C5.09 Temperature changes in reactions
Many 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.
C5.10 Exothermic reactions
An 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.
C5.11 Endothermic reactions
An 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.
C5.12 Bond breaking and making in reactions
During 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
exothermic
endothermic