Energy transfer during exothermic and endothermic reactions: Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a chemical reaction is the same as before the reaction takes place. If a reaction transfers energy to the surroundings the product molecules must have less energy than the reactants, by the amount transferred.

The idea

Chemical reactions rearrange atoms but never create or destroy energy. The total energy of the universe before and after a reaction is identical, so any energy transferred to the surroundings must have come from the reacting system. In an exothermic reaction, the products have less energy than the reactants; the difference is transferred to the surroundings, often as heat, so the surroundings warm up. For example, burning methane transfers energy to the surroundings, and the products carbon dioxide and water have less chemical energy than methane and oxygen. Conversely, in an endothermic reaction, products have more energy than reactants because energy is taken in from the surroundings. Energy diagrams show reactant and product energy levels, with the vertical difference representing the energy transferred.

What earns the marks

• States that energy is conserved in chemical reactions: the total energy of the universe remains constant.
• Explains that if energy is transferred to the surroundings, the products must have less energy than the reactants.
• Uses the idea that the energy transferred equals the difference in energy between reactants and products.
• Applies conservation of energy to a named reaction, such as combustion of methane, identifying that the surroundings gain energy while products have lower energy.
• Interprets an energy level diagram to show that the products are at a lower energy level than the reactants for an exothermic change.
• Distinguishes between energy stored in chemical bonds and energy transferred to the surroundings, avoiding the misconception that energy is used up.

Common mistakes and how to put them right

• Saying energy is 'used up' or 'lost' in a reaction.

  Put it right: Energy is conserved; it is transferred to the surroundings, not destroyed.

• Thinking products always have more energy than reactants.

  Put it right: In an exothermic reaction, products have less energy than reactants by the amount transferred.

• Confusing energy conservation with mass conservation.

  Put it right: Mass is also conserved, but energy conservation refers to total energy before and after the reaction.

• Assuming the surroundings always cool down.

  Put it right: If energy is transferred to the surroundings, the surroundings warm up; cooling occurs when energy is taken in from the surroundings.

Exam tips

• When asked about energy changes, always refer to the total energy of the universe remaining constant and link this to the energy difference between reactants and products.
• Use energy level diagrams to support your answer: label reactants higher than products for an exothermic reaction and state that the gap equals the energy transferred.
• Avoid phrases like 'energy is lost'; instead write 'energy is transferred to the surroundings' to show correct scientific vocabulary.
• If a question gives a temperature change, state whether the surroundings gain or lose energy and relate this to the relative energy of products and reactants.