Energy Changes (Topic 5)

When chemical reactions occur, there are always changes in energy.

Energy is moved in Chemical Reactions

• Chemicals store a certain amount of energy
• If products store more energy than original reactants,then they have taken in the difference in energy between products and reactants from surroundings.
• If they store less, then the excess energy is transfered to surroundings during the reaction.
• The overall amount of energy doesn't change. This is due to the fact that energy is conserved in reactions - it can't be created or destroyed. Only moved around.
• This means that the amount of energy in the universe always stays the same.

An exothermic reaction is one which transfers energy to the surroundings, usually by heating. This is presented by a rise in temperature.

• An example of an exothermic reaction is burning fuels - called combustion.
• Neutralisation reactions are also exothermic.
• Many oxidation reactions are exothermic.
• Can been see in everyday items such as: hand warmers and self heating cans.

An endothermic reaction is one in which takes in energy from the surroundings. This is shown by a fall in temperature.

• Endothermic reactions are less common than exothermic reactions.
• An example of this would be thermal decomposition.

Energy Transfer can be measured.

• You can measure the amount of energy released by a chemical reaction by taking the temperature of the reagents, mixing them in a polysyndetic cup and measuring the temperature of the solution at the end of the reaction.
• The issue with energy measurements is the amount of energy lost to the surroundings.
• You can reduce it slightly by putting the cup in a beaker of cotton wool to give it more insulation and putting on a lid to reduce the amount of energy lost to evaporation.
• This method works for neutralisation reactions.

Energy must always be supplied to break bonds.

• During a chemical reaction, old bonds are broken and new bonds are formed.
• Energy must be supplied to break existing bonds - this is an endothermic process.
• Energy is released when new bonds are formed - this is an exothermic process.
• In exothermic reactions the energy released by forming is greater than the energy used to break them. In endothermic reactions the energy released by forming them.

Reaction profiles show energy changes.

• See page 96 of Chemistry revision guide.

Every chemical bond has a particular bond energy.

• This bond energy varies slightly depending on what compound the bond occurs in.
• You ca use well known bond energies to calculate overall energy charge for a reaction. This 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.
• You can't compare the overall energy changes of reaction unless you know the number differences.

See Page 97 in Chemistry revision guide for an example of this being worked out.