1.8. Kinetics

Question 1

In the reaction A + B -> products, if we doubled the concentration of A and kept B the same, and the rate of the reaction doubled. What could we conclude?

Answer 1

That the rate of the equation is directly proportional to the concentration of A. We say that the reaction is first order in relation to A.

Question 2

In the reaction A + B -> products, if we doubled the concentration of B and kept A the same, and the rate of the reaction increased by a factor of 4. What could we conclude?

Answer 2

That the rate of the equation is proportional to the square of B’s concentration. We say that the reaction is second order in relation to B.

Question 3

What is the rate equation?

Answer 3

rate = k[A]^m[B]^n where m and n are the orders of the reaction with respect to their substances.

i.e. the reaction is m order in respect to A and n order in respect to B.

Question 4

How do you calculate the overall order of a reaction?

Answer 4

Overall order = m + n

Question 5

What is k?

Answer 5

The rate constant.

Question 6

How do we work out k and its units?

Answer 6

k = rate / [A]^m[B]^n

To find out k we put in the concentrations and the rate. To find out its units we put in the units of the reactants and the rate.

An example of finding the units would be if
rate = k[A]^1[B]^2 then k = rate/[A]^1[B]^2.
We substitute the units in for each value:

k = mol l^-1 s^-1 / [mol l^-1][mol l^-1]^2

which simplifies to:

k = mol l^-1 s^-1 / (mol l^-1)(mol^2 l^-2)

The mol l^-1 cancel out leaving:

k = s^-1 / mol^2 l^-2 which is the same as mol^-2 l^2 s^-1

Question 7

How can we work out a reaction mechanism from the rate equation?

Answer 7

The order of the reactants tells us how many moles of that reactant is present in the rate-determining step.