Thermochemistry and Kinetics

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AP Chemistry › Thermochemistry and Kinetics

Questions 1 - 10

For the following cell reaction:

2 Al (s) + 3 Mn2+ (aq) -> 2 Al3+ (aq) + 3 Mn (s)

predict if the cell potential Ecell will be larger, or smaller than Eocell for the following conditions: \[Al3+\] = 1.0 M; \[Mn2+\] = 5.0 M.

Larger

Smaller

Identical

Can not be determined

Explanation

$E_{cell} = E^0_{cell} - \frac{0.0592}{n} : log : Q$

At these concentrations Q will become smaller, and thus log Q will become smaller. This will give rise to a larger cell potential.

Which of the following is a law of thermodynamics?

ΔH (system) + ΔH (surroundings) = ΔH (universe)

ΔS (system) + ΔS (surroundings) = ΔS (universe)

ΔE (surroundings) = ΔE (system)

The entropy of the universe is always decreasing

Explanation

The second law of thermodynamics states that the entropy of the system and the entropy of the surroundings is equal to the entropy of the universe. The rest of the answer choices are not one of the fundamental laws of themodynamics.

Which of the following is a law of thermodynamics?

ΔH (system) + ΔH (surroundings) = ΔH (universe)

ΔS (system) + ΔS (surroundings) = ΔS (universe)

ΔE (surroundings) = ΔE (system)

The entropy of the universe is always decreasing

Explanation

The rate law of the reaction, $A + B \rightarrow C$ , is $\textup{Rate} = k\left [ A\right ]^{3}$ . Which of the following does not increase the rate of the reaction?

Increasing the concentration of

Adding the catalyst to the reaction

Increasing the temperature of the reaction

Increasing the concentration of

Explanation

The reactant is not included in the rate law expression, and therefore altering its concentration does not affect the rate of the reaction. Catalysts always increase the rate of reactions by lowering its activation energy. Increasing temperature (average kinetic energy of the molecules) increases the frequency of collisions, and increases the proportion of collisions that have enough energy to overcome the activation energy and undergo a chemical reaction. Increasing the concentration of will increase the rate of the reaction as indicated by the rate law.

Chaning which of the following factors can alter the rate of a zero-order reaction?

Temperature

Increasing the concentration of reactants

Increasing the concentration of products

Decreasing the concentration of products

Explanation

A zero-order reaction has a rate of formation of product that is independent of changes in concentrations of any of the reactants; however, since the rate constant itself is dependent on temperture, changing the temperature can alter the rate.

$O_{_{3}} \rightarrow O_{_{2}} + O$ (slow)

$O + O_{_{3}} \rightarrow 2O_{_{2}}$ (fast)

The mechanism for decomposition of ozone is shown. What is the intermediate of the process?

$O_{2}$

$2O_{2}$

$O_{3}$

Explanation

Intermediate is created and destroyed, and therefore does not appear on the net equation, which is $2O_{3} \rightarrow 3O_{2}$ . Thus, the intermediate is . Note that when asked for an intermediate, the coefficient in front of it is not used, rather we are looking for the species that is a product of one reaction and a reactant in a subsequent step.

Which of the following is a classic example of a first-order reaction?

Radioactive decay

A collision between 2 reactant molecules

A change in temperature

None of the other answers

Explanation

First order reactions have rates that are directly proportional to only 1 reactant. In radioactive decay, the rate of decrease of a radioactive material is proportional only to the amount of the material.

A chemistry student is trying to calculate how long it will take a power source of to heat a sample of ice from $-35^{o}C$ to $75^{o}C$ . Given that the specific heat capacity of ice is $2.06\frac{J}{g^{o}C}$ , the specific heat capacity of liquid water is $4.184\frac{J}{g^{o}C}$ , and the heat of fusion for water is $334.16\frac{J}{g}$ , how long will this process take?

There is not enough information to determine the amount of time needed for the process described

Explanation

In order to solve this problem, we'll need to break it up into steps.

Step 1: Calculate the amount of energy necessary to raise the sample of ice from $-35^{o}C$ to $0^{o}C$ . To do this, we'll need to use the following equation:

$q_{1}=mc_{H_{2}O(s)}\Delta T$

$q_{1}=( 20\cdot 10^{3}g)\left ( 2.06\frac{J}{g^{0}C} \right )\left ( 35^{o}C \right )=1.44\cdot 10^{6}J$

Step 2: Calculate the amount of energy necessary to convert the sample at $0^{o}C$ from ice to water. We'll need to make use of the following equation:

$q_{2}=m\Delta H_{fus(H_{2}O)}$

$q_{2}=\left ( 20\cdot 10^{3}g \right )\left ( 334.16\frac{J}{g} \right )=6.68\cdot 10^{6}J$

Step 3: Calculate the amount of energy necessary to convert the sample of water from $0^{o}C$ to $75^{o}C$ .

$q_{3}=mc_{H_{2}O(l)}\Delta T$

$q_{3}=\left ( 20\cdot 10^{3}g \right )\left ( 4.184\frac{J}{g^{o}C} \right )\left ( 75^{o}C \right )=6.28\cdot 10^{6}J$

Step 4: Sum the amount of energy from the previous 3 steps. This value is the total amount of energy for the entire process.

$q_{Total}=q_{1}+q_{2}+q_{3}=1.44\cdot 10^{7}J$

Step 5: Now that we know the total amount of energy needed for the process, we need to calculate the time based on the amount of power provided.

$P=\frac{q_{Total}}{t}$

$t=\frac{q_{Total}}{P}=\frac{\left ( 1.44\cdot 10^{7}J \right )}{(100\frac{J}{s}) }=1.44\cdot 10^{5}s$

$t=(1.44\cdot10^{5}s)\left ( \frac{1hr}{3600s} \right )=40hrs$

How many grams of Cr can be obtained by the electrolysis of a Cr(NO3)3 if 10 amps are passed through the cell for 6 hours?

38.8 g

19.4 g

103 g

12.5 g

56.3 g

Explanation

$Cr^{3+} + 3 e- \rightarrow Cr(s)$

$mass = \frac{10 A , x, 6 hr , x , \frac{3600 s}{1 hr}}{ \frac{96485 C}{mol e-} } , x , \frac{1 mol , Cr}{3 mol e-} , x , \frac{51.996 g , Cr}{1 mol , Cr}= 38.8 g$