Reaction complexity Activation energy The state of the reactants The temperature Catalysts Summary - Rate of Reaction

In the first part of Rates of chemical reactions - mechanisms, five factors which affect the speed of a chemical reaction were identified:

1. Reaction complexity.
   
2. Activation energy of the reaction - energy required for the reaction to occur.
   
3. The state of the reactants, i.e. solid, liquid, solution or gas.
   
4. The temperature at which the reaction occurs.
   
5. The presence of a catalyst or enzyme.

Reaction complexity
The more complex the reaction, the slower it tends be. If the reaction involves large molecules and a complex series of transformations, like the formation of protein in a cell, then the reaction tends to be slow. If the reaction is a simple one, involving the collision of two atoms or small molecules and their joining to form a new substance, then the reaction tends to be quick.

The following animation shows a hydrogen atom and chlorine atom combining to form HCl. Although very much simplified compared to the most reactions, this shows the simplist possible type of interaction - two atoms colliding to form a molecule.

Formation of HCL

Activation energy
To allow particles to react with one another, the reacting particles must collide with enough energy to overcome any initial repulsion between themselves and join together. The activation energy is the energy required to overcome this initial repulsion.

Activation energy is quite often depicted as a hill, with only the fastest moving particles able to get over the hill and react. Those with too little energy, simply bounce off each other! The higher the activation energy hill the slower the reaction.

Activation energy hill

The state of the reactants
Put simply, the more movement the reactant particles have, the faster they will react. Gases tend to react faster than liquids and liquids react much faster than solids. Liquid petrol is vaporised before exploding in an internal combustion engine. If you look carefully at a burning candle you will see that the fastest reaction (the burning) occurs around the wick, where the solid wax has been melted to form a liquid and then vaporises before burning.

The temperature
It is true for all chemical reactions that the higher the temperature, the faster they react. The hotter the surroundings of the reaction, the faster the particles will be moving and the more energy they will have and the more particles will collide with enough energy to get over the activation energy hill and react. This all adds up to faster reactions.

Catalysts
Catalysts are substances which speed up chemical reactions without being used up in the reaction themselves. Catalysts work by helping the reactant particles come together in the right orientation for reaction and by lowering the energy required for the reaction to occur. An enzyme is an example of a biological catalyst.

Platinum is an excellent catalyst for many gaseous reactions. Platinum wire placed in a mixture of hydrogen and oxygen gas will make it explode, a dramatic increase in the rate of reaction!

Catalytic reactions

Summary - Rate of reaction

Slow		Fast
complex molecules	liquids	simple ions, atoms and molecules
high activation energy		low activation energy
solids		gases
cold		hot
		catalysts and enzymes

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