Energy changes


The Big Idea – Chemical Reactions involve rearrangement of atoms in substances to form new substances.

The energy of a reaction or its enthalpy is a measure of the energy (heat) of reactions. We know some reactions get hot and release heat to the surroundings (Exothermic) and some reactions absorb heat and the surrounding get cold (Endothermic). This heat comes from making and breaking bonds as the atoms rearrange themselves into new compounds. Making bonds releases heat energy (exothermic) and breaking bonds requires heat energy (endothermic). The balance of these 2 results in an overall endothermic or exothermic reactions.

The Essentials

You must know how to determine if a reaction is endothermic or exothermic from experimental results and how to draw the corresponding reaction profile. This topic includes a REQUIRED practical, so learn how do measure energy of a reaction and how to improve results (for all reactions involving heat, improvements involve insulation of equipment to minimise heat losses)

100% Energy of reactions

100% Practical Energy Changes

Deeper Understanding

For the higher paper you must understand reactions as the breaking and the making of bonds. Each bond has a specific energy. Energy is needed to break bonds and is released when new bonds are made. The overall balance of this breaking and making determines if a reaction is endothermic or exothermic.

Use the equation break – make. If the answer is negative it is an exothermic reaction, if its positive it is an endothermic reaction

Other Links

BBC Bitesize – energy of reactions

MyGCSE – Energy of reactions

Review and Rate your Understanding

After revising the topic and completing the questions of the 100% sheet try the end of topic summary questions (answers given) and then print and complete the checklist to rate your understanding.



Have you learnt all the facts on the 100% sheet?

Have you completed the BBC Bitesize tutorial?

Have you been able to complete all the questions on the 100% sheet?

Let us know how you feel about this topic in the comments section below. Any questions you have, just ask.

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