Ionisation Enthalpy, Oxidation States — Revision Notes

⚡ 30-Second Revision

Ionisation Enthalpy (IE): — Energy to remove an electron.
$IE_1 < IE_2 < IE_3 dots$ — (Successive IEs always increase).
3d Series $IE_1$ Trend: — Generally increases, but irregular.

- **Cr ($3d^54s^1$), Cu ($3d^{10}4s^1$):** Higher $IE_1$ due to stable d-configs after $4s^1$ removal. - **Mn ($3d^54s^2$):** Exceptionally high $IE_3$ ($Mn^{2+}$ is $3d^5$). - **Zn ($3d^{10}4s^2$):** Exceptionally high $IE_3$ ($Zn^{2+}$ is $3d^{10}$).

Down a Group IE: — 5d > 4d due to Lanthanoid Contraction.
Oxidation States (OS): — Hypothetical charge.
Transition Metals: — Variable OS due to small energy difference between $(n-1)d$ and $ns$ orbitals.
Max OS Trend: — Increases up to Mn (+7), then decreases.
Stable OS: — $d^0, d^5, d^{10}$ configurations are highly stable (e.g., $Sc^{3+}$ ($d^0$), $Mn^{2+}$ ($d^5$), $Zn^{2+}$ ($d^{10}$)).
Higher OS Stability: — Favored by highly electronegative ligands (O, F).
Disproportionation: — Element in intermediate OS simultaneously oxidizes and reduces (e.g., $2Cu^+ \rightarrow Cu^{2+} + Cu$).

2-Minute Revision

Ionisation enthalpy (IE) is the energy needed to remove an electron. For transition metals, successive IEs always increase. The first IE ($IE_1$) across the 3d series generally increases but shows irregularities.

Chromium ($3d^54s^1$) and Copper ($3d^{10}4s^1$) have slightly higher $IE_1$ because removing their single $4s$ electron leaves a stable d-configuration. Manganese ($3d^54s^2$) has an exceptionally high *third* IE ($IE_3$) because $Mn^{2+}$ has a stable half-filled $3d^5$ configuration.

Similarly, Zinc ($3d^{10}4s^2$) has a very high $IE_3$ due to the stable $3d^{10}$ configuration of $Zn^{2+}$. Down a group, 5d elements often have higher IEs than 4d elements due to the lanthanoid contraction, which increases the effective nuclear charge.

Transition metals are famous for their variable oxidation states. This is because the energy difference between their $(n-1)d$ and $ns$ orbitals is very small, allowing electrons from both subshells to participate in bonding.

The maximum oxidation state increases from Sc to Mn (+7) and then decreases. Stable oxidation states often correspond to $d^0$, $d^5$, or $d^{10}$ configurations. Higher oxidation states are stabilized by highly electronegative elements like oxygen and fluorine.

Some intermediate oxidation states, like $Cu^+$, can undergo disproportionation in aqueous solution, forming both higher ($Cu^{2+}$) and lower ($Cu$) oxidation states, driven by the greater stability of $Cu^{2+}$ due to its high hydration enthalpy.

5-Minute Revision

Let's quickly review the critical aspects of ionisation enthalpy and oxidation states for transition elements, essential for NEET. Ionisation Enthalpy (IE) is the energy to remove an electron. Remember that successive IEs ($IE_1, IE_2, IE_3 dots$) always increase.

For the 3d series, the $IE_1$ trend is generally increasing but with key irregularities. For instance, Chromium ($[Ar]3d^54s^1$) and Copper ($[Ar]3d^{10}4s^1$) exhibit slightly higher $IE_1$ values because removing their single $4s$ electron leads to a stable half-filled ($3d^5$) or fully-filled ($3d^{10}$) d-subshell, respectively.

A crucial point is the exceptionally high *third* ionisation enthalpy ($IE_3$) for Manganese ($[Ar]3d^54s^2$) and Zinc ($[Ar]3d^{10}4s^2$). For Mn, $Mn^{2+}$ is $[Ar]3d^5$, a very stable half-filled configuration, making the removal of the third electron extremely difficult.

For Zn, $Zn^{2+}$ is $[Ar]3d^{10}$, a highly stable fully-filled configuration, making $IE_3$ prohibitively high. When moving down a group, the 5d transition elements often have higher IEs than their 4d counterparts, a direct consequence of the lanthanoid contraction which increases the effective nuclear charge.

Oxidation States are a defining characteristic of transition metals. They exhibit variable oxidation states because the energy difference between their $(n-1)d$ and $ns$ orbitals is very small, allowing a varying number of electrons from both subshells to participate in bonding.

The range of oxidation states generally increases from Scandium to Manganese (reaching $+7$ in $MnO_4^-$) and then decreases. The stability of a particular oxidation state is heavily influenced by achieving stable electronic configurations like $d^0$ (e.

g., $Sc^{3+}$), $d^5$ (e.g., $Mn^{2+}$, $Fe^{3+}$), or $d^{10}$ (e.g., $Zn^{2+}$). Higher oxidation states are typically stabilized by highly electronegative ligands such as oxygen and fluorine (e.g., $Cr_2O_7^{2-}$).

An important reaction to remember is disproportionation, where an element in an intermediate oxidation state simultaneously oxidizes and reduces. The classic example is $Cu^+$ in aqueous solution, which disproportionates into $Cu^{2+}$ and $Cu$.

This is driven by the significantly higher hydration enthalpy of $Cu^{2+}$ compared to $Cu^+$, making $Cu^{2+}$ more stable in water. Always link these properties back to the electronic configurations and the relative energy levels of the orbitals.

Prelims Revision Notes

Ionisation Enthalpy (IE)

Definition: — Energy to remove an electron from an isolated gaseous atom/ion.
Successive IEs: — $IE_1 < IE_2 < IE_3 dots$ always.
3d Series $IE_1$ Trend: — Generally increases from Sc to Zn, but with irregularities.

* Reasons for irregularities: Interplay of increasing nuclear charge, d-electron shielding, and electron-electron repulsion. * **Cr ($[Ar]3d^54s^1$):** Higher $IE_1$ because removing $4s^1$ leaves stable $3d^5$.

* **Cu ($[Ar]3d^{10}4s^1$):** Higher $IE_1$ because removing $4s^1$ leaves stable $3d^{10}$. * **Mn ($[Ar]3d^54s^2$):** Exceptionally high $IE_3$ because $Mn^{2+}$ is $[Ar]3d^5$ (stable half-filled).

* **Zn ($[Ar]3d^{10}4s^2$):** Exceptionally high $IE_3$ because $Zn^{2+}$ is $[Ar]3d^{10}$ (stable fully-filled).

Down a Group (e.g., 3d to 4d to 5d):

* Generally decreases due to increased size. * Exception: 5d elements often have higher $IE_1$ than 4d elements (e.g., Hf vs Zr) due to Lanthanoid Contraction (poor shielding by 4f electrons leads to increased effective nuclear charge for 5d elements).

Oxidation States (OS)

Definition: — Hypothetical charge if bonds were ionic.
Variable OS: — Characteristic of transition metals.

* Reason: Small energy difference between $(n-1)d$ and $ns$ orbitals, allowing both to participate in bonding.

Common OS in 3d Series:

* Sc: +3 only ($d^0$). * Ti: +2, +3, +4 (most stable +4). * V: +2, +3, +4, +5 (most stable +5). * Cr: +2, +3, +6 (common +3, +6 in $CrO_4^{2-}$). * Mn: +2 to +7 (widest range; +2, +7 common). * Fe: +2, +3 (common +3 due to $d^5$). * Co: +2, +3 (common +2 in simple salts, +3 in complexes). * Ni: +2 (most common), +3, +4 (in complexes). * Cu: +1, +2 (common +2 in aqueous solution). * Zn: +2 only ($d^{10}$).

Maximum OS Trend: — Increases from Sc (+3) to Mn (+7), then decreases.
Stability of OS:

* Electronic Configuration: $d^0, d^5, d^{10}$ are highly stable. * Ligands: Higher OS stabilized by highly electronegative ligands (O, F) (e.g., $MnO_4^-$, $Cr_2O_7^{2-}$). * Aqueous Stability: Influenced by hydration enthalpy (e.g., $Cu^{2+}$ more stable than $Cu^+$ in water).

Disproportionation: — An element in an intermediate OS simultaneously oxidizes and reduces.

* Example: $2Cu^+(aq) \rightarrow Cu^{2+}(aq) + Cu(s)$. Driven by high hydration enthalpy of $Cu^{2+}$.

Vyyuha Quick Recall

In Oxidation, Variable Charges Make Fun Compounds, Never Consistently Zero.

Ionisation Enthalpy: Remember the general increase but focus on Cr, Cu (higher $IE_1$) and Mn, Zn (higher $IE_3$).
Oxidation States: Variable for transition metals.
Charges: Max OS increases up to Mn (+7).
Make: Mn has the widest range.
Fun: Fe commonly +2, +3.
Compounds: Cu shows +1, +2; +1 disproportionates.
Never: Ni mostly +2.
Consistently: Co mostly +2, +3.
Zero: Zn only +2 (stable $d^{10}$). Sc only +3 (stable $d^0$).

This mnemonic helps recall the key elements and their characteristic oxidation state patterns and IE exceptions.