Chemistry: Reaction Mechanisms and Activation Energy Worksheet

Question 1

In your own words, explain what activation energy is and why it matters in a chemical reaction.

Accepted Answer

Activation energy is the minimum energy that reacting particles must have for a successful reaction to occur. It matters because reactions with lower activation energy usually happen faster at the same temperature.

Question 2

A reaction has the following mechanism: Step 1: A + B -> C, Step 2: C + D -> E. Write the overall reaction and identify any intermediate.

Accepted Answer

The overall reaction is A + B + D -> E. C is an intermediate because it is produced in Step 1 and consumed in Step 2, so it does not appear in the overall reaction.

Question 3

A potential energy diagram shows reactants at 40 kJ, a peak at 110 kJ, and products at 25 kJ. What is the activation energy for the forward reaction, and is the reaction exothermic or endothermic?

Accepted Answer

The activation energy for the forward reaction is 70 kJ because 110 kJ - 40 kJ = 70 kJ. The reaction is exothermic because the products have less energy than the reactants.

Question 4

For the mechanism Step 1: NO2 + NO2 -> NO3 + NO, slow, Step 2: NO3 + CO -> NO2 + CO2, fast, write the overall reaction.

Accepted Answer

The overall reaction is NO2 + CO -> NO + CO2. One NO2 and the NO3 cancel because they are produced and consumed within the mechanism.

Question 5

Using the same mechanism, Step 1: NO2 + NO2 -> NO3 + NO, slow, Step 2: NO3 + CO -> NO2 + CO2, fast, what rate law is predicted by the slow elementary step?

Accepted Answer

The predicted rate law is rate = k[NO2]^2. The slow elementary step controls the rate, and it contains two NO2 molecules as reactants.

Question 6

A catalyst is added to a reaction mixture. Explain how the catalyst changes the reaction mechanism and activation energy.

Accepted Answer

A catalyst provides an alternate reaction pathway with lower activation energy. It changes the mechanism but is not consumed in the overall reaction.

Question 7

In a reaction mechanism, how can you tell the difference between a catalyst and an intermediate?

Accepted Answer

An intermediate is formed in one step and used up in a later step. A catalyst is used in an early step and regenerated in a later step, so it appears at the start and end of the mechanism but not in the overall reaction.

Question 8

A reaction has two steps. Step 1 has an activation energy of 35 kJ and Step 2 has an activation energy of 80 kJ. Which step is most likely the rate-determining step? Explain.

Accepted Answer

Step 2 is most likely the rate-determining step because it has the larger activation energy. The step with the largest energy barrier is usually the slowest step.

Question 9

Classify each elementary step by molecularity: Step 1: A -> products, Step 2: A + B -> products, Step 3: 2A + B -> products.

Accepted Answer

Step 1 is unimolecular because one particle reacts. Step 2 is bimolecular because two particles collide. Step 3 is termolecular because three particles are involved in the elementary step.

Question 10

A reaction has reactants at 60 kJ, products at 100 kJ, and a transition state at 150 kJ. Calculate the forward activation energy, reverse activation energy, and enthalpy change.

Accepted Answer

The forward activation energy is 90 kJ because 150 kJ - 60 kJ = 90 kJ. The reverse activation energy is 50 kJ because 150 kJ - 100 kJ = 50 kJ. The enthalpy change is +40 kJ because 100 kJ - 60 kJ = +40 kJ.

Question 11

Explain why increasing temperature usually increases the rate of a chemical reaction.

Accepted Answer

Increasing temperature gives particles more kinetic energy. More particles then have enough energy to overcome the activation energy barrier, so successful collisions happen more often.

Question 12

For the elementary reaction H2 + I2 -> 2HI, write the rate law for this single-step process.

Accepted Answer

The rate law is rate = k[H2][I2]. Since the reaction is elementary, the reactant coefficients can be used as the exponents in the rate law.

Question 13

A student says, "A catalyst increases the amount of product formed at equilibrium because it lowers activation energy." Explain what is correct and what is incorrect in this statement.

Accepted Answer

It is correct that a catalyst lowers activation energy. It is incorrect that a catalyst increases the amount of product at equilibrium, because a catalyst speeds up both the forward and reverse reactions and does not change the equilibrium position.

Question 14

Consider the mechanism Step 1: X + Y -> Z, fast, Step 2: Z + Y -> W, slow. Why might the experimentally measured rate law not be simply rate = k[Z][Y]?

Accepted Answer

The measured rate law usually must be written in terms of reactants in the overall reaction, not intermediates such as Z. Since Z is an intermediate, its concentration may need to be related to X and Y using the earlier fast step.

Question 15

A reaction coordinate diagram has three peaks and two valleys between the reactants and products. What do the peaks and valleys represent in a multi-step mechanism?

Accepted Answer

The three peaks represent transition states for three elementary steps. The two valleys between them represent reaction intermediates formed after one step and consumed in a later step.

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Chemistry: Reaction Mechanisms and Activation Energy

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