Enthalpy of Neutralization of Strong Acid and Strong Base

The standard enthalpy change of neutralization reaction is the enthalpy change that occurs when the solutions of an acid and an alkali react together under conditions to produce 1 mole of water. This reaction is also said to be an exothermic reaction as a high amount of energy is being given out when the neutralization reaction takes place. Enthalpy is also called the heat that is involved when a reaction takes place.

Neutralization Enthalpy of Strong Base and Strong Acid

For a strong acid and a strong base, the neutralization enthalpy is still constant. This is because both strong acids and strong bases are fully ionized in a dilute solution. Neutralization changes in enthalpy are often negative-when an acid and alkali react, heat is released

What is Standard Enthalpy Change?

As solutions of an acid and an alkali react together under normal conditions to produce 1 mole of water, the standard enthalpy change of neutralization is the enthalpy change. Note that the neutralization shift in enthalpy is always measured per mole of water produced. Neutralization alterations in enthalpy are often negative - when an acid and alkali react, heat is released. The values are often very nearly similar for reactions involving strong acids and alkalis, with values between -57 and -58 kJ mol-1. Which varies slightly depending on the combination of acid and alkali.

Neutralization Reaction

When an acid and a base react to form water and salt, a neutralization reaction requires the combination of H+ ions and OH- ions to produce water. There is a pH equal to 7 for the neutralization of a heavy acid and strong base. Neutralizing a strong acid and a weak base would have a pH of less than 7 and, conversely, the resultant pH will be greater than 7 when a strong base neutralizes a weak acid.

It means that salts are formed from equal weights of acid and base when a solution is neutralized. The amount of acid required is the amount that one mole of protons (H+) would give and the amount of base needed is the amount that one mole of protons would give (OH-). Since salts are formed from neutralization reactions of equal acid and base weight concentrations, N parts of the acid will always neutralize N parts of the base.

Why do Strong Acids that React with Strong Alkalis Produce Similar Values?

We assume that strong acids and strong alkalis in the solution are completely ionized and that the ions work independently of each other. In solution, dilute hydrochloric acid, for example, contains hydrogen ions and chloride ions. The sodium hydroxide solution in the solution consists of sodium ions and hydroxide ions. In essence, the equation for any strong acid being neutralized by a strong alkali is just a reaction to make water between hydrogen ions and hydroxide ions. The other ions present (for example, sodium and chloride) are merely spectator ions, which do not participate in the reaction.

The equation of reaction between hydrochloric  acid and sodium hydroxide solution is:

                        NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

But the actual happening is different:

                       OH−(aq) + H+(aq) → H2O(l)

If the reaction is the same in both a strong acid and strong alkali, then it is not surprising that enthalpy change is similar.

Anything about 99% of the acid is not naturally ionized in a weak acid such as acetic acid at ordinary concentrations. This implies the other enthalpy terms involved in ionizing the acid as well as the reaction between the hydrogen ions and hydroxide ions would include the enthalpy shift of neutralization. And ammonia is also present primarily as ammonia molecules in solution in a weak alkali like ammonia solution. 

Again, apart from the basic form of water from hydrogen ions and hydroxide ions, there may be other enthalpy modifications involved. The calculated enthalpy shift of neutralization for reactions involving acetic acid or ammonia is a few kJ less exothermic than with solid acids and bases.

One source that provides the enthalpy shift of sodium hydroxide solution neutralization with HCl as-57.9 kJ mol-1:

                 NaOH(aq) + HCl(aq) → Na+(aq) + Cl−(aq) + H2O

The neutralization enthalpy change for acetic acid-neutralizing sodium hydroxide solution is -56.1 kJ mol-1:

                 NaOH(aq) + CH3COOH(aq) → Na+(aq) + CH3COO−(aq) + H2O

For very weak acids, such as cyanide hydrogen solution, the neutralization shift of enthalpy can be much less. The value of the hydrogen cyanide solution being neutralized by potassium hydroxide solution as -11.7 kJ mol-1, for example, is given by another source.

                    NaOH(aq) + HCN(aq) → Na + (aq) + CN − (aq)+H2O

Experiment to understand the Enthalpy of Neutralization of Strong Acid and Strong Base

The experiment can be conducted between a strong acid and a strong base by titration process. The temperature that is evolved while the reaction is proceeding to equilibrium is noted down and then the heat value is calculated from the same.

Precautions to be taken while Performing the Experiment for Neutralization Reaction

When performing the experiment in the lab it is necessary to take some precautions which can be provided as follows:

1. Due to radiation, there is some heat lost to the environment and hence the reaction flask can be a bit hot. Handling hot things must be done carefully.

2. The solution density must be equal to 1 g/ml.

3. The hydrochloric acid that is the strong acid and the sodium hydroxide that is the strong base ionization is considered to be 100 per cent.

4. The specific heat of the solution is taken as 4.189 J/g.

5. The mixture that contains both HCl and NaOH must be stirred properly to get accurate results.