Problem 12 - Chemistry in 30 days

Question

Which of the following compounds on reacting with \(\mathrm{NaOH}\) and \(\mathrm{H}_{2} \mathrm{O}_{2}\) gives a yellow colour? (a) \(\mathrm{Cr}(\mathrm{OH})_{3}\) (b) \(\mathrm{Zn}(\mathrm{OH})_{2}\) (c) \(\mathrm{Al}(\mathrm{OH})_{3}\) (d) \(\mathrm{Fe}(\mathrm{OH})_{3}\)

Step-by-Step Solution

Answer

\( \mathrm{Cr(OH)}_{3} \) gives a yellow color when reacted with \( \mathrm{NaOH} \) and \( \mathrm{H}_{2} \mathrm{O}_{2} \).

Step 1: Identify the Reactants

The question asks which compound gives a yellow color when reacting with sodium hydroxide (\(\mathrm{NaOH}\)) and hydrogen peroxide (\(\mathrm{H}_{2} \mathrm{O}_{2}\)). We must understand the role of these reactants in forming colored compounds.

Step 2: Reactivity with NaOH and H₂O₂

Compounds that can oxidize when treated with \(\mathrm{NaOH}\) and \(\mathrm{H}_{2} \mathrm{O}_{2}\) may change color. Specifically, chromate ions \(\text{(yellow in color)}\), are formed when chromium (III) hydroxide \(\mathrm{Cr(OH)}_{3}\) reacts with these agents.

Step 3: Evaluate Each Option

- \(\mathrm{Cr(OH)}_{3}\): Reacts with \(\mathrm{NaOH}\) and \(\mathrm{H}_{2} \mathrm{O}_{2}\) to form yellow chromate ions.- \(\mathrm{Zn(OH)}_{2}\): Does not form any colored solutions.- \(\mathrm{Al(OH)}_{3}\): Does not produce colored species in reaction.- \(\mathrm{Fe(OH)}_{3}\): Tends to produce rust-colored compounds but not yellow.

Step 4: Select the Correct Compound

The compound \(\mathrm{Cr(OH)}_{3}\) forms yellow chromate ions on reacting with \(\mathrm{NaOH}\) and \(\mathrm{H}_{2} \mathrm{O}_{2}\). Therefore, it is the compound that gives a yellow color.

Key Concepts

Sodium Hydroxide Reactivity

Sodium hydroxide, commonly known as lye or caustic soda, is a strong base used in many chemical reactions. It is highly reactive and can cause significant changes in the chemical structures it interacts with. When it comes into contact with metal hydroxides like \[\mathrm{Cr(OH)}_{3}\], sodium hydroxide can dissolve these hydroxides, forming soluble complexes.

This characteristic is particularly important when considering its reaction with chromium compounds. In such reactions, sodium hydroxide helps to dissolve and destabilize the existing hydroxide compound, setting the stage for further reactions like oxidation.

Oxidation with Hydrogen Peroxide

Hydrogen peroxide \((\mathrm{H}_2\mathrm{O}_2)\) is a well-known oxidizing agent. It readily takes part in chemical reactions that involve electron transfer, meaning it can help convert substances to a higher oxidation state by providing oxygen atoms. In the context of the exercise, hydrogen peroxide interacts with the soluble chromium complexes created by sodium hydroxide.
When these react, hydrogen peroxide oxidizes \(\mathrm{Cr(OH)}_{3}\), which cues the color change associated with the formation of chromate ions. These ions are distinctive because of their bright yellow hue. This oxidation process is crucial for forming the colored chromate ions from otherwise colorless or differently colored substances.

Chromium Compounds

Chromium, a transition metal, can form a variety of compounds, each with diverse properties and colors. The state and appearance of these compounds depend heavily on the oxidation states of chromium.

• The \[\mathrm{Cr(OH)}_{3}\] begins the reaction in a chromium III hydroxide form, which is relatively stable and insoluble at room temperature.
• However, its reactivity alters significantly when exposed to strong bases such as sodium hydroxide coupled with the oxidizing influence of hydrogen peroxide, leading to the formation of chromate ions.
• The chromate ion \([\mathrm{CrO}_4^{2-}]\) is the vibrant yellow ion that results from these reactions, a visual indicator of its successful formation.

Colored Ion Formation

Color changes in chemical reactions often signify the formation or transformation of ions that can absorb visible light. Such is the case with the \([\mathrm{CrO}_4^{2-}]\) chromate ions, which absorb certain wavelengths of light, reflecting the rest to create a yellow color.

This quality arises because of the interaction between these ions and light, where specific light wavelengths are absorbed. Complex ion formation, like that observed here, often results in distinct colors. The chemistry behind this involves the d-orbitals available in transition metals like chromium. These orbitals allow the electrons to transition between energy levels, absorbing and reflecting specific light wavelengths, resulting in colored compounds.
In practical applications, these visually vibrant reactions are informative. They allow chemists to detect specific ions and gain insight into reaction pathways and material composition just by observing color changes.