Reaction profile diagrams
There are two simple ways of showing the energy changes that take place in a chemical reaction:
- an Chart showing the energy in the reactants and products, and the difference in energy between them.
- a Chart showing how the energy of reactants and products changes during a reaction.
Reaction profiles
A reaction profile shows how the The capacity of a system to do work or the quantity required for mechanical work to take place. Measured in joules (J). For example, a man transfers 100 J of energy when moving a wheelbarrow. of the A substance that reacts together with another substance to form products during a chemical reaction. and A substance formed in a chemical reaction. changes during a reaction. It includes the The minimum amount of energy that colliding particles must have for them to react. - the minimum energy needed for a reaction to start. The activation energy is shown as a 'hump' in the line which:
- starts at the energy of the reactants
- is equal to the difference in energy between the top of the 'hump' and the reactants
The overall change in energy in a reaction is the difference between the energy of the reactants and products.
Exothermic reactions
The diagram shows a reaction profile for an Reaction in which energy is given out to the surroundings. The surroundings then have more energy than they started with so the temperature increases. reaction.
Question
Describe how you can see from a reaction profile that a reaction is exothermic.
In an exothermic reaction, the energy change is negative. This is because the products have less energy than the reactants and the arrow showing the overall change in energy points downwards.
Endothermic reactions
The diagram shows a reaction profile for an Reaction in which energy is taken in. reaction.
Question
Describe how you can see from a reaction profile that a reaction is endothermic.
In an endothermic reaction, the energy change is positive. This is because the products have more energy than the reactants and the arrow showing the overall change in energy points upwards.
The effect of catalysts
A A substance that changes the rate of a chemical reaction without being changed by the reaction itself. is a substance that:
- speeds up the rate of a reaction
- does not alter the products of the reaction
- is unchanged chemically and in The amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). at the end of the reaction
A catalyst allows an alternative The sequence of reactions needed to produce a desired product from a particular set of raw materials. that has a lower activation energy than the uncatalysed reaction. This is seen in the reaction profile as a lower 'hump' representing the activation energy.