Salt Hydrolysis and Solubility of Salts for JEE

When an acid reacts with a base, salt and water are formed and the reaction is called neutralisation. Salts completely dissociate in aqueous solutions to give their constituent ions. The ions so produced are hydrated in water. In certain cases, the cation, anion or both react with water and the reaction is called salt hydrolysis. Hence, salt hydrolysis is the reverse of the neutralisation reaction.

Salts of Strong Base and Strong Acid

Let us consider the reaction between NaOH and nitric acid to give sodium nitrate and water.

$\mathrm{NaOH}(\mathrm{aq})+\mathrm{HNO}_{3}(\mathrm{aq}) \rightarrow \mathrm{NaNO}_{3}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})$

The salt NaNO₃ completely dissociates in water to produce Na⁺ and NO₃⁻ ions.

$\mathrm{NaNO}_{3}(\mathrm{aq}) \rightarrow \mathrm{Na}^{+}(\mathrm{aq})+\mathrm{NO}_{3}^{-}(\mathrm{aq})$

Water dissociates to a small extent as

$\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \leftrightarrows \mathrm{H}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})$

Since $[H^+]=[OH^–]$, water is neutral

NO₃^– ions are the conjugate base of the strong acid HNO₃ and Na⁺ is the conjugate acid of the strong base NaOH.

Solubility Product

In our day to day life, We have come across many precipitation reactions in inorganic qualitative analysis. For example, Dilute Hydrochloric acid is used to precipitate Pb²⁺ ions as PbCl₂ where the solubility of salt in water is less. Kidney stones are developed over a period of time due to the precipitation of Ca²⁺ or as Calcium oxalate etc. Inorder to understand the precipitation of salts, let us consider the solubility equilibria that exist between the undissociated sparingly soluble salt and its constituent ions in solution.

For a general salt X_mY_n ,

$X_{m} Y_{n}(s){\overset{H_{2}O}{\rightleftarrows}} X^{n+}(a q)+n Y^{m-}(a q)$

The equilibrium constant for the above is

$K=\dfrac{\left[X^{n+}\right]^m\left[Y^{m-}\right]^{n}}{\left[X_{m} Y_{n}\right]}$

In solubility equilibria, the equilibrium constant is referred to as solubility product constant or otherwise known as Solubility product.

In such heterogeneous equilibria, the concentration of the solid is a constant and is omitted in the above expression can be written as

$K_{s p}=\left[X^{n+}\right]^m\left[Y^{m-}\right]^n$

The solubility product of a compound can be defined as the product of the molar concentration of the constituent ions, each raised to the power of its stoichiometric coefficient in a balanced equation that has attained equilibrium. The term Solubility is useful to decide whether an ionic compound can get precipitated when a solution that contains the constituent ions are mixed together.

When the product of molar concentration of the constituent ions i.e., ionic product of the constituent ions, exceeds the solubility product then the compound has the possibility to get precipitated. The expression for the solubility product and the ionic product appears to be the same but they aren’t same, because in the solubility product expression, the molar concentration represents the equilibrium concentration and in the ionic product, the initial concentration (or) concentration at a given time ‘t’ is used.

In general we can summarise as,

Ionic product > K_sp , precipitation will occur and the solution is supersaturated.

Ionic product < K_sp , no precipitation and the solution is unsaturated.

Ionic product = K_sp , equilibrium exists and the solution is saturated.

Calculation of solubility of sparingly soluble salts

Substances like Silver Chloride, Lead Sulphate etc., are sparingly soluble in water. The solubility product of such substances can be determined using the conductivity measurements.

Let us consider AgCl as an example

AgCl (s) ⇆ Ag⁺ + Cl^–

K_sp = [Ag⁺] [Cl^-]

Let the concentration of $[Ag^+]$ be ‘C’ molL^–1.

As per the stoichiometry, if $[Ag^+]$ = C, the $[Cl^–]$ also equal to ‘C’ molL^–1.

$\mathrm{K}_{\mathrm{sp}}=\mathrm{C} \cdot \mathrm{C} \Rightarrow \mathrm{C}^{2}$

We know that the concentration (in terms of mol dm^–3) is related to the molar and the specific conductance by the following expressions.

$\Lambda_{\circ}=\dfrac{\kappa \times 10^{-3}}{C\left(\mathrm{in}~\mathrm{mol}~L^{-1}\right)}$

(or)

$C=\dfrac{\kappa \times 10^{-3}}{\Lambda}$

Substitute the concentration value in the relation $K_{sp}=C^{2}$

$C=\left(\dfrac{\kappa \times 10^{-3}}{\Lambda_{o}}\right)^{2}$

Soluble Salts

Compounds that are soluble in water at room temperature are called Soluble Salts. These salt compounds readily get dissolved in water because they can form intermolecular attractions within the molecules of water. We know that the Water molecules are always polar in nature. Therefore, water can be termed as polar solvent, and polar salts can get dissolved in water.

Since salts are known to be ionic compounds, they readily get dissolved in water because water molecules tend to attract the ions in the compound, which makes them get separated from each other, which results in the dissolution of the salt. Here, the dissolution of the salt forms ionic species in water, which makes the newly formed aqueous solution to behave highly conductive. The ionic species dissolved in water may conduct electricity through it. An example of a soluble salt is table salt or sodium chloride (NaCl). The aqueous solution of NaCl contains sodium ions and chloride ions. Soluble Salts examples: NaCl, KCl, MgCl₂ etc.

Insoluble Salts

Insoluble salts are those salt compounds that are insoluble in water at room temperature. These are insoluble in water because of the fact that water molecules cannot attract the ions in the salt compound. Therefore, we cannot see any intermolecular interactions between water molecules and the insoluble salt compounds.

Furthermore, insoluble salts are generally nonpolar compounds. Unlike the soluble salts, the mixing of insoluble salts with water does not make the solution conductive because the salt does not separate into ions in this case. Insoluble Salt Example: AgCl, PbNO₃, PbCl₄.

Difference Between Soluble and Insoluble Salts

Conclusion

We can classify the salt compounds into two types depending on their water solubility. They are soluble and insoluble salts. The key difference between soluble and insoluble salts is that the soluble salts can get dissolved in water at room temperature, whereas the insoluble salts cannot. Moreover, the soluble salts are polar; that is why they can get dissolved in water, which is a polar solvent. Contrary to this, insoluble salts are nonpolar. So, this is another prominent difference between soluble and insoluble salts.