Question

Write a balanced equation describing the dissolution of calcium hydroxide in water

Acording to solubility rules, should Ca(OH)2 be soluble or insoluble in water. Explain

Predict the sign +- of delta G for the process.

Predict the sign + - of delta S for this process.

Answer 1

1 mol Ca(OH)₂ contains 1 mol Ca²⁺ and 2 mol OH^- ions. So the equation for dissolution of Ca(OH)₂ in water is as shown

Ca(OH)2(s) → Cal+ (aq) + 20H-(aq)

As per the solubility rules most hydroxides are slightly soluble. Hydroxides of group II elements (Ca, Sr, Ba) are sparingly insoluble. So according to solubility rules Ca(OH)₂ is sparingly soluble in water.

As Ca(OH)₂ is sparingly soluble in water, this means the process of dissolution of Ca(OH)₂ in water is spontaneous. Also we know that the sign of free energy change $\Delta G$ is negative for a spontaneous process. So sign of $\Delta G$ is negative.

We know that when a solid substance (Ca(OH)₂in this case) goes into aqueous phase by dissolving in water as ions, the disorder of the system increases. So entropy of the system increases when Ca(OH)₂ dissolves in water, i.e. final entropy is more than the initial entropy. So the sign of entropy change $\Delta S$ is positive.