Remember order of reaction and rate of reaction in A-Level Chemistry? Let's learn how to know the initial rate of reaction:
with a reaction involving reactants A & B:
Respect to A:
· Measure the rate of reaction by timing how long it takes for a measurable change to occur.
· Repeat the experiment changing the initial concentration of A but keeping the initial concentration of B constant.
As can be seen, when the [A] doubles, the rate doubles, therefore a change in the rate is proportional to the change in concentration. Thus in the rate equation A is first order.
Respect to B:
· Measure the rate of reaction by timing how long it takes for a measurable change to occur.
· Repeat the experiment changingthe initial concentration of B but keeping the initial concentration of A constant.
When [B] is doubled, the rate quadruples.This change in rate is proportional to the change in the concentration squared and B is second order.
Hence the rate equation when A & B react together is:
Rate= k[A][B]2
Examples of measurable change to measure rate of reaction:
- gas syringe method (collect and measure gas produced)
- change in mass (gas produced escaped)
- precipitation
Units of rate constant:
given the information that rate equation is Rate= k[A][B]2 and the following data:
the unit of k:
mol dm-3s-1= k[mol dm-3] [ mol dm-3]2
k= mol dm-3 s-1 / [mol dm-3] [ mol dm-3]2
k =mol-2dm6s-1
Value of k:
0.18 = k(0.5)(3.0)2
k =0.04
Hence the units of k depend on the overall order of reaction
· The rate of reaction will always increase when temperature increases.
· This is best illustrated usinga Maxwell-Boltzmann distribution.
At higher temperatures, more molecules will have energy greater than or equal to the activation energy, which results in more successful collisions.
If the concentrations of reactants are kept constant as temperature increases, k must increase as the rate increases. The increase of k with temperature is exponential.
Drafted by Eunice (Chemistry)