3.2.1 Exothermic & Endothermic Reactions

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 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
• The energy comes from the chemical bonds themselves which could be considered as tiny stores of chemical energy
• In the majority of reactions, the energy is in the form of heat energy, although sometimes other types of energy are produced such as light or sound

Exothermic Reactions

• In exothermic reactions energy is given out to the surroundings so the temperature of the surroundings increases
  
  • This energy is transferred from the chemical energy store of the chemical system and so the energy of the system falls
  • 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 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
  • 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 treatment 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

Measuring Energy Transfer

• The changes in heat content can be determined and measured with a thermometer or temperature probe
• Note that the overall amount of energy does not change as energy is conserved in reactions
• This means that it cannot be created or destroyed but it can only be transferred
• So, if energy is transferred to the surroundings during a chemical reaction, then the products formed must have less energy than the reactants by the same amount as that transferred

Activation Energy

• For atoms or particles to react with each other in a chemical system they must first of all come into contact with each other in a collision
• A number of factors come into play when analysing collisions such as energy, orientation, and number of collisions per second (the frequency of collisions)
• In terms of the energy of the collision, there is a minimum amount of energy required for the collision to be successful, that is for the particles to react together
• This minimum amount of energy is called the activation energy (E^a)
• Different reactions have different activation energies, depending on the chemical identities involved
• Reactions which have higher activation energies require more energy to start than those with lower activation energies

Reaction Profiles

• Reaction profiles (sometimes called 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.
• Arrows on the diagrams indicate whether the reaction is exothermic (downwards pointing) or endothermic (upwards pointing)
• The initial increase in energy represents the activation energy required to start the reaction
• The greater the initial rise then the more energy that is required to get the reaction going e.g., more heat needed

The energy level diagram for exothermic reactions. The overall energy change is negative because the system (reactants) loses energy to the surroundings.

The energy level diagram for endothermic reactions. The system (reactant substances) have gained energy so the overall energy change is positive.

• Energy is given out in exothermic reactions
  
  • The energy of the products will be lower than the energy of the reactants, so the change in energy is negative
  • This is represented on the reaction profile with a downwards-arrow as the energy of the products is lower than the reactants
• Energy is taken in endothermic reactions
  
  • The energy of the products will be higher than the energy of the reactants, so the change in energy is positive
  • This is represented on the reaction profile with an upwards-arrow as the energy of the products is higher than the reactants