Chemistry: Kinetics: Reaction Rates and Activation Energy Worksheet

Question 1

A reactant has a concentration of 0.80 M at 0 seconds and 0.20 M at 30 seconds. Calculate the average rate of disappearance of the reactant.

Accepted Answer

The average rate of disappearance is 0.020 M/s. This is found by subtracting 0.20 M from 0.80 M to get 0.60 M, then dividing by 30 s.

Question 2

A product concentration increases from 0.00 M to 0.45 M in 15 seconds. Calculate the average rate of formation of the product.

Accepted Answer

The average rate of formation is 0.030 M/s. The concentration changes by 0.45 M over 15 s, so 0.45 M divided by 15 s equals 0.030 M/s.

Question 3

In an energy diagram, the reactants are at 40 kJ, the peak of the curve is at 125 kJ, and the products are at 70 kJ. Find the activation energy for the forward reaction and state whether the reaction is endothermic or exothermic.

Accepted Answer

The activation energy for the forward reaction is 85 kJ because 125 kJ minus 40 kJ equals 85 kJ. The reaction is endothermic because the products are higher in energy than the reactants by 30 kJ.

Question 4

Explain how a catalyst changes the rate of a chemical reaction.

Accepted Answer

A catalyst increases the reaction rate by providing an alternate pathway with a lower activation energy. The catalyst is not used up during the reaction and does not change the overall energy difference between reactants and products.

Question 5

List three factors that can increase the rate of a chemical reaction and briefly explain why each one works.

Accepted Answer

Increasing temperature can increase reaction rate because particles move faster and collide with more energy. Increasing concentration can increase reaction rate because more particles are available to collide. Increasing surface area can increase reaction rate because more reactant particles are exposed for collisions.

Question 6

A solid chunk of calcium carbonate reacts with hydrochloric acid. The same mass of powdered calcium carbonate reacts with the same acid. Which form reacts faster, and why?

Accepted Answer

The powdered calcium carbonate reacts faster because it has a greater surface area. More particles are exposed to the acid, so there are more frequent collisions between reactant particles.

Question 7

The concentration of reactant A is 0.100 M at 0 seconds and 0.060 M at 20 seconds. Calculate the average rate of disappearance of A during this time interval.

Accepted Answer

The average rate of disappearance of A is 0.0020 M/s. The concentration decreases by 0.040 M, and 0.040 M divided by 20 s equals 0.0020 M/s.

Question 8

Reaction 1 has an activation energy of 35 kJ/mol. Reaction 2 has an activation energy of 85 kJ/mol. If both reactions are run at the same temperature and have similar collision frequencies, which reaction is faster? Explain your reasoning.

Accepted Answer

Reaction 1 is faster because it has the lower activation energy. A larger fraction of colliding particles will have enough energy to react when the activation energy is lower.

Question 9

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

Accepted Answer

Increasing temperature gives particles more kinetic energy. The particles move faster, collide more often, and a greater fraction of collisions have enough energy to overcome the activation energy.

Question 10

Initial rate data show that doubling the concentration of A doubles the reaction rate, while doubling the concentration of B makes the reaction rate four times larger. Write the rate law for the reaction in terms of A and B.

Accepted Answer

The rate law is rate = k[A][B]^2. The reaction is first order in A because doubling A doubles the rate, and second order in B because doubling B quadruples the rate.

Question 11

For a reaction with rate law rate = k[A]^2, the rate has units of M/s and concentration has units of M. Determine the units of the rate constant k.

Accepted Answer

The units of k are 1/(M s), or M^-1 s^-1. Since rate = k[A]^2, k equals rate divided by M^2, so (M/s)/M^2 = M^-1 s^-1.

Question 12

Food spoils more slowly in a refrigerator than at room temperature. Use reaction kinetics to explain why.

Accepted Answer

Food spoils more slowly in a refrigerator because the lower temperature reduces the rates of chemical reactions and biological processes. Particles have less kinetic energy, so fewer collisions have enough energy to overcome activation energy.

Chemistry: Kinetics: Reaction Rates and Activation Energy

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