Enzymes and Catalysts
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AP Chemistry › Enzymes and Catalysts
Name the catalyst in each reaction.
I.
II.
I: Z
II: A
I: Z
II: Z
I: A
II: A
I: Z
II: B
I: A
II: C
Explanation
A catalyst will not be consumed during a reaction, so the catalyst will be whichever chemical is found both on the reactant side of the equation and on the product side.
For equation 1 that is compound Z; for equation 2 that is compound A.
Which of the following is true of catalysts?
They increase reaction rate
They increase reaction activation energy
They are changed during the course of the reaction
They are always in the same phase as the reactants
They raise the equilibrium product concentration
Explanation
Catalysts are substances that increase reaction rates without being consumed in the reaction. They decrease the activation energy needed, and they do not always need to be in the same phase as the reactants. In heterogeneous catalysis, the catalyst is in a different phase than the reactants. Equilibrium concentrations of both reactants and products are unchanged by the addition of a catalyst.
A catalyst increases the rate of a reaction by which of the following mechanisms?
Lowering the activation energy
Making the reactants less energetically stable
Making the products more energetically stable
Making the products less energetically stable
Elevating the temperature of the reaction
Explanation
A catalyst has no effect on the relative stability of the reactants or products, nor does it effect the temperature of a reaction.
Instead, catalysts lower the energy of transition states, increasing their stability, to lower the overall activation energy of the reaction. When the reaction requires less energy, it proceeds at a faster rate.
Enzymes are very physiologically and industrially important molecules. From cheese-making to the metabolism of toxins, we need enzymes for life and society as we know it.
Which of the following is not true of enzymes?
Enzymes increase the equilibrium product concentration
Enzymes decrease the activation energy of a reaction
Enzymes form complexes with their substrates
Enzymes interact with substrates through active sites
None of these
Explanation
Enzymes function by forming complexes with their substrates at active sites. This interaction is often thought of as a lock and key mechanism, in the sense that the active site is shaped to fit a substrate.
Enzymes are biological catalysts and therefore they work to lower the energy barrier or activation energy that prevents a reaction from proceeding to equilibrium. In other words: enzymes and catalysts in general make a reaction reach equilibrium faster.
Therefore enzymes do not change the equilibrium product concentration, just the time it takes to get to equilibrium.
Which of the following is not true of catalysts?
They cannot be in a separate phase than the reactants.
They increase reaction rate.
They are regenerated in the reaction.
They lower the activation energy needed for the reaction to proceed.
Explanation
All of the choices are true, except that catalysts can be in distinct phases than the reactants. These are known as heterogenous catalysts.
What is the function of a catalyst in a chemical reaction?
Catalysts speed up the reaction without being consumed
Catalysts slow down the reaction without being consumed
Catalysts increase the amount of product produced by the reaction
Catalysts decrease the amount of product produced by the reaction
Catalysts increase the amount of reactant available
Explanation
Catalysts increase the reaction rate without being consumed during the reaction. They don't cause the reaction to make more product, but since the catalyst won't be used up in the reaction.
Which of the following statements regarding catalysts is true?
Catalysts decrease the activation energy of both forward and reverse reactions
Catalysts decrease the 
Catalysts decrease the activation energy of the fast step of a reaction
Catalysts are included in the overall net reaction equation
Explanation
Note that when catalyst decreases the activation energy (Ea), 
How does adding a catalyst affect a reaction?
It lowers the energy required to have reactants react with one another
It increases the amount of product created in a reaction
It increases the kinetic energy of the reactants, resulting in more collisions
It lowers the internal energy of the products, resulting in more stable bonds
Explanation
A catalyst is a substance that increases the rate of a reaction, typically by lowering the activation energy required to initiate a reaction. The catalyst does not affect the equilibrium of a reaction, and is not consumed during the reaction.