EXOTHERMIC AND ENDOTHERMIC REACTIONS
Examples of Exothermic Reactions:
- Neutralisation reactions
- Hydration of anhydrous copper sulphate
Examples of Endothermic Reactions:
- Dissolving certain salts in water (e.g. potassium chloride, ammonium nitrate)
- Thermal decomposition (e.g. converting calcium carbonate into calcium oxide (quicklime))
ΔH (read as 'delta H') is the symbol for the enthalpy change of a reaction. Enthalpy change refers to the change in energy in a reaction. ΔH is negative for an exothermic reaction (i.e. the products have less energy than the reactants) and positive for an endothermic reaction (i.e. the product have more energy than the reactants).
- ΔH is negative for exothermic reactions
- The difference in energy is given out as heat.
- Therefore, the temperature rises.
- ΔH is positive for endothermic reactions
- The extra energy needed to form the products is taken in from the surroundings.
- Therefore, the temperature falls.
Energy Level Diagrams:
Enthalpy change can be illustrated using an energy level diagram:
Sometimes, a simplified version of the above diagrams are drawn, without the reaction pathway curve and activation energy drawn on.
Formula for Energy Change:
Specific Heat Capacity (c) is the quantity of energy, in Joules (J), needed to change the temperature of one gram (g) of a substance by one degree Celsius (oC).
e.g. The specific heat capacity of water is 4.18 J/g/oC
Formula for Enthalpy Change:
We can work out the energy needed to break different chemical bonds. This is called the bond energy. These bond energies can be used to calculate ΔH for a reaction.
Bond energies are average values for the bonds in different molecules. Therefore, these calculations only give us a rough value for ΔH.
Making bonds is EXOTHERMIC
Breaking bonds is ENDOTHERMIC