Question

The number of metal ions among the following which are coloured and have tendency to oxidize are

$\text{Ti}^{2+}$, $\text{V}^{2+}$, $\text{Cr}^{2+}$, $\text{Mn}^{2+}$, $\text{Fe}^{2+}$, $\text{Co}^{2+}$

Answer 1

4

Answer 2

To determine the number of metal ions that are both coloured and have a tendency to oxidize, we will analyze each ion based on its electronic configuration and standard electrode potential.

1. Coloured Ions: Transition metal ions are generally coloured if they have a partially filled d-subshell (i.e., d$^1$ to d$^9$ configurations), which allows for d-d electronic transitions upon absorption of visible light. Ions with d$^0$ or d$^{10}$ configurations are typically colourless.

Let's determine the electronic configuration for each ion:

• Ti (Z=22): [Ar] 3d$^2$ 4s$^2$. $\text{Ti}^{2+}$ is [Ar] 3d$^2$. (Partially filled d-subshell, so coloured).
• V (Z=23): [Ar] 3d$^3$ 4s$^2$. $\text{V}^{2+}$ is [Ar] 3d$^3$. (Partially filled d-subshell, so coloured).
• Cr (Z=24): [Ar] 3d$^5$ 4s$^1$. $\text{Cr}^{2+}$ is [Ar] 3d$^4$. (Partially filled d-subshell, so coloured).
• Mn (Z=25): [Ar] 3d$^5$ 4s$^2$. $\text{Mn}^{2+}$ is [Ar] 3d$^5$. (Partially filled d-subshell, so coloured).
• Fe (Z=26): [Ar] 3d$^6$ 4s$^2$. $\text{Fe}^{2+}$ is [Ar] 3d$^6$. (Partially filled d-subshell, so coloured).
• Co (Z=27): [Ar] 3d$^7$ 4s$^2$. $\text{Co}^{2+}$ is [Ar] 3d$^7$. (Partially filled d-subshell, so coloured).

All the given ions have partially filled d-subshells (from d$^2$ to d$^7$), so all of them are coloured.

2. Tendency to Oxidize: An ion has a tendency to oxidize if it acts as a reducing agent, meaning it readily loses electrons to get oxidized to a higher oxidation state. This can be evaluated by looking at the standard electrode potentials (E$^\circ$) for the M$^{3+}$/M$^{2+}$ couple. A more negative or less positive E$^\circ$ value indicates a stronger tendency for the M$^{2+}$ ion to be oxidized to M$^{3+}$.

Let's examine the standard reduction potentials for the M$^{3+}$/M$^{2+}$ couple:

• $\text{Ti}^{3+}$/$\text{Ti}^{2+}$: E$^\circ = -0.37$ V. Since the potential is negative, $\text{Ti}^{2+}$ is a good reducing agent and readily oxidizes to $\text{Ti}^{3+}$ (or even $\text{Ti}^{4+}$).
• $\text{V}^{3+}$/$\text{V}^{2+}$: E$^\circ = -0.26$ V. Since the potential is negative, $\text{V}^{2+}$ is a good reducing agent and readily oxidizes to $\text{V}^{3+}$ (or higher states like $\text{V}^{4+}$ or $\text{V}^{5+}$).
• $\text{Cr}^{3+}$/$\text{Cr}^{2+}$: E$^\circ = -0.41$ V. This is a highly negative potential, indicating that $\text{Cr}^{2+}$ is a very strong reducing agent and is easily oxidized to $\text{Cr}^{3+}$. The stability of $\text{Cr}^{3+}$ (3d$^3$) contributes to this.
• $\text{Mn}^{3+}$/$\text{Mn}^{2+}$: E$^\circ = +1.57$ V. This highly positive potential indicates that $\text{Mn}^{3+}$ is a strong oxidizing agent, meaning $\text{Mn}^{2+}$ is very stable (due to its half-filled 3d$^5$ configuration) and does not readily oxidize to $\text{Mn}^{3+}$.
• $\text{Fe}^{3+}$/$\text{Fe}^{2+}$: E$^\circ = +0.77$ V. This positive potential indicates that $\text{Fe}^{2+}$ can be oxidized to $\text{Fe}^{3+}$ by moderate oxidizing agents. So, it has a tendency to oxidize.
• $\text{Co}^{3+}$/$\text{Co}^{2+}$: E$^\circ = +1.82$ V. This highly positive potential indicates that $\text{Co}^{3+}$ is an extremely strong oxidizing agent, meaning $\text{Co}^{2+}$ is very stable and does not readily oxidize to $\text{Co}^{3+}$ in aqueous solution.

Summary:

Ion	Electronic Configuration	Coloured?	Tendency to Oxidize (Reducing Agent)?	Meets both criteria?
$\text{Ti}^{2+}$	3d$^2$	Yes	Yes (E$^\circ$ = -0.37 V)	Yes
$\text{V}^{2+}$	3d$^3$	Yes	Yes (E$^\circ$ = -0.26 V)	Yes
$\text{Cr}^{2+}$	3d$^4$	Yes	Yes (E$^\circ$ = -0.41 V)	Yes
$\text{Mn}^{2+}$	3d$^5$	Yes	No (E$^\circ$ = +1.57 V)	No
$\text{Fe}^{2+}$	3d$^6$	Yes	Yes (E$^\circ$ = +0.77 V)	Yes
$\text{Co}^{2+}$	3d$^7$	Yes	No (E$^\circ$ = +1.82 V)	No

The metal ions that are both coloured and have a tendency to oxidize are $\text{Ti}^{2+}$, $\text{V}^{2+}$, $\text{Cr}^{2+}$, and $\text{Fe}^{2+}$. The number of such metal ions is 4.