Rates Of Reaction
Chemical Reactions only occur when reacting particles collide with each other with the sufficient energy.
The minimum amount of energy rquired to cause a reaction is called the activation energy.
There are four important factors that affect the rate of reaction:
- Surface Area
- Use of a Catalyst
In a cold reaction the mixture the particles move quite slowly. They collide less often, with less energy, so fewer collisions are successful.
In a hot reaction mixture the particles move more quickly. They collide more often, with greater energy, so more collsions are successful.
In a low concentration reaction, the particles are spread out. They collide less often, so there are fewer successful collisions.
In a high concentration reaction, the particles are crowded close together. They collide more often, so there are more successful collisions.
Increasing the pressure of reacting gases also increases the frequency of collisions.
Concentrations of solutions are given in moles per cubic decimtre (mol/dm)
Equal volumes of solutions of the same molar concentration contain the same number of moles of solute, i.e the same number of particles.
Equal volumes of gases at the same temperature and pressure contain the same number of particles.
Large pieces of a solid reactant have a smaller surface area in relation to their volume.
Fewer particles are exposed and avaliable for collisions. This results in fewer collisions and a slower reaction.
Small pieces of a solid reactant have a large surface area in relation to their volume, so more particles are exposed and available for collisions.
This means more collisions and a faster reaction.
Using a Catalyst
A catalyst is a substance that changes the rate of a chemical reaction without being used up or altered in the process. A catalyst:
- reduces the amount of energy needed for a successful collision.
- makes more collsions successful.
- speeds up the reaction.
- provides a surface for the moleculs to attach to, which increase thier chances of a collsion.
Different reactions need different catalysts. For example...
- the cracking of hydrocarbons uses broken pottery.
- the manufacture of ammonia uses iron.
Increasing the rates of chemical reactions is important in industry because it helps to reduce costs.
Rate of Reaction= Amount of reactant used OR product formed / Time
The rate of a chemical reaction can be found in two ways:
1. Measuring the amount of reactants used. If one of the products is gas, you could weigh the reaction mixture before and after the reaction takes place. The mass of the mixture will decrease.
2. Measuring the amount of products formed. You could use a gas syringe to measure the total volume of gas produced at timed intervals.
Plotting Reaction Rates.
Graphs can be plotted to show progress of a chemical reaction. There are three things to remember:
1. The steeper the line the faster the reaction
2. When one of the reactants is used up the reaction stops (the line becomes horizontal)
3. The same amount of product is formed from the same amount of reactants, irrespective of rates.