Exothermic and Endothermic Reactions
Ever noticed how some chemical reactions get hot? These are exothermic reactions which release thermal energy into their surroundings, causing temperature increases. Common examples include combustion (burning fuels), respiration in your cells, neutralisation reactions, and when metals react with acids.
In contrast, endothermic reactions absorb energy from their surroundings, making them feel cold. These include thermal decomposition (like limestone in a blast furnace), photosynthesis in plants, and certain types of electrolysis. The fizzing sensation in sherbet occurs due to an endothermic reaction!
The energy changes in reactions can be understood through bond energies - the energy needed to break bonds between atoms. When bonds break, energy is absorbed (endothermic process), and when new bonds form, energy is released (exothermic process). The overall energy change depends on which process requires more energy.
Remember this! In an exothermic reaction, more energy is released when forming new bonds than is absorbed when breaking existing bonds. The opposite is true for endothermic reactions.
Chemists visualize these energy changes using reaction profiles, which show the energy pathway from reactants to products. All reactions require some initial energy to get started - the activation energy - even if they ultimately release energy overall. This explains why even highly exothermic reactions like combustion need a spark to begin.
