Question

$KI$ and $CuS{O_4}$ solution when mixed give:
A. $Cu{I_2} + {K_2}S{O_4}$
B. $C{u_2}{I_2} + {K_2}S{O_4}$
C. $C{u_2}{I_2} + {K_2}S{O_4} + {I_2}$
D. $Cu{I_2} + {K_2}S{O_4} + {I_2}$

Answer 1

Hint: Try to recall that when $KI$ and $CuS{O_4}$ are mixed together then they undergo a redox reaction and not precipitation or double decomposition because it involves oxidizing agents like ${I^ - }$ and reducing agent like $C{u^{2 + }}$. Now, by using this you can easily find the correct option for the given question.

Complete step by step solution:
It is already known to you that iodide ion (${I^ - }$) is a good reducing agent and cupric ion ($C{u^{2 + }}$) is a good oxidizing agent.
So, when potassium iodide,$KI$ is added to the copper sulphate, $CuS{O_4}$ solution, then they will undergo redox reaction in which iodide ion reduces cupric ion, $C{u^{2 + }}$ to cuprous ion, $C{u^{2 + }}$ (blue to white precipitate)and it themselves get oxidized to iodine gas.

The best way to look at the above reaction is via the half equations:
Reduction half equation: $2C{u^{ + 2}} + 2{e^ - } \to 2C{u^{ + 1}}$
Oxidation half equation: ${I^{ - 1}} \to {I_2} + {e^ - }$.
To balance these both half equations, double the oxidation half equation (to equalise electrons) and then add both reduction and half equations together.
$({I^{ - 1}} \to {I_2} + {e^ - }) \times 2$
$2C{u^{ + 2}} + 2{e^ - } \to 2C{u^{ + 1}}$
-------------------Add
$2C{u^{ + 2}} + {I^{ - 1}} \to 2C{u^ + } + {I_2}$
Now, add in the spectator ions:
$2CuS{O_4} + 2KI \to C{u_2}{I_2} + {K_2}S{O_4} + {I_2}$
So, the brownish color of solution is due to the liberation of iodine gas.
Therefore, from above we can easily conclude that option C is the correct option to the given option.

Note: It should be remembered to you that the iodine formed in the above reaction reacts with potassium ions in the reaction mixture to give potassium triiodide ($K{I_3}$).
Also, you should remember that the liberated iodine gas can be titrated against a standardized solution of sodium thiosulphate with starch as an indicator to determine the amount of copper sulphate present in the solution.