H2S gas when passed through a solution of cations containing HCl precipitates the cations of second group of qualitative analysis but not those belonging to the fourth group. It is because:
presence of HCl decreases the sulphide ion concentration
presence of HCl increases the sulphide ion concentration
solubility product of group II sulphides is more than that of group IV sulphides
sulphides of group IV cations are unstable in HCl
The Correct Option is A
Solution and Explanation
To understand why H2S gas precipitates the cations of the second group but not the fourth group when passed through a solution containing HCl, we need to consider the effects of HCl on the concentration of sulphide ions.
- H2S gas in water partially dissociates to form sulphide ions (S2-): H_2S \rightleftharpoons 2H^+ + S^{2-}
- The presence of HCl contributes to the concentration of H+ ions in the solution.
- This increased concentration of H+ ions shifts the equilibrium of the above reaction to the left, due to Le Chatelier's Principle, thus reducing the S2- ion concentration in the solution.
- In qualitative analysis, cations of Group II precipitate as their respective sulphides under acidic conditions, where the concentration of S2- is lower, because Group II sulphides have very low solubility products (Ksp). Therefore, even the low concentration of S2- is sufficient for their precipitation.
- On the contrary, cations of Group IV require a much higher concentration of S2- for precipitation as their sulphides have higher solubility products.
Thus, it is the reduction of sulphide ion concentration due to the presence of HCl that prevents the precipitation of Group IV cations, while allowing the precipitation of Group II cations.
Hence, the correct answer is: presence of HCl decreases the sulphide ion concentration.
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Concepts Used:
Equilibrium
An equilibrium represents a state in a process when the observable properties such as color, temperature, pressure, concentration etc do not show any change.
The word equilibrium means ‘balance’ which indicates that a chemical reaction represents a balance between the reactants and products taking part in the reaction. The equilibrium state is also noticed in certain physical processes such as the melting point of ice at 0℃, both ice and water are present at equilibrium.
In the case of physical processes such as the melting of solid, dissolution of salt in water etc., the equilibrium is called physical equilibrium while the equilibrium associated with chemical reaction is known as chemical equilibrium.
Equilibrium in Chemical changes
The chemical equilibrium in a reversible reaction is the state at which both forward and backward reactions occur at the same speed.
The stage of the reversible reaction at which the concentration of the reactants and products do not change with time is called the equilibrium state.
Read More: Calculating Equilibrium Concentration
Types of Chemical Equilibrium
There are two types of chemical equilibrium:
- Homogeneous Equilibrium
- Heterogeneous Equilibrium
Homogenous Chemical Equilibrium
In this type, the reactants and the products of chemical equilibrium are all in the same phase. Homogenous equilibrium can be further divided into two types: Reactions in which the number of molecules of the products is equal to the number of molecules of the reactants. For example,
- H2 (g) + I2 (g) ⇌ 2HI (g)
- N2 (g) + O2 (g) ⇌ 2NO (g)
Reactions in which the number of molecules of the products is not equal to the total number of reactant molecules. For example,
- 2SO2 (g) + O2 (g) ⇌ 2SO3 (g)
- COCl2 (g) ⇌ CO (g) + Cl2 (g)
Heterogeneous Chemical Equilibrium
In this type, the reactants and the products of chemical equilibrium are present in different phases. A few examples of heterogeneous equilibrium are listed below.
- CO2 (g) + C (s) ⇌ 2CO (g)
- CaCO3 (s) ⇌ CaO (s) + CO2 (g)
Thus, the different types of chemical equilibrium are based on the phase of the reactants and products.
Check Out: Equilibrium Important Questions