Animated Solution for Chemistry - d and f-Block Elements: In the given chemical reaction, colours of the ${\text{Fe}}^{2+}$ and ${\text{Fe}}^{3+}$ ions, are respectively $5{\text{Fe}}^{2+}+{\text{MnO}}_4^{-}+8{\text{H}}^{+}⟶{\text{Mn}}^{2+}+4{\text{H}}_2\text{O}+5{\text{Fe}}^{3+}$

Have you ever wondered why chemistry labs are filled with such vibrant, colorful solutions? The secret often lies within the d-block elements! In this problem, we are looking at a classic redox titration reaction between ferrous ions and permanganate ions in an acidic medium.

This equation might look like a mouthful, but it tells a beautiful story of electron transfer. The permanganate ion (${\text{MnO}}_4^{-}$) acts as a powerful oxidizing agent. It aggressively pulls electrons away from the iron, forcing it to jump from a $+2$ oxidation state to a $+3$ oxidation state.

A Pale Green

Let's focus on our starting material, the ferrous ion (${\text{Fe}}^{2+}$). When you dissolve a ferrous salt, like Mohr's salt or ferrous sulfate, in water, you are greeted with a characteristic pale green solution.

Why green? It all comes down to the electronic configuration. The ${\text{Fe}}^{2+}$ ion has a $[\text{Ar}]3{\text{d}}^6$ configuration. When water molecules surround this ion, they split the energy levels of those d-orbitals. As light passes through the solution, the electrons absorb specific wavelengths to jump between these split levels (a phenomenon known as d-d transitions). The light that isn't absorbed—the light that reaches our eyes—appears green.

A Shift to Yellow

As the reaction proceeds, the ${\text{Fe}}^{2+}$ loses an electron and transforms into the ferric ion (${\text{Fe}}^{3+}$). This tiny loss of a single electron has a dramatic visual effect!

The new configuration is $[\text{Ar}]3{\text{d}}^5$. Because the number of electrons has changed, the energy gap between the split d-orbitals also changes. The ion now absorbs a different set of wavelengths from the visible spectrum. Consequently, aqueous solutions containing ${\text{Fe}}^{3+}$ ions typically exhibit a yellow or yellowish-brown color.

By simply tracking the oxidation states and recalling our laboratory observations, we can confidently determine the colors. The reactant ${\text{Fe}}^{2+}$ is green, and the product ${\text{Fe}}^{3+}$ is yellow.

Therefore, the correct sequence of colors is green, yellow, which perfectly matches option (d).

Bonus Insight: While the question focuses on iron, don't forget the star of the show—the permanganate ion! It starts as a brilliant, deep purple and turns practically colorless as it reduces to ${\text{Mn}}^{2+}$. This dramatic color change is exactly why ${\text{KMnO}}_4$ is used as a self-indicator in redox titrations. No extra indicator drops needed!