Physical Chemistry : Catalysts and Enzymes

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #1 : Catalysts And Enzymes

Upon addition of a catalyst, which of the following are expected to happen to a catalytic reaction?

I. The amount of products produced will increase

II. The amount of reactant molecules reaching the activation energy will increase

III. The rate constant of the reaction will increase

Possible Answers:

III only

II and III

I and III

I only

Correct answer:

II and III

Explanation:

Catalysts are added to a reaction to decrease the activation energy. This will allow for reactants to easily overcome the energy barrier (activation energy) and carry out the reaction; therefore, the reaction will happen quicker and the rate of reaction will increase. Recall that the rate constant quantifies the rate of a reaction. Since rate of reaction and rate constant are directly proportional, addition of catalyst will lower the activation energy, increase the rate of reaction, and, subsequently, increase the rate constant.

It’s important to remember that catalysts only alter the rate of a reaction (speeds up the reaction). They have no effect on thermodynamics/equilibrium (the amount of products produced).

Example Question #1 : Catalysts And Enzymes

Which of the following molecules will increase the speed of a biological reaction?

Possible Answers:

More than one of these

DNAase

Histones

ATP

Correct answer:

DNAase

Explanation:

The speed of a reaction is increased when the amount of reactants reaching the activation energy (energy barrier) is increased. This can be done via two ways: increasing temperature or adding a catalyst. Increasing temperature will add kinetic energy to the reactant and increase the amount of reactants reaching the energy barrier. Adding a catalyst will decrease the activation energy and, subsequently, increase the amount of reactants reaching the energy barrier.

Since the temperature is kept relatively constant in the human body (due to homeostasis), the most common way human body increases the speed of a reaction is by using catalysts. Biological catalysts are called enzymes and they are usually named with the suffix -ase. The only molecule that is an enzyme in this question is DNAase, which catalyzes the hydrolysis of phosphodiester bonds in the backbone of DNA.

ATP provides energy for active reactions but it cannot speed up the reaction. Histones are proteins found in nucleus that are involved in DNA packaging. They are irrelevant to this question.

Example Question #1 : Catalysts And Enzymes

Which of the following parameters is unaltered by the addition of a catalyst?

Possible Answers:

More than one of these remain unaltered

Enthalpy

Equilibrium constant

Activation energy

Correct answer:

More than one of these remain unaltered

Explanation:

Catalysts are chemicals that function to speed up a reaction. They do not, however, change the amount of products produced (the equilibrium of the reaction). They just make it so that the equilibrium is reached at a faster rate. The main way catalysts decrease reaction rate is by lowering the activation energy. This is the energy peak required to create transition states (molecules that are in between reactants and products). Once past this peak, the transition states convert into the products. Catalysts lower this peak so that it is easier to create transition states.

As mentioned, equilibrium constant will not change because catalysts do not alter the equilibrium of the reaction. Enthalpy (change in heat) of the reaction will also remain constant because it is not altered by the catalyst.

Example Question #11 : Equilibrium And Kinetics

Which of the following parameter(s) is/are unaltered in an adiabatic reaction?

Possible Answers:

Heat in the surroundings

Neither of these

Both of these

Heat inside the system

Correct answer:

Both of these

Explanation:

An adiabatic reaction is characterized as a reaction that neither gains nor loses net heat. This means that the process of converting the reactants to products does not alter the heat in the system (reaction) or the surroundings; therefore, both the heat inside and outside the system will be constant.

Example Question #11 : Equilibrium And Kinetics

Enzymes __________ the rate of reaction by __________ activation energy.

Possible Answers:

increase . . . decreasing

decrease . . . decreasing

decrease . . . increasing

increase . . . increasing

Correct answer:

increase . . . decreasing

Explanation:

Enzymes are biological catalysts that speed up many reactions essential for the human body. Their chemistry is the same as catalysts. They speed up reactions by lowering the activation energy of reactions. This allows reactions to easily overcome the energy barrier and create the necessary products from reactants.

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