Chemistry: Kinetics: Reaction Rates and Activation Energy
Calculating reaction rates and explaining energy barriers
Chemistry: Kinetics: Reaction Rates and Activation Energy
Calculating reaction rates and explaining energy barriers
Chemistry - Grade 9-12
- 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.
Use rate = change in concentration divided by change in time.
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. - 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.
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. - 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.
Activation energy is the energy of the peak minus the energy of the reactants.
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. - 4
Explain how a catalyst changes the rate of a chemical reaction.
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. - 5
List three factors that can increase the rate of a chemical reaction and briefly explain why each one works.
Think about what makes collisions more frequent or more effective.
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. - 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?
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. - 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.
Use a positive value when reporting the rate of disappearance.
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. - 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.
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. - 9
Explain why increasing temperature usually increases the rate of a reaction.
Temperature affects both particle speed and the energy of collisions.
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. - 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.
A factor of 2 in concentration causing a factor of 4 in rate means second order.
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. - 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.
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. - 12
Food spoils more slowly in a refrigerator than at room temperature. Use reaction kinetics to explain why.
Connect lower temperature to particle motion and successful collisions.
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.