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up to 4 minutes only!

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GCSE Energy changes in reactions. Exothermic. Endothermic. Breaking / making bonds.

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Includes calculations and energy level diagrams.

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GCSE CHEMISTRY - What are Energy Level Diagrams? - What is the Energy Level Diagram for an Exothermic Reaction? - GCSE SCIENCE.

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The energy level diagram shows the change in energy as reactants turn into products. The difference in energy is given the name delta H.

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Simple energy level diagrams only show the energy levels at the beginning and end of a reaction. Energy levels change gradually during a reaction, and this can be shown using a curve between the reactant and product energy levels.

This is an exothermic reaction because the energy level of the reactants is higher than the energy level of the products.

However, the energy curve goes up from the reactants' energy level to begin with, then drops to the products' energy level. This is because many reactions need an input of energy to start the reaction off. This is energy is called the activation energy. It is represented on an energy level diagram as the difference between the reactants' energy level and the top of the curve.

For example, burning methane in a Bunsen burner:

methane + oxygen → carbon dioxide + water

The activation energy must be supplied in the form of a flame or a spark to get the methane to ignite. Once the reaction begins, it gives out energy to the surroundings so it is exothermic.

Catalysts

A catalyst is a substance that speeds up the rate of a chemical reaction without being used up in the reaction.

Catalysts can do this because they provide a different pathway for the reaction to follow. This pathway has lower activation energy than the one followed by the uncatalysed reaction. As a result, a greater proportion of reacting particles have enough energy to react.

The energy level diagram below shows the effect of a catalyst in lowering activation energy.

Lowering the activation energy has many advantages. It means that reactions happen more quickly and are more economical in terms of the energy required for industrial-scale reactions.

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BBC - GCSE Bitesize Science - Energy from reactions : Revision, Page 5

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Simple energy level diagrams only show the energy levels at the beginning and end of a reaction. Energy levels change gradually during a reaction, and this can be shown using a curve between the reactant and product energy levels. This is an exothermic reaction because the energy level of the reactants is higher than the energy level of the products.

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During a chemical reaction:

Energy is needed to break bonds, and energy is released when bonds are made.

Exothermic reactions

Exothermic reactions give out heat energy to the surroundings. Exothermic reactions have a negative energy change. This is shown in the energy level diagram below.

Some examples of exothermic reactions are:

Endothermic reactions

Endothermic reactions absorb heat energy from the surroundings, making the temperature of the surroundings cooler. Endothermic reactions have a positive energy change. This is shown in the energy level diagram below.

Some examples of endothermic reactions are:

Bond breaking and making - Higher tier

In an exothermic reaction, more energy is released when new bonds are made than is needed to break existing bonds.

In an endothermic reaction, more energy is needed to break existing bonds than is released when new bonds are made.

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BBC - GCSE Bitesize Science - Energy from reactions : Revision, Page 4

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During a chemical reaction: Energy is needed to break bonds, and energy is released when bonds are made. Exothermic reactions give out heat energy to the surroundings. Exothermic reactions have a negative energy change. This is shown in the energy level diagram below.