Question

H₂S will precipitate the sulphides of all the metals from the solution of chlorides of Cu , Zn and Cd if:
(A) The solution is aqueous
(B) The solution is acidic
(C) The solution is dilute acidic
(D) Any of the above solutions is present.

Answer 1

Hint: The act or process of precipitating a material from a solution is known as precipitation. If the solution of , and chlorides is aqueous, will precipitate the sulphides of all the metals from the solution. , , and can all form sulphide precipitates when is present in a neutral media.

Complete Step by Step Solution:
When two ionic bonds combine and an insoluble salt precipitates out of an aqueous solution, this is known as the precipitation reaction. The insoluble salts produced by precipitation processes are known as precipitates. Precipitation is a prime example of a double displacement reaction.
We know that, belongs to group 4, while and both belong to group 2. is employed for both group 2 and group 4 sulphides, however the nature of the medium—acidic or basic—differs. reacts with group 2 cations to create sulphides, however these products have very poor solubility values ( solubility product). Therefore, if we utilise acidic (diluted) media for group 2, and will dissociate into their constituent ions. This will result in the common ion effect, which will lower the concentration of ion. Due to their low values, group 2 elements also precipitate out at low concentrations.
However, group number 4's sulphides have a larger value than group number 2. So in this situation, basic media is used because the sulphide ion concentration there is higher.
All reactions take place in aqueous mediums since only and precipitate can form in an acidic solution. Acidic medium will not produce a precipitate.
Hence, A. is the correct option.

Note: We do not use strong acid for most reactions. Also keep in mind that alkaline solution in this case will have a very high concentration of ion so both group 2 and group 4 sulphides will precipitate in the conditions given in the question. Hence, the solution should be aqueous and/or alkaline.