Why should you never add water to the strong acid while diluting the acid? Why should you add the strong acid to the water?

In the previous article, we addressed the reason behind never adding water to hot oil. Similarly, we have to avoid adding water to strong acids. Whenever you want to dilute a strong acid, add the strong acid to a beaker containing water, not the reverse.

The dissolution of strong acids in water generates heat (exothermic reaction). This heat is generated due to the enthalpy. The enthalpy of a solution can either be zero (athermic reactions- involving no exchange of energy), positive (endothermic reactions), or negative (exothermic reactions).

Why does this happen? What happens at the microscopic level and causes energy release?

As an Arrhenius acid molecules, the dissolution of a strong acid (HA) in water (H₂O) releases a hydrogen ion/poton (H⁺). This hydrogen ion (H⁺) cannot exist by its own in the solution. When released, it directly combines with a water molecule (H₂O) to form a hydronium ion H₃O⁺

Hence, the dissolution of a strong acid (HA) in water is represented by the following equation:

HA +H₂O -> H₃O⁺ + A^–

For example, hydrochloric acid, a strong acid, dissociates in water according to the following equation:

HCl +H₂O -> H₃O⁺ + Cl^–

The heat is generated due to the hydration of the proton (H⁺) and specifically due to the hydrogen bond formation.

However, heat is generated in both scenarios: when you add acid to water or when you add water to acid. Why is it pronounced in the case of adding water to strong acid?

The moment you add water to the strong acid, you are creating a concentrated solution. Why is it considered concentrated? In this instance, the amount of water is small as compared to the amount of strong acid. This is not a scientific definition! However, it makes the reader imagine what is happening in the beaker at the microscopic level. The dissolution of water produces a large amount of heat energy that causes the water to boil and splash out of the container.

On the other hand, when you add a strong acid to the water, at first you are creating a very dilute solution (again, this is relative to the previous scenario). When heat is released in the diluted solution. It is dissipated in the large volume of water, and its effect almost vanishes.