The amount of energy in the universe at the end of a chemical reaction is…

The idea

This statement applies energy conservation to the whole universe, treated as a closed system. During a reaction, energy may move between the reacting chemicals and the surroundings, but the total energy of the universe remains constant. For example, in an exothermic reaction, the chemical store of the reactants decreases, while the thermal store of the surroundings increases by the same amount, so the total is unchanged. This idea supports understanding of exothermic and endothermic reactions and reaction profile diagrams, where the energy difference is shown as a transfer to or from the surroundings.

What earns the marks

• States that the total energy of the universe is constant before and after a reaction.
• Explains that energy is transferred between the reaction mixture and the surroundings, not created or destroyed.
• Uses the universe as a closed system to account for all energy changes.
• Applies the idea to a specific reaction, identifying the energy transfer direction.
• Recognises that any apparent energy change in the chemicals is balanced by an opposite change in the surroundings.

Common mistakes and how to put them right

• Believing that energy disappears in an endothermic reaction.

  Put it right by stating energy is transferred from the surroundings to the chemicals.

• Thinking the universe's energy increases when a fuel burns.

  Put it right by explaining energy is redistributed, not added.

• Confusing the system (reactants and products) with the surroundings.

  Put it right by defining the system and surroundings clearly.

Exam tips

• Refer to the universe as a closed system when explaining conservation.
• Use 'energy transferred to the surroundings' for exothermic and 'energy transferred from the surroundings' for endothermic.
• Check that your explanation accounts for both the chemicals and the surroundings.