ENERGY CHANGES CALCULATING ENERGY CHANGES FURTHER PRACTICE
YOU SHOULD be able to: ■ Distinguish between exothermic and endothermic reactions in terms of energy content of products and reactants. ■ Draw an energy profile diagram to illustrate endothermic and exothermic change. ■ Calculate energy changes from experiments or from experimental data.
QUICK POINTS
All reactions involve energy changes; energy is absorbed when bonds are broken and released when bonds are formed.
When energy is released in a reaction, it is said to be exothermic and Δ H is negative.
When energy is absorbed in a reaction, it is said to be endothermic and Δ H is positive.
ENERGY CHANGES
The heat content of a substance is called its enthalpy (H). The change in enthalpy, or Δ H, is the difference between the energy content of the products and the energy content of the reactants. Δ H rxn = H products – H reactants.
For a reaction to take place, reactants must achieve a minimum energy in order for products to form. This is called the activation energy, or energy barrier.
The difference between the energy of the products and the reactants is the overall energy change Δ H.
In exothermic reactions, energy is released, leading to an increase in the temperature of the surroundings. Objects feel hotter as temperature increases.
In endothermic reactions, energy is absorbed, leading to a decrease in the temperature of the surroundings. Objects feel colder as temperature decreases.
Catalysts work by lowering the activation energy of the reaction.
Energy profile of a catalysed and uncatalysed reaction. Energy changes for different reactions can be calculated by noting heat lost or gained throughout the reaction. These are usually expressed as joules per mole of reactant or product formed. Heat of combustion refers to the energy change that occurs when one mole of a substance is burnt completely in oxygen.
Heat of solution is the energy change that occurs when one mole of a solute dissolves in a given volume of solvent until there is no further heat change.
Heat of neutralisation refers to the energy change when one mole of water is formed from the reaction between an acid and a base.