Rates of Reaction
The faster molecules collide with each other and the more energy with which they collide, the faster the rate of reaction. Factors such as concentration, pressure, temperature, surface area and addition of a catalyst affect reaction rate.
Collisions
Just placing two different molecules together doesn’t necessarily mean a reaction will happen. In order for particles to react, they need to collide with enough energy. The minimum amount of energy needed for particles to react is called the activation energy.
If particles collide with enough energy they will react to form products - we call this a ‘successful collision’. The more successful collisions there are in a given time, the faster the rate of reaction. There are a number of factors which affect the number of collisions and therefore reaction rate.
Activation energy
There is an ‘energy barrier’ that molecules need to overcome in order to react. This barrier, or threshold, is known as the activation energy. A reaction with a high activation energy will be less likely to happen than one with low activation energy. Adding a catalyst speeds up the rate of reaction by lowering the activation energy. It does this by providing an alternative reaction pathway - a bit like showing you a short-cut from your house to the supermarket when usually you’d go the long way around. We can show how adding a catalyst lowers the activation energy on a reaction profile like the one below:
Factors which affect reaction rate
There are two ways in which the factors below may increase reaction rate:
1. By increasing the frequency of collisions (molecules bump into one another more often)
2. By giving the molecules more energy - this means that a higher number of molecules will have more energy than the activation energy so there will be more successful collisions
Increasing the concentration of a solution or increasing the pressure of a gas means that the reacting particles are closer together (there are more particles in the same volume). The frequency of collisions between reactant particles increases, increasing reaction rate.
Increasing the surface area : volume ratio of a reacting solid means that more reacting particles are exposed at the surface. The frequency of collisions between reactant particles increases, increasing reaction rate.
Increasing the temperature of the reactants causes them to move more quickly and increases the energy of the particles. The frequency of collisions increases and the proportion of successful collisions increases (since more molecules will now have energy that is greater than the activation energy). Therefore reaction rate increases.
Adding a catalyst increases the rate of reaction by providing an alternative pathway with a lower activation energy. Since the ‘energy barrier’ for the reaction to happen has been reduced, a larger number of particles will have sufficient energy to react and there will be more successful collisions. Catalysts are chemically unchanged at the end of the reaction and can be reused.
Experiments to investigate reaction rate
You can measure the rate of reaction by measuring the change in mass of a reactant or product and dividing by the time taken.
Depending on what time of experiment you’re doing, there’s different ways of measuring reaction rate:
For reactions where a solid (a precipitate) is formed from two colourless solutions, you can place the solutions over a piece of paper with a cross drawn on the bottom. You time how long it takes for the cross to disappear as the product forms above it. This method is not very accurate because different people may have a different opinion of when the cross is no longer visible.
If you have a reaction where a gas is given off, you can measure the loss in mass of the reactants. You place the conical flask containing the reactants on a measuring balance and when the mass of the reactants stops decreasing, the reaction will be finished. Record the total decrease in mass over time.
If you have a reaction where a gas is given off, you can also measure the volume of gas produced using a gas syringe. Record the total volume of gas produced over time.
Did you know..
Enzymes are catalysts which are found in living organisms and just like their chemical counterparts they speed up reactions inside our bodies. An enzyme known as phosphatase catalyses a reaction in less than ten milliseconds. Without the enzyme, the uncatalysed reaction would take one trillion years.
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