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^{-3}s^{-1}= k[mol dm^{-3}] [ mol dm^{-3}]^{2}

k= mol dm^{-3 }s^{-1} / [mol dm^{-3}] [ mol dm^{-3}]^{2}

k =mol^{-2}dm^{6}s^{-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)