CHEMCA Free Chemistry Notes for JEE NEET Chemistry MadeEasy: Reactivity & Interstitial Compounds

Transition metals exhibit variable reactivity primarily governed by their ionization enthalpies and hydration enthalpies. Additionally, their crystal lattices allow small atoms to occupy voids, forming unique Interstitial Compounds.

1. Standard Electrode Potentials ($M^{2+}/M$)

Trends and Anomalies

The standard electrode potential ($E^\circ$) determines the reducing power of a metal. It depends on three factors:

$$ M(s) \xrightarrow{\Delta_{sub}H} M(g) \xrightarrow{IE_1+IE_2} M^{2+}(g) \xrightarrow{\Delta_{hyd}H} M^{2+}(aq) $$

General Trend: $E^\circ$ values become less negative across the series (Ti to Zn), indicating decreasing reducing power.

The Copper Anomaly ($E^\circ = +0.34V$):
Copper is the only 3d metal with a positive $E^\circ$.
Reason: The high energy required to transform $Cu(s)$ to $Cu^{2+}(g)$ (high Enthalpy of Atomization and Ionization) is not balanced by its Hydration Enthalpy.
Consequence: Cu does not liberate $H_2$ from acids.

Stability of Mn and Zn: Values for Mn (-1.18V) and Zn (-0.76V) are more negative than expected.
Reason: Stability of half-filled $d^5$ ($Mn^{2+}$) and fully-filled $d^{10}$ ($Zn^{2+}$) configurations.

2. Trends in $M^{3+}/M^{2+}$ Potentials

Relative Stability of Oxidation States

A low (or negative) value of $E^\circ$ indicates that the $M^{3+}$ state is stable. A high positive value indicates that $M^{3+}$ is a strong oxidizing agent (wants to reduce to $M^{2+}$).

Couple	Value	Implication
$Sc^{3+}/Sc^{2+}$	Very Low	$Sc^{3+}$ ($d^0$) is highly stable. Sc does not form +2.
$Fe^{3+}/Fe^{2+}$	+0.77 V (Low)	$Fe^{3+}$ ($d^5$) is relatively stable.
$Mn^{3+}/Mn^{2+}$	+1.57 V (High)	$Mn^{3+}$ is a strong oxidizer because $Mn^{2+}$ ($d^5$) is much more stable.

3. Chemical Reactivity

General Reactivity

Most transition metals are sufficiently electropositive to dissolve in mineral acids, liberating $H_2$.
Passivity: Metals like Titanium (Ti) and Chromium (Cr) are thermodynamically reactive but kinetically inert due to the formation of a protective oxide layer on their surface.
Reducing Character: Decreases along the series ($Ti > V > Cr > Mn > Fe > Co > Ni > Cu$).

4. Interstitial Compounds

Trapped Atoms

Transition metals form interstitial compounds when small atoms like Hydrogen (H), Carbon (C), Boron (B), or Nitrogen (N) get trapped inside the crystal lattice voids (interstices) of the metal.

Examples: $TiC, Mn_4N, Fe_3H, VH_{0.56}, TiH_{1.7}$

Characteristics

Non-stoichiometric: Their composition does not correspond to normal oxidation states (e.g., $VH_{0.56}$).
High Melting Points: Higher than those of pure metals.
Hardness: They are extremely hard (some borides approach diamond hardness).
Conductivity: They retain metallic conductivity.
Inertness: They are chemically inert.

Reactivity & Interstitial Compounds | chemca

Reactivity, $E^\circ$ Values & Interstitial Compounds