Table of Contents


AQA | Unit 1 | Chemistry 1

Page 1 | Atoms, periodic table, chemical reactions

Page 2 | Limestone and Building Materials

Page 3 | Metal and their uses

Page 4 | Crude oil and fuels

Page 5 | Other useful substances from crude oil

Page 6 | Plant oils and their uses

Page 7 | Changes in the earth and its atmosphere

AQA | Unit 2 | Chemistry 2

Page 1 | Structure and Bonding 

 Page 2  | Atomic structure, analysis and quantitative chemistry

Page 3 | Rates of Reactions

Page 4 | Exothermic and Endothermic Reaction

Page 5 | Acids, Bases and Salts

Page 6 | Electrolysis

AQA | Unit 3 | Chemistry 3

Page 1 | The periodic table

Page 2 | Water

Page 3 | Calculating and explaining energy change

Page 4 | Further analysis and quantitative chemistry

Page 5 | The production of ammonia

Page 6 | Alcohols, carboxylic acids and esters


Exothermic and endothermic reaction

Learning objectives:

To learn about energy changes in exothermic and endothermic reactions how the arrangement of electrons 


Exothermic reactions

When a chemical reaction occurs, energy s either taken in or given out

Exothermic reactions transfer energy to the surrounding in the form of heat. This is usually indicated by a rise in temperature on a thermometer.
Most common exothermic reactions are combustion, oxidation or neutralization reactions
In terms of bonds, energy is releases when new bonds form, bond making is an exothermic process.


Endothermic reactions

These reactions will take in energy from the surroundings, therefore when measuring with a thermometer the temperature will go down. Examples of endothermic reactions are electrolysis and thermal decomposition reactions
In terms of bonds, energy is absorbed to break bonds so bond breaking is an exothermic process.


Energy diagrams

These diagrams show the different levels of energy in the reactants and the products. The bigger the difference the bigger the energy change. Energy diagrams help to illustrate whether a reaction is endothermic or exothermic

This is illustrated below

Energy diagrams


enthalpy change

In an endothermic reaction, energy is absorbed so the products will always have more energy than the reactants, therefore the H is always positive.


Bond energy calculations

You can calculate the bond energy by measuring the total energy of the reactants and the total energy in the products; to calculate the change in energy you simply follow this equation

Worked example

H –H +Cl-Cl 2 x (H-Cl)

BondBond Energy




Energy of reactants = 436 + 243 = 679
Energy of products = 2 x 432 = 864

Energy change = reactants – products = 679 – 864 = -185

This shows that the reaction was exothermic


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