Kinetics

Question 1

What is the rate equation?

Answer 1

Rate = k[A]^m [B]ⁿ

mol dm^-3 s^-1 = k [mol dm^-3]^m x [mol dm-3]ⁿ

• Rate = mol dm^-3 s^-1
• k = temperature dependant constant; work out units by cancelling out sides. (always has s^-1)
• m and n are the orders of reaction with respect to reactants [A] and [B] (m, n restricted to values 1, 2 or 0)
• overall order = m + n

If the order of reaction with respect to A is 0 (zero), this means that the concentration of A doesn’t affect the rate of reaction.

Question 2

What does this show?

Answer 2

The rate of reaction is proportional to the square of the concentration of A.

This means that if you doubled the concentration of A, the rate would go up 4 times (2²). If you tripled the concentration of A, the rate would increase 9 times (3²).

Question 3

Derive the rate equation for a reaction from data
relating initial rate to the concentrations of the different
reactants.

Answer 3

If a rate is shown to be 9 times faster with all the other concentrations remaining constant and NO (for example) having increased x3; 9 = 3² so the reaction is second order W.R.T. NO.

If a concentration doubles for CO (e.g.) with all other concentrations remaining constant but the rate staying the same, then it is zero order W.R.T. CO.

rate = k[NO]²

Question 4

Explain the qualitative effect of changes in
temperature on the rate constant k.

Answer 4

Small change produces large changes in reaction rates; a much greater proportion of molecules have enough energy to react (having reached E_a) hence the increase in reaction rate with temperature.

Question 5

U.nderstand that the orders of reactions with respect to
reactants can be used to provide information about the rate
determining/limiting step of a reaction

Answer 5

• Step with slower rate = rate-determining step.
• If a reactant appears in the rate equation, it must affect the rate. Hence this or a derivative of must be in the rate-determining step.
• If a reactant doesn’t appear in the rate equation, then it won’t be involved in the rate-determining step.
• The order of a reaction W.R.T. a reactant shows the number of molecules of that reactant that are involved in the rate-determining step.
• Consider what appears in the rate equation, and then match the reactants that appear in a step(s) involved.