reaction.

Question 14

A reaction is found to be second order overall. Which of the following could be its rate law?

Accepted Answer

$$	ext{Rate} = k[A][B]$$

Question 15

Which of the following best describes the difference between heat capacity ($$C) $$and specific heat capacity ($$c$$)?

Accepted Answer

$$C$$ depends on the amount of substance, while $$c is $$independent of the amount.

Question 16

A sample of gas has a volume of 10.0 L at 27°C and 1.00 atm. What will be its volume at 127°C and 1.00 atm?

Accepted Answer

13.4 L

Question 17

Which of the following is the correct formula for calculating the standard enthalpy change of a reaction ($$\Delta H_{rxn}^\circ) $$from standard enthalpies of formation?

Accepted Answer

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

Question 18

A reaction is carried out in a coffee-cup calorimeter using 100.0 g of solution (assume $$c = 4.18\ \mathrm{J/g\cdot ^\circ C}). If $$the temperature increases by 2.50°C, what is the heat released by the reaction?

Accepted Answer

$$-1,045 J$$

Question 19

Which of the following best explains why the volume of a gas increases when the temperature increases at constant pressure (Charles's Law)?

Accepted Answer

The gas molecules move faster and collide with the container walls more frequently and forcefully, causing the volume to expand.

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