energetics- IGCSE / GCSE

Energy changes accompanying reactions

Temperature changes in reactionsMany reactions give off heat such as the burning of wood which causes a temperature increase. Other reactions take in energy and cause a temperature fall.

Exothermic reactionsAn Exothermic reaction is one which gives out energy to the surroundings, usually in the form of heat and usually shown by a rise in temperature. An example of an exothermic process is the burning of fuels.

Endothermic reactionsAn Endothermic reaction is one which takes in energy from the surroundings, usually in the form of heat and usually shown by a fall in temperature. An example of an endothermic is photosynthesis. This is because it takes in energy from the sun in the form of light.

Bond breaking and making in reactionsDuring a chemical reaction, old bonds are broken and new ones formed. Energy must be supplied to break existing bonds and so this is endothermic. Energy is released when new bonds are formed and so the formation of bonds is exothermic.Hydrogen reacts with oxygen to form water.

O=O + H-H + H-H --->O +O + H +H + H +H --->H-O-H + H-O-H Old bonds break single atoms with new bonds form in oxygen and no bonds in water molecules hydrogen molecules exothermicendothermic