Endothermic & Exothermic Reactions (CIE IGCSE Chemistry)

Heat Exchange in Reactions

• Chemical reactions occur so that elements can achieve a more stable energy state by gaining a full outer shell of electrons
• This is done by chemical bonding (we have already seen ionic and covalent bonding) where old bonds are broken, and new bonds are formed)
• This process involves the transfer of thermal energy into and out of reaction mixtures
• The terms used to describe this are the system (what happens in the chemical reaction) and the surroundings (anything other than the chemicals reacting)
• The energy within the system comes from the chemical bonds themselves which could be considered as tiny stores of chemical energy

Exothermic Reactions

• In exothermic reactions thermal energy is transferred to the surroundings so the temperature of the surroundings increases
• This energy is transferred from the chemical energy store of the chemical system to the surroundings and so the energy of the system falls - this means that the energy change is negative
• The overall transfer is from the system to the surroundings
• Combustion, oxidation, and neutralisation reactions are typical exothermic reactions
• Hand warmers used in the wintertime are based on the release of heat from an exothermic reaction
• Self-heating cans of food and drinks such as coffee and hot chocolate also use exothermic reactions in the bases of the containers

Diagram showing the transfer of heat energy outwards from an exothermic reaction

Endothermic Reactions

• In endothermic reactions thermal energy is taken in from the surroundings so the temperature of the surroundings decreases
• This energy is transferred to the chemical energy store of the chemical system and so the energy of the system increases - this means the energy change is positive
• The overall transfer is from the surroundings to the system
• These types of reactions are much less common than the exothermic reactions
• Electrolysis, thermal decomposition reactions and the first stages of photosynthesis are typical endothermic reactions
• Sports injury treatments often use cold packs based on endothermic reactions to take heat away from a recently injured area to prevent swelling

Diagram showing the transfer of heat energy from the surroundings into an endothermic reaction

Reaction Pathway Diagrams

• Reaction pathway diagrams (a.k.a energy level diagrams) are graphical representations of the relative energies of the reactants and products in chemical reactions
• The energy of the reactants and products are displayed on the y-axis and the reaction pathway (a bit like time) is shown on the x-axis
• The difference in height between the energy of reactants and products represents the overall energy change of a reaction.
  • This is usually a sketch but can be drawn to scale if data is provided
• Arrows on the diagrams indicate whether the reaction is exothermic (overall reaction arrow is downwards pointing, showing that the system has lost energy) or endothermic (overall reaction arrow is upwards pointing, showing that the system has gained energy)
• The initial increase in energy represents the activation energy (Ea), which is the minimum energy that colliding particles must have in order to react
• The greater the initial rise, the more energy that is required to get the reaction going e.g. more heat needed
• You can identify a reaction pathway diagram for an exothermic reaction as the energy of the product is lower than the reactants (as thermal energy has been transferred to the surroundings) 
• You can identify a reaction pathway diagram for an endothermic reaction as the energy of the product is higher than the reactants (as thermal energy has been taken in from the surroundings) 

Reaction pathway diagram of an exothermic reaction and an endothermic reaction