Energy transfer in reactions:
During a chemical reaction old bonds are broken and new bonds are formed. To do this energy must be supplied to break existing bonds and energy is released when new bonds are formed.
When bonds break it is an endothermic reaction, where heat energy is taken from the surrondings to break the bonds and is shown by a fall in temperature. These are much less common e.g. photosynthesis uses light energy from the sun.
When new bonds are formed this is an exothermic reaction, which is where…