Define a transition element, explain the existence of variable oxidation states from the close energies of the 3d and 4s subshells, and describe their use as catalysts and the role of variable oxidation states in catalysis

What this dot point is asking

SEAB wants you to define a transition element precisely, explain why these elements show variable oxidation states in terms of the close energies of the $3d$ and $4s$ subshells, and describe how that variability makes them effective catalysts (both homogeneous and heterogeneous). The definition and the catalysis-by-oxidation-state-change explanation are dependable Paper 2 questions.

The answer

Defining a transition element

A transition element is a $d$-block element that forms at least one stable ion with a partially filled $d$ subshell. Two $d$-block elements fail this test:

• Scandium: its common ion $\text{Sc}^{3+}$ is $[\text{Ar}]\,3d^0$ (empty $d$).
• Zinc: its only ion $\text{Zn}^{2+}$ is $[\text{Ar}]\,3d^{10}$ (full $d$).

So Sc and Zn are not typical transition elements and lack the characteristic properties.

The characteristic properties

Transition elements show:

1. Variable oxidation states.
2. Coloured compounds and ions.
3. Catalytic activity.
4. The formation of complex ions.

All four stem from the partially filled $3d$ subshell.

Why variable oxidation states arise

The $3d$ and $4s$ subshells are very close in energy. When a transition element ionises, the $4s$ electrons are removed first, then a variable number of $3d$ electrons can be removed or used in bonding with only small energy changes. Because successive ionisation energies rise gently across the $3d$ electrons, several oxidation states are energetically accessible.

For example, iron commonly shows $+2$ and $+3$, manganese ranges from $+2$ to $+7$ (as in $\text{MnO}_4^-$), and chromium shows $+3$ and $+6$ (as in $\text{Cr}_2\text{O}_7^{2-}$).

Catalysis through variable oxidation states

A catalyst provides an alternative route of lower activation energy. Transition metals and their ions do this in two ways:

Homogeneous catalysis (catalyst in the same phase). A transition-metal ion cycles between two oxidation states, being oxidised in one step and reduced back in the next, so it is regenerated. The classic example is $\text{Fe}^{2+}$ or $\text{Fe}^{3+}$ catalysing the otherwise slow reaction between the two anions $\text{S}_2\text{O}_8^{2-}$ and $\text{I}^-$.

Heterogeneous catalysis (catalyst in a different phase, usually a solid surface). The metal uses its partially filled $d$ orbitals to adsorb reactant molecules onto its surface, weakening their bonds and bringing them close together. Examples: iron in the Haber process, vanadium(V) oxide in the Contact process, and nickel in the hydrogenation of alkenes.

Examples in context

Example 1. Autocatalysis in the manganate(VII)-ethanedioate titration. The reaction of $\text{MnO}_4^-$ with ethanedioate is slow at first but speeds up as $\text{Mn}^{2+}$ forms, because $\text{Mn}^{2+}$ catalyses the reaction by cycling between oxidation states. This autocatalysis is a favourite SEAB context for testing the link between variable oxidation states and homogeneous catalysis.

Example 2. Catalytic converters. Platinum, palladium and rhodium in a car's catalytic converter heterogeneously catalyse the conversion of carbon monoxide and nitrogen oxides to carbon dioxide and nitrogen. The metals adsorb the gases on their surfaces using $d$ orbitals, lowering the activation energy. SEAB uses this everyday application to test heterogeneous catalysis.

Try this

Q1. Write the electronic configurations of $\text{Mn}^{2+}$ and $\text{Cr}^{3+}$. [2 marks]

• Cue. $\text{Mn}^{2+} = [\text{Ar}]\,3d^5$; $\text{Cr}^{3+} = [\text{Ar}]\,3d^3$.

Q2. Explain why transition elements can show several oxidation states. [2 marks]

• Cue. The $3d$ and $4s$ subshells are close in energy, so a variable number of $3d$ electrons can be lost after the $4s$, with small energy differences.

Q3. Name the catalyst and state its type (homogeneous or heterogeneous) for (a) the Contact process and (b) the $\text{S}_2\text{O}_8^{2-}$ + $\text{I}^-$ reaction. [2 marks]

• Cue. (a) $\text{V}_2\text{O}_5$, heterogeneous. (b) $\text{Fe}^{2+}$ (or $\text{Fe}^{3+}$), homogeneous.