Problem 130 - A Complete Resource Book in Chemistry for JEE Main

Question

$$ \begin{aligned} &\text { Match the following }\\\ &\begin{array}{ll} \hline \text { Column-I (Inorganic ions) } & \begin{array}{l} \text { Column-II (can } \\ \text { get tested using } \\ \text { reagent) } \end{array} \\ \hline \text { (a) } \mathrm{Co}^{2+} & \text { (p) } \mathrm{K}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right] \\ \text { (b) } \mathrm{Cu}^{2+} & \text { (q) } \mathrm{KSCN} \\ \text { (c) } \mathrm{Fe}^{3+} & \text { (r) } \mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right] \\ \text { (d) } \mathrm{Zn}^{2+} & \text { (s) } \mathrm{KNO}_{2}+ \\ & \mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H} \\ & \text { (t) } \mathrm{K}_{2}\left[\mathrm{Hg}(\mathrm{SCN})_{4}\right] \\ \hline \end{array} \end{aligned} $$

Step-by-Step Solution

Answer

(a) -> (s), (b) -> (p), (c) -> (q), (d) -> (t).

Step 1: Analyze Co^{2+} Ion

Co^{2+} ions react with KNO2 and CH3CO2H to form a yellow precipitate of Co(NO2)3. Hence, (a) Co^{2+} matches with (s) KNO2 + CH3CO2H.

Step 2: Analyze Cu^{2+} Ion

Cu^{2+} ions react with K4[Fe(CN)6] to produce a reddish-brown precipitate of Cu2[Fe(CN)6]. Thus, (b) Cu^{2+} matches with (p) K4[Fe(CN)6].

Step 3: Analyze Fe^{3+} Ion

Fe^{3+} ions react with KSCN to form a blood-red complex, Fe(SCN)3. Therefore, (c) Fe^{3+} pairs with (q) KSCN.

Step 4: Analyze Zn^{2+} Ion

Zn^{2+} ions form a white precipitate of ZnHg(SCN)4 when reacted with K2[Hg(SCN)4]. Consequently, (d) Zn^{2+} matches with (t) K2[Hg(SCN)4].

Step 5: Conclusion of Matches

The pairs are: (a) with (s), (b) with (p), (c) with (q), and (d) with (t).

Key Concepts

Reactivity of Co^{2+}

Cobalt ions ( Co^{2+} ) offer interesting reactivity characteristics when they come in contact with certain reagents, making them easy to distinguish in chemical tests. A common chemical test involves Co^{2+} ions being mixed with potassium nitrite ( KNO_2 ) and acetic acid ( CH_3CO_2H ). This reaction results in forming a yellow precipitate of cobalt nitrite, denoted as Co(NO_2)_3 .
This characteristic reaction can help identify cobalt ions in a mixture. An easy way to remember this test is to associate the yellow color of the precipitate with the word 'golden', which sounds similar to 'cobalt'. This mnemonic can aid in recalling that Co^{2+} reacts with KNO_2 and acetic acid.

Reactivity of Cu^{2+}

Copper ions ( Cu^{2+} ) are well-known for their beautiful and distinctive colors when reacting with certain compounds. When Cu^{2+} ions encounter potassium hexacyanoferrate(II) ( K_4[Fe(CN)_6] ), they form a reddish-brown precipitate. The result of this reaction is copper(II) hexacyanoferrate, Cu_2[Fe(CN)_6] , which can serve as a distinct marker for the presence of copper ions.
Remembering the color change from blue to reddish-brown can help you visualize the conversion taking place due to the reaction. This test is important in analytical chemistry for verifying the presence of copper ions in a sample.

Reactivity of Fe^{3+}

Iron(III) ions ( Fe^{3+} ) have a special affinity for thiocyanate ions. When Fe^{3+} interacts with potassium thiocyanate ( KSCN ), it forms a complex that has a striking blood-red color. The complex produced is iron(III) thiocyanate ( Fe(SCN)_3 ), which is notable for its intense color.
This reaction is extremely useful for detecting iron(III) ions, as the blood-red color is a highly visible and definitive indicator. A fun fact to help remember this reaction is to think of the 'red' color of 'red blood cells', which can hint at the vibrant red color produced.

Reactivity of Zn^{2+}

Zinc ions ( Zn^{2+} ) provide an easy identification method due to their reaction with specific reagents. When Zn^{2+} ions are combined with mercury thiocyanate complex ( K_2[Hg(SCN)_4] ), a white precipitate forms. This product, zinc mercury thiocyanate ( ZnHg(SCN)_4 ), serves as an indicator for the presence of zinc ions in solution.
The formation of a white precipitate is a simple observation that can decisively identify zinc ions. This reaction is key in qualitative analysis, helping separate Zn^{2+} from multivalent metallic ions in a solution.