Rates of Reaction
A chemical reaction can only occur between particles
when they collide (hit each other).
Particles may be atoms, ions or molecules.
There is a minimum amount of energy
which colliding particles need in order to react with each other.
If the colliding particles have less than this minimum energy
then they just bounce off each other and no reaction occurs.
This minimum energy is called the activation energy.
The faster the particles are going, the more energy they have.
Fast moving particles are more likely to react when they collide.
You can make particles move more quickly by heating them up
(raising the temperature).
Changing the Rate of a Reaction.
There are 5 ways to increase the rate of a chemical reaction.
They are all understood in terms of collision theory.
The rate of a chemical reaction may be increased by
1) Raising the temperature.
2) Increasing the concentration (in solution).
3) Increasing the pressure (in gases).
4) Increasing the surface area of a solid.
5) Use a catalyst.
Measuring the Rate.
The reaction between sodium thiosulphate solution
and dilute hydrochloric acid.
HCl + sodium thiosulphatearrowsodium chloride + sulphur dioxide +
sulphur + water.
HCl(aq) + Na2S2O3(aq) arrow NaCl(aq) + SO2(g) + S(s) + H2O(l)
The solid sulphur (S(s)) formed in this reaction
makes the colorless solution go cloudy.
The reaction is usually carried out in a flask
placed on a piece of white paper which has a black cross on it.
At the beginning of the reaction, the cross can be seen easily.
As the flask becomes more and more cloudy
the cross gets harder to see.
You can measure the time from the start of the reaction
until the cross can no longer be seen.
This is a way of measuring the rate of formation of sulphur.
The reaction between magnesium and dilute acid...