Question

What is the method to be followed while diluting an acid?

Answer 1

Hint: We know that the aqueous solution, dilute sulphuric acid dissociates to form protons and sulphate ions. Also, the autoionization of water gives protons and hydroxide ions. Thus, the dilute aqueous sulphuric acid solution contains protons, hydroxide ions and sulphate ions.

Complete answer:
As we know that when hydroxide ions are discharged, oxygen gas is liberated at anode. However, if sulphate ions are discharged, Sulphur dioxide gas is liberated. Since the value of the electrode potential for the discharge of hydroxide ions is less than that of sulphate ions, the hydroxide ions will get discharged preferentially at anode. Due to this, oxygen gas will liberate in preference to Sulphur dioxide gas at anode.
Also, if we add water to acid, you form an extremely concentrated solution of acid initially and the solution may boil very violently, splashing concentrated acid. If you add acid to water, the solution that forms is very dilute and the small amount of heat released is not enough to vaporize and spatter it. When acid is poured into water, it flows down the flask and mixes much better, so no boiling occurs. Addition of water to acid for dilution forms an extremely concentrated solution of acid initially. So much heat is released that the solution may boil very violently, splashing concentrated acid out of the container and all this because the reaction is exothermic. The reason this occurs is due to the large amount of energy released in the hydration reaction of concentrated ions. So, it is always safer to add acid to water dropwise with constant stirring and not water to acid directly.

Note:
Remember that when hydroxide ions are discharged, oxygen gas is liberated at anode. However, if sulphate ions are discharged, Sulphur dioxide gas is liberated. Since the value of the electrode potential for the discharge of hydroxide ions is less than that of sulphate ions.