When solutions of acids and bases are mixed, reactions occur in which the acidic and basic properties of the reactants are nullified. Such acid-base reactions are known as neutralisation reactions.
The Reactions of Acids
The reactions of acids with metal hydroxides, metal oxides, carbonates and hydrogen carbonates can all be considered acid-base reactions or neutralisation reactions because they involve the transfer of protons from one species to another.
If we look at the reaction between nitric acid and sodium hydroxide again, nitric acid has transferred a proton to sodium hydroxide.
These reactions are exothermic – heat is given out, is negative. You should be familiar with the reaction
In neutralisation reaction, hydroxide ions react with the hydrogen ions to form water.
This is an exothermic reaction, and the heat released has much the same value for all reactions between strong acids and bases. This value ranges between 57 and 58 of water formed. We usually write the above reaction as:
net ionic equations
Notice that for both Reactions 1 and 2 the net ionic equations are the same. This accounts for the heat released by Reactions 1 and 2 being very nearly the same even though different combinations of acids and bases react. In these types of neutralisation reactions, the heat of neutralisation is the heat produced per mole of water that is formed.
Heats of Reaction
Heats of reaction are measured using a calorimeter. There are different types of calorimeters but for reactions carried out in solution, such as neutralisation reactions, a crude calorimeter can be constructed from a polystyrene cup as shown in figure below. The heat energy released or absorbed by these reacting systems increases or decreases the temperature of the solution (the surroundings). The polystyrene cup is assumed to have minimal heat capacity and the heat of solution is calculated from measurements of the temperature change of the solution and the specific heat of water.