Energy is conserved in chemical reactions.

The idea

Energy conservation means the total energy of a closed system stays constant during any chemical change. In a reaction, bonds in reactants break, absorbing energy, and new bonds in products form, releasing energy. The difference between these energy changes appears as energy transferred to or from the surroundings, often as heat. For example, burning methane releases energy to the surroundings, but the total energy of the universe remains unchanged; the chemical store decreases while the thermal store of the surroundings increases by the same amount. This principle underpins exothermic and endothermic definitions and reaction profile diagrams.

What earns the marks

• States that energy cannot be created or destroyed in a chemical reaction.
• Explains that energy is transferred between the chemical store of reactants/products and the thermal store of the surroundings.
• Uses the idea that bond breaking absorbs energy and bond making releases energy.
• Applies conservation to a named reaction, identifying where energy is transferred.
• Recognises that the total energy before and after a reaction is equal.

Common mistakes and how to put them right

• Thinking energy is used up in exothermic reactions.

  Put it right by stating energy is transferred to the surroundings, not destroyed.

• Assuming energy can be lost completely.

  Put it right by explaining it is transferred to other stores, such as thermal energy in the surroundings.

• Confusing energy conservation with mass conservation.

  Put it right by distinguishing energy stores from atoms, which are also conserved but separately.

Exam tips

• Use the phrase 'energy is conserved' and refer to energy transfer to or from the surroundings.
• When explaining a reaction, name the energy store that decreases and the one that increases.
• Avoid saying energy is 'produced' or 'used up'; say 'transferred' or 'released to the surroundings'.