College Chemistry : Kinetics
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Example Questions
Example Question #21 : Thermodynamics And Kinetics
What are the units for the rate constant 
Because the rate of a chemical reaction is always in 
If we replace the variables above with their units, the equation will look like this:
Therefore, in order to match, 
Example Question #22 : Thermodynamics And Kinetics
If the reaction:
Follows the mechanism:
What is the rate order with respect to 
Because B is in the rate determining step (the slow step) then we know that it is not zero order with respect to the concentration of B.
However, because there is only one molecule of B reacting in the rate determining step, we know that this reaction is first order with respect to the concentration of B.
Example Question #11 : Kinetics
If the reaction:
Follows the mechanism:
What is the rate order with respect to 
The rate law is determined by the slowest step in a reaction mechanism. Because B is not a reactant in the slow step, then it will not appear in the reaction mechanism.
Therefore, the reaction is zeroth order in relation to the concentration of B.
Example Question #1 : Enzymes, Catalysts, And Activation Energy
Which of the following is true regarding an enzyme-catalyzed reaction?
The reaction becomes more exergonic
The reaction is driven towards the left
The reaction is driven towards the right
The rate of the forward reaction and the reverse reaction both increase
The rate of the forward reaction and the reverse reaction both increase
For this question, we need to determine a true statement with regard to reactions that are catalyzed by enzymes.
To answer this, it's important to distinguish between the thermodynamics of a reaction and the kinetics. When adding an enzyme, the activation energy for the reaction is lowered. What this means is that it becomes easier for the reactants to achieve the high-energy transition state on their way to becoming product. Thus, enzymes affect the kinetics of a reaction by increasing both the forward rate and the reverse rate.
But if we look at the thermodynamics of an enzyme-catalyzed reaction, there is no change. In other words, the difference in free energy between the reactants and the products remains the same, regardless of the presence of enzyme. Thus, an enzyme will not cause a reaction to change its equilibrium position; it cannot shift a reaction towards the left or towards the right.
Because the thermodynamics of the reaction remain unchanged, the reaction will not become more exergonic, nor will it become more endergonic. Furthermore, the equilibrium of the reaction will not shift.
Example Question #2 : Enzymes, Catalysts, And Activation Energy
Which of the following statements is true with respect to reactions involving enzymes?
In the presence of enzyme, the rate constant is increased
In the presence of enzyme, the rate constant is decreased
In the presence of enzyme, the equilibrium is shifted to the left
In the presence of enzyme, the equilibrium is shifted to the right
In the presence of enzyme, the rate constant is increased
For this question, we're asked to select a true statement regarding reactions that are influenced by enzymes.
When thinking about how enzymes affect reactions, its important to draw a distinction between kinetics and thermodynamics. Enzymes increase a reaction's rate, but they don't change the equilibrium of a reaction. In other words, they make reactions go faster, but they don't make reactions go.
Since enzymes don't affect equilibrium, we can rule out two of the answer choices. Moreover, we know that enzymes increase the reaction rate by lower the activation energy. Consequently, this raises the rate constant for the reaction.
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