Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

A blue colouration is obtained in which case? (1) on dissolving \(\mathrm{NH}_{4} \mathrm{OH}\) in copper sulphate. (2) on reacting \(\mathrm{CuSO}_{4}\) solution with potassium ferrocyanide. (3) on reacting \(\mathrm{FeCl}_{3}\) with sodium ferrocyanide. (4) on dissolving anhydrous \(\mathrm{CuSO}_{4}\) in water. (a) 2,4 only (b) \(1,3,4\) (c) \(1,2,4\) (d) 3,4 only

Step-by-Step Solution

Answer

None of the options correctly identify reactions 1 and 4 as producing a blue solution.

Step 1: Analyze Each Chemical Reaction

Examine each option to determine the color produced: 1. **NH4OH with Copper Sulphate**: When ammonium hydroxide (NH4OH) is added to copper sulphate (CuSO4), it forms a complex that turns into a deep blue solution. 2. **CuSO4 with Potassium Ferrocyanide**: Reaction between copper sulphate and potassium ferrocyanide produces a brown precipitate, not a blue solution. 3. **FeCl3 with Sodium Ferrocyanide**: Iron(III) chloride reacts with sodium ferrocyanide to form a blue precipitate known as Prussian blue, not a solution. 4. **Anhydrous CuSO4 with Water**: Dissolving anhydrous copper sulphate in water results in a blue solution as the water hydrates CuSO4, forming CuSO4·5H2O.

Step 2: Determine Which Reactions Result in a Blue Solution

Identify the reactions that lead to a blue solution based on the analysis: - Reaction 1 forms a deep blue solution. - Reaction 2 does not form a blue solution. - Reaction 3 forms a blue precipitate, not a solution. - Reaction 4 forms a blue solution. Therefore, reactions 1 and 4 produce a blue solution.

Step 3: Compare to Given Options

Look at the given options to see which one matches the results from Step 2: - Option (a): 2,4 only – Incorrect, since reaction 2 does not produce a blue solution. - Option (b): 1,3,4 – Incorrect, since reaction 3 produces a blue precipitate, not a solution. - Option (c): 1,2,4 – Incorrect, as reaction 2 does not produce a blue solution. - Option (d): 3,4 only – Incorrect, as reaction 3 involves a blue precipitate. None of the options correctly match the reactions resulting in a blue solution.

Key Concepts

Complex Ions and Color

Complex ions are fascinating as they often lead to striking color changes in solutions. When certain ligands like ammonia (NH₄OH) react with metal ions such as copper(II) sulfate (CuSO₄), they form complex ions. In our case of mixing NH₄OH with CuSO₄, the formation of the complex ion \([Cu(NH_3)_4]^{2+}\) gives the solution a deep blue color. This happens because the ligands cause splitting in the d-orbitals of the metal ion. Electrons absorb specific wavelengths of light, resulting in the vibrant appearance. Not all complexes will result in color changes. Some can be colorless depending on the nature of the metal ion and the ligands involved. Generally, transition metals form these complexes, leading to rich colors ranging across the spectrum depending on the composition.

Transition Metal Compounds

Transition metals are unique elements that have electrons filling their d-orbitals. This electron configuration allows them to form various compounds, often resulting in vivid colors. For example, copper in \( ext{CuSO}_4\) can create different colors based on its environment. When CuSO₄ dissolves in water, it forms a blue solution due to the presence of hydrated copper(II) ions which absorb certain wavelengths of light. Unlike most elements, transition metals like iron and copper often exist in several oxidation states, each with distinct colors. Transition metal compounds are important in everyday life. They are used in dyes and pigments and play a crucial role in biological systems and industrial catalysis.

Precipitation Reactions

Precipitation reactions occur when two soluble salts react in solution to form an insoluble solid, known as a precipitate. The color of the precipitate can be used to identify components in a reaction. In the exercise example, mixing \( ext{FeCl}_3\) with sodium ferrocyanide generates a blue precipitate known as Prussian blue. This happens because an insoluble compound forms that comes out of solution. Precipitation reactions serve various purposes in analytical chemistry; they are key for isolating and identifying substances. For example, one can determine the presence of specific ions by the color of the precipitate. This method is highly visual and one of the simplest ways to conduct chemical analysis.