Problem 117 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{ZnS}\) is not precipitated by passing \(\mathrm{H}_{2} \mathrm{~S}\) in acidic medium but \(\mathrm{CuS}\) is precipitated. The reason for it is (a) \(\mathrm{Ksp} \mathrm{CuS}<<\mathrm{Ksp} \mathrm{ZnS}\) (b) \(\mathrm{Ksp} \mathrm{CuS}>>\mathrm{Ksp} \mathrm{ZnS}\) (c) \(\mathrm{Ksp} \mathrm{CuS}=\mathrm{Ksp} \mathrm{ZnS}\) (d) none of these

Step-by-Step Solution

Answer

(a) K_{sp} CuS << K_{sp} ZnS.

Step 1: Understanding Solubility Product

The solubility product, K_{sp} , is an equilibrium constant that refers to the solubility of a compound under given conditions. If a compound has a smaller K_{sp} , it is less soluble compared to a compound with a larger K_{sp} .

Step 2: Reviewing Given Information

We are given that ZnS does not precipitate in an acidic medium with H_2S , but CuS does form a precipitate. This indicates that under the same acidic conditions, CuS reaches its solubility limit and precipitates, while ZnS does not.

Step 3: Analyzing Solubility in Acidic Medium

In acidic solutions, a lower K_{sp} value means a compound is more likely to precipitate since it is less soluble. Therefore, for CuS to precipitate while ZnS does not in acidic conditions, it suggests that K_{sp} for CuS is much smaller than that for ZnS .

Step 4: Conclusion based on Solubility Product

Based on the analysis, since K_{sp} of CuS is much smaller than that of ZnS , it indicates that choice (a) is the correct answer: K_{sp} CuS << K_{sp} ZnS. This is why CuS precipitates but ZnS does not in an acidic medium.

Key Concepts

Precipitation Reactions

Precipitation reactions are types of chemical reactions where two soluble salts are mixed together in a solution, and an insoluble salt, known as the precipitate, forms. When you see two clear solutions turn cloudy, that's often the formation of a precipitate, which can then settle out of the solution. Precipitation reactions are important in chemistry because they allow us to isolate and identify different compounds based on their solubility properties. Understanding precipitation reactions is key to topics like solubility equilibrium and the behavior of substances under different conditions. Such reactions are driven by the solubility product ( K_{sp} ), which determines whether or not a compound will precipitate under specific conditions. Essentially, if the product of the ion concentrations in a solution exceeds the K_{sp} of a compound, it will precipitate. These reactions are useful in processes like water purification, forming crystals, and in some analytical techniques. To predict if a substance like CuS will form as a precipitate, chemists analyze the K_{sp} as compared to other factors in the solution like pH.

Solubility Equilibrium

Solubility equilibrium involves a balance between dissolved ions in a solution and the undissolved solid form of those ions. When a substance dissolves, it reaches a state where some part remains dissolved while some rematerializes as a solid, forming a dynamic equilibrium. The solubility product constant ( K_{sp} ) reflects this state. The smaller the K_{sp} , the less soluble the substance is, meaning it will precipitate sooner when conditions like concentration or temperature change. For example, in the case of CuS and ZnS , CuS has a lower K_{sp} , indicating it reaches saturation and precipitates more easily under the same conditions that keep ZnS dissolved. This concept is crucial to understanding how certain factors, like changing the acidity or presence of other ions in the solution, can affect the solubility and thus the formation of precipitates. It also explains why certain compounds behave differently in various chemical processes, such as extraction or separation techniques.

Acidic Medium Chemistry

In chemistry, the nature of the medium, whether acidic or basic, can significantly impact the behavior of chemical reactions. In an acidic medium, certain compounds that might otherwise remain dissolved will precipitate due to changes in ion concentration and K_{sp} values. For example, when H_2S gas is passed through an acidic solution, the availability of hydrogen ions can affect the solubility of sulfide ions. In the case of ZnS and CuS , these interactions mean that despite sulfide ions being present, only CuS precipitates because it has a significantly lower K_{sp} than ZnS . Acidic conditions often result in the increased presence of hydrogen ions, which can shift the solubility equilibrium, affecting which compounds form a precipitate. This principle underlies a range of processes in chemical research and industry, from ore extraction to environmental chemistry, where controlling the medium's properties is key to achieving the desired chemical outcome.