MCAT Physical : Biochemistry, Organic Chemistry, and Other Concepts
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Example Questions
Example Question #111 : Biochemistry, Organic Chemistry, And Other Concepts
A kinetics experiment is constructed, and the following rates of product formation are observed after adding various substrates. The results are recorded below. What is the equation for the rate law?
|
A |
B |
C |
Initial Rate (M/s) |
|
0.1M |
0.2M |
0.2M |
1.5 |
|
0.2M |
0.2M |
0.2M |
3 |
|
0.2M |
0.2M |
0.4M |
3 |
|
0.2M |
0.4M |
0.2M |
12 |
Rate = k[A][B]2
Rate = k[A]2[B]
Rate = k[A][B][C]
Rate = k[A][B]
Rate = k[A][B]2
We can look at the differences between trial 1 and trial 2 to see that the reaction is first order with respect to [A], since the rate doubles when the reactant concentration doubles. Reactants are considered first order when this 1:1 relationship between concentration and rate is observed.
We can look at trials 2 and 3 to see that [C] has no effect on the reaction, making it a zeroth order reactant and simply equal to 1 in the rate law.
Trials 3 and 4 show that when [B] doubles, the rate quadruples. This means it is second order with respect to [B], increasing the rate by the square of the increase in concentration. Since we already know that reactant C does not affect the rate, it is irrelevant that its concentration is halved.
This gives us Rate = k[A][B]2.
Example Question #12 : 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.
What factors, not depicted in the reaction diagram, are most likely to influence reaction rate?
Reactant concentrations
Free energy change
Intermediate stability
Reactant stability
Product stability
Reactant concentrations
Reactant concentration, along with activation energy and temperature, is a major determinant in reaction rate, as demonstrated in Le Chatlier's principle.
Example Question #2 : Factors Affecting Reaction Rate
Given the above equation for a reaction, which statement must ALWAYS be true?
If 
If 
If 
If 
If
This question can be approached two different ways. One way is by looking at the equation, and the other is understanding what the variables in the equation represent.
1) The first method is understanding the equation. Because we must understand basic log and natural log functions, we know that the natural log of a fraction is a negative number. Additionally, because we know that a reaction is spontaneous when 
2) A second approach to this question is understanding the variables in the equation. Keq is the ratio of product to reactant at equilibrium. Q is the reaction quotient, and represents the ratio of product to reactant at a given point in time. If Q is less than Keq, the reaction is still too far to the left, and is still reacting to reach equilibrium. Because the reaction is trying to reach equilibrium, it will continue to spontaneously react until it reaches that point.
Example Question #3 : Factors Affecting Reaction Rate
Consider the energy diagram of a three-step chemical process shown below. Which step would have the fastest rate?
The reaction with the fastest rate will have the lowest activation energy; the energy difference between the starting compound and the top of the energy barrier that must be overcome to proceed to the product should be the smallest. Of the given choices, the reaction proceeding from C to D has the lowest energy barrier, and hence will have the lowest activation energy and fastest reaction rate.
In contrast, the energy required to move from point A to point B has the largest energy barrier, and can be assumed to be the rate-limiting step.
Example Question #13 : Reaction Kinetics
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.
Reaction rates depend on reaction concentrations, as well as the reaction constant 
I. Temperature
II. Activation energy
III. Reactant concentrations
II and III
I and II
I only
I, II, and III
III only
I and II
If you thought all of these factors are used in the Arrhenius equation, consider that the rate constant is independent of the reactant concentrations. The concentration effect is considered in the rate law by multiplying the concentrations by the constant, but the concentrations themselves are independent of the actual Arrhenius calculation.
The calculation shows that 
Example Question #14 : Reaction Kinetics
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.
The scientist in the passage calculates that, under experimental conditions, the gievn reaction releases 
Under the exact same conditions, another unknown reaction is studied. This second reaction releases 
Which of the following must be true?
The first reaction will progress faster at the experimental conditions, but may be slower at other conditions
The second reaction will progress faster at the experimental conditions, but may be slower at other conditions
We cannot predict the relative rates of these two reactions with the information given
The second reaction will always progress faster
The first reaction will always progress faster
We cannot predict the relative rates of these two reactions with the information given
The amount of energy released or absorbed by a reaction is independent of its rate. Rate is principally determined by the activation energy, while the inherent bond energies of reactants and products dictate thermodynamic changes. One cannot be used to predict the other.
Example Question #12 : Reaction Kinetics
In any given reaction, the rate-determining step __________.
may change if temperature is changed or pressure is changed
represents the step with the lowest activation energy
represents the endothermic component of the reaction
cannot be modified by use of a catalyst
may change if temperature is changed or pressure is changed
Reaction rates, including the rate-determining step, can be altered with a change in conditions. This includes a change in temperature, a change in pressure, or the addition of a catalyst.
Example Question #112 : Biochemistry, Organic Chemistry, And Other Concepts
For a multi-step chemical reaction, which of the following will reduce the reaction rate?
A step with a high activation energy
A step that involves the excess reagent
A step that involves the limiting reagent
A step with a high energy transition state
A step with a high activation energy
For a multi-step reaction, the rate-determining step is the step with the highest activation energy. Be careful not to confuse this with the energy transition state.
Example Question #113 : Biochemistry, Organic Chemistry, And Other Concepts
What is the role of a catalyst in an reaction?
To increase the rate of reaction by lowering the activation energy
To decrease the rate of reaction by lowering the activation energy
To increase the rate of reaction by increasing the activation energy
To decrease the rate of reaction by increasing the activation energy
To increase the rate of reaction by lowering the activation energy
A catalyst is used to increase the rate of a reaction. By definition, it does so without being consumed during the reaction. Thus the choices that talk about decreasing the rate of reaction are wrong. Catalysts decrease the activation energy of a reaction.
Example Question #1 : Catalysts, Transition States, And Activation Energy
Carbonic anhydrase is an important enzyme that allows CO2 and H2O to be converted into H2CO3. In addition to allowing CO2 to be dissolved into the blood and transported to the lungs for exhalation, the products of the carbonic anhydrase reaction, H2CO3 and a related compound HCO3-, also serve to control the pH of the blood to prevent acidosis or alkalosis. The carbonic anhydrase reaction and acid-base reaction are presented below.
CO2 + H2O 
H2CO3 
Increasing the concentration of the carbonic anhydrase would __________ the rate constant of the forward reaction.
decrease
increase
not affect
not affect
The concentration of the enzyme is independent of the rate constant because the enzyme can only catalyze the conversion of reactants to products at a specific rate. Increasing the concentration of the enzyme, however, would increase the absolute number of reactions occurring simultaneously; thus, the rate of reaction (but not the rate constant) would increase.
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