MCAT Physical : Physical Chemistry

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

Example Question #5 : Catalysts, Transition States, And Activation Energy

Which of the following statements is false about catalysts?

Possible Answers:

Catalysts lower the activation energy (Ea) of certain reactions

Catalysts shift the equilibrium position of a reaction in favor of the products

All of these statements are true

Catalysts change the rate of the reaction

Correct answer:

Catalysts shift the equilibrium position of a reaction in favor of the products

Explanation:

Catalysts do not shift the equilibrium position of a reaction in favor of the products.

Catalysts speed up chemical reactions by lowering the activation energy (Ea) of reactions, but do not affect the equilibrium position since the change in rate from reactants to products speeds up proportionally to the change in rate from products to reactants (the same Keq will be achieved whether a catalyst is used or not).

Example Question #6 : Catalysts, Transition States, And Activation Energy

If the reactants and/or products in a chemical reaction are gases, the reaction rate can be determined by measuring the change of pressure as the reaction proceeds. Consider the following reaction and pressure vs. reaction rate data below.

 


Trial

PXY(torr)

PZ(torr)

Rate (torr/s)

1

100

200

0.16

2

200

200

0.32

3

200

100

0.04

4

200

150

0.14

If an inhibitory catalyst were added to the reaction __________.

Possible Answers:

the concentration of YZ would decrease at equilibrium

the concentration of YZ would increase at equilibrium

the activation energy would decrease

the activation energy would increase

Correct answer:

the activation energy would increase

Explanation:

A catalyst affects activation energy; an inhibitory catalyst increases activation energy. Catalysts do not affect equilibrium concentrations of products or reactants.

Example Question #7 : Catalysts, Transition States, And Activation Energy

Which of the following is false about catalyzed reactions?

Possible Answers:

Catalysts increase the forward rate, while reducing the reverse rate.

Catalysts lower the activation energy for the reaction.

Catalysts will not alter the equilibrium of the reaction.

Catalysts do not alter the energy change between the products and reactants.

Correct answer:

Catalysts increase the forward rate, while reducing the reverse rate.

Explanation:

A catalyst is a substance that increases the rate of a reaction without being altered or used up in the reaction. Both the forward and reverse rates of the reaction are accelerated by a catalyst. Slowing the reverse rate, with an increase in forward rate, would result in a shift in equilibrium. Remember that a catalyst will never change the equilibrium constant (Keq) of a reaction.

Example Question #1 : Catalysts, Transition States, And Activation Energy

Suppose a catalyst is added to the equation. How will this affect ?

Possible Answers:

It will be decreased.

It will be unaffected.

It will be increased.

A catalyst will not accelerate this reaction.

Correct answer:

It will be unaffected.

Explanation:

Remember that catalysts affect the kinetics of a reaction, but will not affect the equilibrium. As a result,  will remain the same in the presence of a catalyst, though the reaction rate will accelerate.

Example Question #2 : Catalysts, Transition States, And Activation Energy

A scientist is studying a reaction, and places the reactants in a beaker at room temperature. The reaction progresses, and she analyzes the products via NMR. Based on the NMR readout, she determines the reaction proceeds as follows:

In an attempt to better understand the reaction process, she varies the concentrations of the reactants and studies how the rate of the reaction changes. The table below shows the reaction concentrations as she makes modifications in three experimental trials.

 

A different scientist runs the same reaction described in the passage. In this trial, however, the scientist adds a catalyst. With the addition of a catalyst, which of the following must be true?

I. The overall order of the reaction increases

II. The rate constant increases

III. The activation energy decreases

Possible Answers:

II and III

III only

I, II, and III

II only

I and II

Correct answer:

II and III

Explanation:

The addition of a catalyst lowers the activation energy of a reaction. This means that the rate constant will increase, as the activation energy is a term used to calculate this value.

The Arrhenius equation shows that , where is the activation energy.

The order of the reaction, however, does not increase.

Example Question #3 : Catalysts, Transition States, And Activation Energy

Acids and bases can be described in three principal ways. The Arrhenius definition is the most restrictive. It limits acids and bases to species that donate protons and hydroxide ions in solution, respectively. Examples of such acids include HCl and HBr, while KOH and NaOH are examples of bases. When in aqueous solution, these acids proceed to an equilibrium state through a dissociation reaction.

All of the bases proceed in a similar fashion.

 

The Brønsted-Lowry definition of an acid is a more inclusive approach. All Arrhenius acids and bases are also Brønsted-Lowry acids and bases, but the converse is not true. Brønsted-Lowry acids still reach equilibrium through the same dissociation reaction as Arrhenius acids, but the acid character is defined by different parameters. The Brønsted-Lowry definition considers bases to be hydroxide donors, like the Arrhenius definition, but also includes conjugate bases such as the A- in the above reaction. In the reverse reaction, A- accepts the proton to regenerate HA. The Brønsted-Lowry definition thus defines bases as proton acceptors, and acids as proton donors.

Acids and bases can be used as catalysts to promote faster reactions of various types. Which of the following will be be true of these catalytic reactions?

I. The acid or base will be regenerated by the reaction

II. The acid or base will increase the activation energy of the reaction

III. The acid or base will increase the amount of energy released by exothermic reactions

Possible Answers:

I, II, and III

I only

I and II

I and III

II and III

Correct answer:

I only

Explanation:

A catalyst must be regenerated in its original form for it to be considered as such. A catalyst is never consumed or produced by a reaction. At once, a catalyst cannot change the thermodynamic properties of a reaction, including the energy absorbed or released. Catalysts typically function by lowering the activation energy, not increasing it.

Example Question #11 : Catalysts, Transition States, And Activation Energy

As the above reaction progresses, a catalyst is added to the system. When the reaction reaches equilibrium, the amount of catalyst is doubled. What effect will this addition have on the reaction?

Possible Answers:

The rate of the forward reaction will increase and the rate of the reverse reaction will decrease

The rate of the forward reaction will increase

The rate of the reverse reaction will decrease

The rate of the forward reaction will increase and the rate of the reverse reaction will increase

Correct answer:

The rate of the forward reaction will increase and the rate of the reverse reaction will increase

Explanation:

Catalysts affect reaction rate by lowering the energy of the transition state. In doing so, they lower the activation energy for the reaction and allow it to proceed faster. Catalysts do not affect the equilibrium constant, the equilibrium concentration of reactions, or the equilibrium concentration of products.

Chemical equilibrium is a dynamic state. The reaction still progresses, but in such a manner that the forward and reverse reaction rates are equal, creating the illusion of static chemical concentrations. Adding catalyst to the reaction at equilibrium will still impact reaction rate, but will not impact the chemical concentrations. For this to be achieved, the catalyst accelerates both the forward and reverse reactions, causing the increases to cancel out and leave the equilibrium concentrations unaffected.

Example Question #12 : Catalysts, Transition States, And Activation Energy

Reaction:

Step 1: (fast)

Step 2: (slow)

In the reaction above, which step is the rate-determining step and what is the intermediate?

Possible Answers:

Step 1 is the rate-determining step and BCis the intermediate

Step 2 is the rate-determining step and B is the intermediate

Step 2 is the rate-determining step and BCis the intermediate

Step 1 is the rate-determining step and B is the intermediate

Correct answer:

Step 2 is the rate-determining step and BCis the intermediate

Explanation:

In this reaction, step 2 is the rate-determining step and BCis the intermediate. The rate-determining step is always the slowest step in the reaction; the rate of the entire reaction depends on the speed of this step. BC2 is the intermediate because intermediates are not found in the overall reaction; they are produced and then immediately consumed.

Example Question #13 : Catalysts, Transition States, And Activation Energy

A scientist is studying a reaction, and places the reactants in a beaker at room temperature. The reaction progresses, and she analyzes the products via NMR. Based on the NMR readout, she determines the reaction proceeds as follows:

In an attempt to better understand the reaction process, she varies the concentrations of the reactants and studies how the rate of the reaction changes. The table below shows the reaction concentrations as she makes modifications in three experimental trials.

 

Consider that the reaction in the passage is the first step of a larger process. This process then goes on to a second, faster reaction between water and a solid metal. What is true of the overall rate of this entire process?

Possible Answers:

It must be equal to the difference in the rates of both reactions

It must be equal to the rate of the second reaction

It is a new rate, unrelated to either individual reaction rate

It must be equal to the rate of the reaction in the passage

It must be equal to the sum of the rates of both reactions

Correct answer:

It must be equal to the rate of the reaction in the passage

Explanation:

The question specifies that the reaction in the passage is the slowest step of this overall process. This means that it is the rate-limiting step, and its rate defines the overall reaction rate of the process.

Water is formed according to the rate in the passage. The second step cannot continue until this water is formed; thus, the second step cannot work at a faster rate than the first. The overall reaction only goes as fast as the rate of the limiting step.

Example Question #14 : Catalysts, Transition States, And Activation Energy

Which of the following is true about the rate-determining step of a reaction that contains two transition states?

Possible Answers:

The rate-determining step has the highest activation energy

The rate determining step is the fastest step in the overall reaction

The rate-determining step cannot be sped up with an enzyme

The rate determining step can only be one step in the reaction

The rate-determining step has the lowest activation energy

Correct answer:

The rate-determining step has the highest activation energy

Explanation:

The rate-determining step is the slowest step in the reaction; however, there can be more than one rate-determining step if there are two steps that have high activation energies. Enzymes help lower the activation energy for a particular step, and thus diminish the effect of rate-determining steps to speed up the reaction. The rate-determining step generally precedes the highest energy transition state.

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