Q: The rate constant $$k$$ for a reaction is given by the equation $$k = \frac{k_B T}{h} e$$^{-\Delta G^{\ddagger}$$/RT}. If $$the activation free energy $$\Delta G$$^{\ddagger}$$ is decreased by 10 kJ/mol at 298 K, by what factor does the rate constant increase? (R = 8.314$ J/mol·K)

By a factor of $$e^{10,000/(8.314 \times 298)}$$ \approx 14$$

Q: Suppose an enzyme binds its substrate with three hydrogen bonds, each contributing $$-20 kJ/$$mol to the binding energy. At $$25^\circC$$ ($$298 K$$), what is the expected rate enhancement due to these hydrogen bonds? ($$R = 8.314 J/$$mol·K)

Rate enhancement $$= e$$^{(60,000)/(8.314 \times 298)}$$ \approx 1.1 \times 10$$^{11}$$

Question 1

Which of the following statements best distinguishes between thermodynamics and kinetics in a chemical reaction?

Accepted Answer

Thermodynamics determines the extent to which a reaction proceeds, while kinetics determines the rate at which a reaction occurs.

Question 2

A reaction has a large negative $$\Delta G^\circ$$ but proceeds extremely slowly at room temperature. What is the most likely explanation?

Accepted Answer

The reaction has a high activation energy barrier.

Question 3

Which of the following is NOT a way that enzymes increase the rate of a chemical reaction?

Accepted Answer

Enzymes increase the equilibrium constant of the reaction.

Question 4

Consider the following reaction: $$N_2 + 3H_2$$ \rightarrow 2NH_3$$ with $$\Delta G^\circ = -33.5$$ kJ/mol. The reaction is thermodynamically favorable but requires high temperature and pressure in the Haber-Bosch process. Why?

Accepted Answer

The reaction has a very high activation energy due to the strong triple bond in $$N_2.$$

Question 5

Which of the following best describes the effect of an enzyme on the activation energy and equilibrium of a reaction?

Accepted Answer

An enzyme lowers the activation energy but does not affect the equilibrium constant.

Question 6

The rate constant $$k$$ for a reaction is given by the equation $$k = \frac{k_B T}{h} e$$^{-\Delta G^{\ddagger}$$/RT}. If $$the activation free energy $$\Delta G$$^{\ddagger}$$ is decreased by 10 kJ/mol at 298 K, by what factor does the rate constant increase? (R = 8.314$ J/mol·K)

Accepted Answer

By a factor of $$e^{10,000/(8.314 	imes 298)}$$ \approx 14$$

Question 7

Which of the following is true regarding the rate-limiting step in an enzyme-catalyzed reaction pathway?

Accepted Answer

It is the step with the highest activation energy.

Question 8

Suppose an enzyme binds its substrate with three hydrogen bonds, each contributing $$-20 kJ/$$mol to the binding energy. At $$25^\circC$$ ($$298 K$$), what is the expected rate enhancement due to these hydrogen bonds? ($$R = 8.314 J/$$mol·K)

Accepted Answer

Rate enhancement $$= e$$^{(60,000)/(8.314 	imes 298)}$$ \approx 1.1 	imes 10$$^{11}$$

Question 9

Which of the following best explains why enzymes are highly specific for their substrates?

Accepted Answer

Enzyme active sites have both steric and chemical complementarity to their substrates.

Question 10

A student claims that an enzyme increases the equilibrium constant $$K_{eq} of a $$reaction. How should you respond?

Accepted Answer

The student is incorrect; enzymes do not affect $$K_{eq}$$, only the rate at which equilibrium is reached.

Question 11

Which of the following is a correct statement about the relationship between the forward and reverse rate constants and the equilibrium constant?

Accepted Answer

$$K_{eq} = \frac{k_{forward}}{k_{reverse}}$$

Question 12

Which of the following best describes the role of binding energy in enzyme catalysis?

Accepted Answer

Binding energy is used to lower the activation energy by stabilizing the transition state.

Question 13

Which of the following is a real-world example of a reaction that is thermodynamically favorable but kinetically slow, requiring a catalyst for practical use?

Accepted Answer

The fixation of nitrogen ($$N_2 + 3H_2$$ \rightarrow 2NH_3$$) in the atmosphere.

Question 14

If the activation energy for the reverse reaction is greater than that for the forward reaction, what can be said about the standard free energy change $$\Delta G^\circ$$?

Accepted Answer

$$\Delta G^\circ is $$negative.

Question 15

Which of the following is NOT a property of enzyme active sites?

Accepted Answer

They always contain metal ions as cofactors.

Question 16

The rate constant for a reaction is given by $$k = \frac{k_B T}{h} e$$^{-\Delta G^{\ddagger}$$/RT}. $$Which of the following changes will increase the rate constant $$k$$?

Accepted Answer

Decreasing $$\Delta G$$^{\ddagger}$$.

Question 17

Which of the following best explains why enzymes can catalyze reactions under mild conditions of pH and temperature?

Accepted Answer

Enzymes lower the activation energy, allowing reactions to proceed rapidly without the need for extreme conditions.

Question 18

Which of the following is a correct statement about the effect of an enzyme on the forward and reverse rates of a reaction?

Accepted Answer

An enzyme increases the rates of both the forward and reverse reactions.

Enzymes I: Introduction to Enzymes – Thermodynamics, Kinetics, and Catalysis

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