Catalytic Properties & Electrode Potentials

CHEPosition & General Properties

Transition metals and their compounds are widespread catalysts, and their redox behaviour is read from standard electrode potentials. The reasoning ties together variable oxidation states, surface adsorption, and the stability of half-filled configurations.

Electrode potential as a redox-strength measure

M^{n+} + e^- \rightarrow M^{(n-1)+} \quad\Rightarrow\quad \text{more positive } E^\circ = \text{stronger oxidiser}

a high

E^\circ(M^{n+}/M^{(n-1)+})

means the higher state is readily reduced.

Key examples for Mn couples

E^\circ(MnO_4^-/Mn^{2+}) = +1.51\ \text{V} \qquad E^\circ(Mn^{3+}/Mn^{2+}) = +1.57\ \text{V}

the large positive value reflects the stability of half-filled

d^5

Mn^{2+}

Transition metals catalyse reactions because their variable oxidation states let them form unstable intermediate compounds and provide alternative low-energy pathways.
They also catalyse by adsorbing reactants on their surfaces (free valencies / vacant $d$ -orbitals), increasing concentration and weakening bonds at the active sites.
Examples: $Fe$ in the Haber process ( $N_2 + 3H_2 \rightarrow 2NH_3$ ), $V_2O_5$ in the Contact process, $Ni$ in hydrogenation.
$Mn^{2+}$ ( $3d^5$ , half-filled) resists oxidation to $Mn^{3+}$ ( $3d^4$ ) because losing an electron breaks the extra-stable half-filled set; hence $E^\circ(Mn^{3+}/Mn^{2+})$ is large and positive ( $+1.57$ V).
Standard electrode potentials measure oxidising/reducing strength: a more positive $E^\circ$ for $M^{n+}/M^{(n-1)+}$ means the higher state is the stronger oxidising agent.
Irregular $E^\circ(M^{2+}/M)$ values across the $3d$ series reflect the combined effect of ionisation enthalpy, atomisation enthalpy and hydration enthalpy, not a single smooth trend.
Homogeneous catalysis (catalyst in the same phase, e.g. $Mn^{2+}$ autocatalysing the $KMnO_4$ –oxalate reaction) and heterogeneous catalysis (solid metal surface, e.g. Fe in Haber) are both common for $d$ -block species.
$E^\circ(Cu^{2+}/Cu)$ is positive ( $+0.34$ V), unusual for the series, so copper does not displace hydrogen from dilute acids — its high $(IE_1+IE_2)$ and atomisation enthalpy are not offset by hydration enthalpy.
The unexpectedly less negative $E^\circ(Mn^{2+}/Mn)$ and $E^\circ(Zn^{2+}/Zn)$ arise from the extra stability of the $d^5$ and $d^{10}$ product configurations respectively.
Transition-metal catalysts can be regenerated (cycled between oxidation states), which is why a small amount suffices, and they are often poisoned by impurities that block surface sites.
$E^\circ(Fe^{3+}/Fe^{2+}) = +0.77$ V is moderate, consistent with the comparable stability of $d^5$ ( $Fe^{3+}$ ) and $d^6$ ( $Fe^{2+}$ ) and the easy interconversion that underlies Fe-based redox catalysis.

Where the marks go

Saying a more negative $E^\circ(M^{n+}/M^{(n-1)+})$ makes a stronger oxidiser; it is the more positive value that does.
Explaining catalysis only by adsorption and forgetting the variable-oxidation-state (intermediate-compound) mechanism, or vice versa — quote both.
Predicting a smooth trend in $E^\circ(M^{2+}/M)$ across the series; the values are irregular because three enthalpy terms combine differently for each metal.
Claiming copper displaces $H_2$ from dilute acids; its positive $E^\circ(Cu^{2+}/Cu)$ means it does not.
Attributing the high $E^\circ(Mn^{3+}/Mn^{2+})$ to size or charge alone; it is mainly the stability of the half-filled $3d^5$ $Mn^{2+}$ that resists further oxidation.

How the board asks it

Give reasonsvariable oxidation states and surface adsorption
Account for the fact that transition metals and their compounds act as good catalysts. Explain with reference to both the variable-oxidation-state and adsorption mechanisms, citing $Fe$ in the Haber process.
Give reasonsstability of half-filled $3d^5$ configuration
Give reasons: $E^\circ(Mn^{3+}/Mn^{2+}) = +1.57$ V is exceptionally large and positive, whereas $E^\circ(Fe^{3+}/Fe^{2+})$ is only $+0.77$ V.
Give reasonspositive $E^\circ(Cu^{2+}/Cu)$ and enthalpy factors
Although copper is a transition metal, it does not displace hydrogen from dilute acids. Explain this in terms of $E^\circ(Cu^{2+}/Cu) = +0.34$ V and the enthalpy factors involved.
Distinguish $E^\circ$ as a measure of oxidising strength
Given $E^\circ(Mn^{3+}/Mn^{2+}) = +1.57$ V and $E^\circ(Fe^{3+}/Fe^{2+}) = +0.77$ V, state which of $Mn^{3+}$ and $Fe^{3+}$ is the stronger oxidising agent and justify your answer.
Distinguishhomogeneous vs heterogeneous catalysis
Distinguish between homogeneous and heterogeneous catalysis by transition-metal species, giving one $d$ -block example of each ( $Mn^{2+}$ in the $KMnO_4$ –oxalate reaction and $Fe$ in the Haber process).
Identify / classifyindustrial catalyst examples
Name the transition metal or its compound used as a catalyst in (i) the Contact process, (ii) the Haber process, and (iii) the hydrogenation of vegetable oils.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.