Reaction profiles
For a chemical reaction to occur between two particles, they must collide with enough energy. The minimum amount of energy needed by particles when they collide in order to cause a reaction is called the The minimum amount of energy that colliding particles must have for them to react..
An Chart showing the energy in the reactants and products, and the difference in energy between them. shows whether a reaction is Reaction in which energy is given out to the surroundings. The surroundings then have more energy than they started with so the temperature increases. or Reaction in which energy is taken in.. It shows 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., and the difference in energy between them.
Exothermic reaction
The energy level decreases in an exothermic reaction. This is because energy is given out to the surroundings.
Endothermic reaction
The energy level increases in an endothermic reaction. This is because energy is taken in from the surroundings.
It is usually more helpful to describe how the energy of the chemicals changes during the reaction, so a Chart showing how the energy of reactants and products changes during a reaction. is more useful than an energy level diagram.
A reaction profile includes the activation energy, which is the minimum energy needed by particles when they collide for a reaction to occur. 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 reactant
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 exothermic reaction.
Question
Describe how you can tell from a reaction profile that a reaction is exothermic.
In the profile 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 endothermic reaction.
Question
Describe how you can tell from a reaction profile that a reaction is endothermic.
In the profile 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.
A study of the energy changes involved in chemical reactions, whether exothermic or endothermic
