These are detailed Revision Notes for Topic Five (Energy Changes) of AQA GCSE Chemistry. They are written by me, using a combination of class notes, text books and revision guides. I have also uploaded the other chapters in my store.
Energy Changes
Exothermic and Endothermic reactions:
In exothermic reactions thermal energy is transferred from the
chemicals to the surroundings and so the temperature increased.
In endothermic reactions thermal energy is transferred from the
surrounding to the chemicals and so the temperature decreases.
Reaction Profiles:
Chemical reaction can only occur when particles collide with
each other with enough energy to react. The minimum energy
particles must have to react is called the activation energy.
Exothermic reactions ~ the products are at a lower energy than the
reactants. The difference in height represents the overall energy
change in the reaction per mole. The initial rise in energy
represents the energy need to start the reaction. This is the
activation energy. The activation energy is the minimum
amount of energy the reactants need to collide with each other and
react. The greater the activation energy, the more energy needed to
start the reaction – this has to be supplied e.g. by heating the reaction
mixture.
Endothermic reaction ~ the products are at a higher energy than the
reactants. The difference in height represents the overall energy
change during the reaction.
Bond Energies:
During a chemical reaction, old bonds are broken, and new bonds are
formed.
Energy must be supplied to break existing bonds – so bonds breaking is
an endothermic process.
Energy is released when new bonds are formed – so bond formation is
an exothermic process.
In exothermic reactions the energy released by forming bonds is
greater than the energy used to break them. In endothermic reactions
the energy used to break bonds is greater than the energy released by
forming them.
Every chemical bond has a particular bond energy associated with it.
The bond energies varied slightly depending on the compounds the
bond occurs in. You can use these known bond energies to calculate
the overall energy change for a reaction. The overall energy change is
the sum of the energies needed to break bonds in the reactants minus
the energy released when the new bonds are formed in the products.
Chemical and fuel cells:
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