Problem 79 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The aqueous solutions of the following salts will be coloured in the case of (a) \(\mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}\) (b) \(\mathrm{LiNO}_{3}\) (c) \(\mathrm{Co}\left(\mathrm{NO}_{3}\right)_{2}\) (d) \(\mathrm{CrCl}_{3}\) (e) Potash alum

Step-by-Step Solution

Answer

Coloured solutions: (c) \(\mathrm{Co}\left(\mathrm{NO}_{3}\right)_{2}\) and (d) \(\mathrm{CrCl}_{3}\).

Step 1: Understanding Colour in Aqueous Solutions

The colour of an aqueous solution generally results from the presence of transition metal ions that have unpaired electrons. These electrons can absorb light and move between different electron levels, often producing colours in the visible spectrum.

Step 2: Identifying Transition Metals

Analyze each salt to determine the presence of transition metals. Transition metals are typically found in the middle of the periodic table (d-block). The salts provided are: (a) \(\mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}\) (b) \(\mathrm{LiNO}_{3}\) (c) \(\mathrm{Co}\left(\mathrm{NO}_{3}\right)_{2}\) (d) \(\mathrm{CrCl}_{3}\) (e) Potash alum. Zinc (Zn) and lithium (Li) are not transition metals. Cobalt (Co) and chromium (Cr) are transition metals. Potash alum contains potassium and aluminum ions, which are not transition metals, but it also includes sulfate ions.

Step 3: Predict Colour Based on Transition Metal Presence

For an aqueous solution to be coloured: - (a) \(\mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}\): Zinc has a full \(d\)-subshell, hence no colour. - (b) \(\mathrm{LiNO}_{3}\): Lithium is not a transition metal, hence no colour.- (c) \(\mathrm{Co}\left(\mathrm{NO}_{3}\right)_{2}\): Cobalt is a transition metal with unpaired electrons, typically resulting in a pink to red colour. - (d) \(\mathrm{CrCl}_{3}\): Chromium is a transition metal with unpaired electrons, typically green or purple in solutions. - (e) Potash alum: Mostly colorless due to lack of transition metals; however, in some conditions, it might show a faint tint, but it is typically considered colourless in standard solutions.

Step 4: Summarizing the Colour of Aqueous Solutions

From the analysis, the salts that will result in coloured solutions due to the presence of transition metals are \(\mathrm{Co}\left(\mathrm{NO}_{3}\right)_{2}\) and \(\mathrm{CrCl}_{3}\) due to the cobalt and chromium ions, respectively.

Key Concepts

Aqueous Solutions

When we dissolve a salt in water, we get an aqueous solution. These solutions often appear colored if they contain certain elements. Transition metals, with their unique electron configurations, frequently produce such colors. This is because transition metals can have unpaired electrons. As light shines on the solution, these unpaired electrons can absorb specific wavelengths of light. This absorption causes the solution to reflect other wavelengths, giving it a distinct color.

Let's consider some examples:

• - Salts like cobalt nitrate contain cobalt ions, which are known to cause a pink to reddish hue.
• - Chromium chloride solutions often appear green or purple.

Understanding why some solutions are colored helps us appreciate the fascinating role of transition metals in chemistry.

Unpaired Electrons

Unpaired electrons are a key concept in understanding colored solutions from transition metals. In simpler terms, electrons exist in pairs within an atom. However, in transition metals, it's common to have electrons that aren't paired. These unpaired electrons are especially important because they can jump between different energy levels.

When certain wavelengths of light hit the solution, these electrons absorb energy and move up to a different level. This process is called electronic transition. After absorbing energy, what remains visible to our eyes is the complementary color of the absorbed wavelength.

• Cobalt typically has unpaired electrons, leading to its distinct color in solutions.
• Chromium too, with its various oxidation states, exhibits complex coloring due to unpaired electrons.

Pondering this concept gives insight into how intriguing transition metals really are!

Periodic Table

The periodic table is not just a chart; it is a powerful tool for understanding element properties. Transition metals, the stars of our discussion here, sit prominently in the d-block of the periodic table. Typical metals found in this section have partially filled d-orbitals.

Recognizing these metals helps in predicting solution colors. Transition metals are known for having multiple oxidation states and complex ion formations. These properties influence chemical behavior and the physical appearance of their solutions.

• Zinc, although in the d-block, is not considered a transition metal because its d-orbital is full.
• Cobalt and chromium, on the other hand, are classic examples of transition metals with partially filled d-orbitals.

When working with problems about colored solutions, referring to the periodic table is an invaluable strategy for students.