Activation energy
When a fuel burns, a spark is needed to start the reaction. A minimum amount of 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. is needed in order for a fuel to burn.
The minimum amount of energy needed for a reaction to take place is called the The minimum energy required for a chemical reaction to take place..
High and low activation energy
In order for two substances to react, their A general term for a small piece of matter. For example, protons, neutrons, electrons, atoms, ions or molecules. must collide with enough energy. The particles in a substance have a range of different energies.
- A low activation energy means that a lot of the particles will collide with enough energy to react. The reaction will be fast.
- A high activation energy means that far fewer particles will collide with enough energy. The reaction will be slow.
Reaction profiles
A Chart showing how the energy of reactants and products changes during a reaction. shows how the energy in 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 activation energy.
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 a reaction profile shows that a reaction is exothermic.
In the reaction for an exothermic reaction, the overall change is negative. You can tell this 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 a reaction profile shows that a reaction is endothermic.
In the reaction for an endothermic reaction, the overall change is positive. You can tell this 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.