Reactivity, $E^\circ$ Values & Interstitial Compounds
Understanding Thermodynamic Stability and Crystal Defects in Transition Metals.
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:
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.
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.
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.
Knowledge Check
Test your understanding of Reactivity & Compounds
No comments:
Post a Comment