MCAT Physical : Reaction Kinetics
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Example Questions
Example Question #128 : Biochemistry, Organic Chemistry, And Other Concepts
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
I, II, and III
II and III
II only
III only
I and II
II and III
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 
The order of the reaction, however, does not increase.
Example Question #129 : Biochemistry, Organic Chemistry, And Other Concepts
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
I only
II and III
I, II, and III
I and II
I and III
I only
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 #130 : Biochemistry, Organic Chemistry, And Other Concepts
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?
The rate of the forward reaction will increase and the rate of the reverse reaction will increase
The rate of the forward reaction will increase
The rate of the forward reaction will increase and the rate of the reverse reaction will decrease
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
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 #31 : Reaction Kinetics
Reaction:
Step 1: 
Step 2: 
In the reaction above, which step is the rate-determining step and what is the intermediate?
Step 2 is the rate-determining step and BC2 is the intermediate
Step 2 is the rate-determining step and B is the intermediate
Step 1 is the rate-determining step and B is the intermediate
Step 1 is the rate-determining step and BC2 is the intermediate
Step 2 is the rate-determining step and BC2 is the intermediate
In this reaction, step 2 is the rate-determining step and BC2 is 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 #12 : 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?
It must be equal to the sum of the rates of both reactions
It must be equal to the difference in the rates of both reactions
It must be equal to the rate of the reaction in the passage
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
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 #32 : Reaction Kinetics
Which of the following is true about the rate-determining step of a reaction that contains two transition states?
The rate determining step can only be one step in the reaction
The rate-determining step has the lowest 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 has the highest activation energy
The rate-determining step has the highest activation energy
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.
Example Question #33 : Reaction Kinetics
Which of the following are true concerning the rate-limiting step of a chemical reaction?
I. It is the slowest step
II. Cannot be altered by a catalyst or enzyme
III. Involves the transition state with the highest activation energy
IV. Is un-affected by a change in reactant concentration or reaction temperature
III and IV
I, II, and III
I, II, III, and IV
I only
I and III
I and III
The rate-limiting step of any reaction will always have the highest activation energy, and thus be the slowest step in a chemical reaction (hence the name). It can be altered with the help of a catalyst, by changing the concentration of reactions, and by altering the reaction temperature. These changes can affect the energy of a transition state or chemical equilibrium, altering the rate-limiting step. Only choices I and III are true.
Example Question #34 : Reaction Kinetics
Consider the following overall reaction:
The rate law for this reaction is determined to be:
What can we conclude about the reaction, based on the rate law?
Carbon monoxide is a solid, so it is not included in the rate law
The overall reaction must be done twice
The slow step involves two nitrogen dioxide molecules
The fast step involves two nitrogen dioxide molecules
The slow step involves two nitrogen dioxide molecules
Since the rate law does not match the overall reaction, we can assume that the reaction has multiple steps.
In a multistep reaction, the slowest step will determine the rate law. As a result, we can conclude that the overall reaction has a slow step, in which two nitrogen dioxide molecules react. The coefficient becomes the exponent. Note that this is only the case because we are working with the identified slow step, which can only be determined experimentally.
Carbon monoxide only enters into the equation in the faster step, and is not included in the overall rate law.
Example Question #35 : Reaction Kinetics
For any given chemical reaction, one can draw an energy diagram. Energy diagrams depict the energy levels of the different steps in a reaction, while also indicating the net change in energy and giving clues to relative reaction rate.
Below, a reaction diagram is shown for a reaction that a scientist is studying in a lab. A student began the reaction the evening before, but the scientist is unsure as to the type of the reaction. He cannot find the student’s notes, except for the reaction diagram below.
At which point(s) on the above graph would you expect to find bonds forming and breaking?
2 and 4
2, 3, and 4
2 and 3
3 and 4
3 only
2 and 4
It may be tempting to include points 2, 3, and 4, but only points 2 and 4 actually have bonds in the process of forming and breaking. This lack of stability is what gives these points the highest relative energy levels on the diagram. Point 3 is a relatively stable intermediate, stabilized by the fact that it does not have bonds in active flux.
Example Question #36 : Reaction Kinetics
For any given chemical reaction, one can draw an energy diagram. Energy diagrams depict the energy levels of the different steps in a reaction, while also indicating the net change in energy and giving clues to relative reaction rate.
Below, a reaction diagram is shown for a reaction that a scientist is studying in a lab. A student began the reaction the evening before, but the scientist is unsure as to the type of the reaction. He cannot find the student’s notes, except for the reaction diagram below.
In the reaction diagram, which point is most instrumental in determining the rate of the forward reaction?
3
5
1
2
4
2
Point 2 is at the highest peak on the chart. The difference between the energy level at this point and the energy level of the reactants is the activation energy. This is the energy that the reactants must overcome to proceed to the lower energy products. Overcoming this point takes energy, and is the primary limiting factor in the speed of any reaction.
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