6.2.7 Catalysis

Catalysis by Transition Metals

Transition metals and their compounds are highly effective catalysts due to their variable oxidation states, which allow them to easily gain or lose electrons. This enables them to catalyze redox reactions either as heterogeneous or homogeneous catalysts.

Heterogeneous Catalysts

In heterogeneous catalysis, the catalyst is in a different phase from the reactants. The reaction takes place on the catalyst's surface at specific active sites.

Mechanism of Action:

Adsorption: Reactants are adsorbed onto the catalyst's surface.
Bond Weakening: Bonds within the reactant molecules weaken, facilitating the reaction.
Favourable Orientation: Reactants are held in an optimal orientation for the reaction.
Desorption: Products are released, freeing up active sites. Support Medium: To increase efficiency, catalysts are often spread over a support medium, maximizing surface area and reducing costs.

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Example: The Contact Process

Catalyst: Vanadium(V) oxide ( $\text{V}_2\text{O}_5$ )

Reaction:

2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3

Mechanism:

Step 1:

\text{SO}_2 + \text{V}_2\text{O}_5 \rightarrow \text{SO}_3 + \text{V}_2\text{O}_4

Step 2:

\text{V}_2\text{O}_4 + \text{O}_2 \rightarrow \text{V}_2\text{O}_5

In the Contact Process, $\text{V}_2\text{O}_5$ reduces to $\text{V}_2\text{O}_4$ and is then regenerated, facilitating continuous conversion of $\text{SO}_2$ to $\text{SO}_3$

Catalyst Poisoning

Impurities in reactants can poison a heterogeneous catalyst by blocking its active sites. This reduces the catalyst's efficiency and increases replacement costs.

Homogeneous Catalysts

A homogeneous catalyst is in the same phase as the reactants, usually in solution. These catalysts work by forming an intermediate species that reacts to form the products and regenerates the catalyst.

Intermediate Formation: The catalyst reacts with one reactant to form an intermediate.
Product Formation: The intermediate then reacts with another reactant to form the product and regenerates the catalyst.

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Example: Catalysis by $\text{Fe}^{2+}$ Ions

Reaction:

2\text{I}^- + \text{S}_2\text{O}_8^{2-} \rightarrow \text{I}_2 + 2\text{SO}_4^{2-}

This reaction is slow due to the repulsion between the negatively charged ions.

Adding $\text{Fe}^{2+}$ ions accelerates the reaction as follows:

Step 1:

2\text{Fe}^{2+} + \text{S}_2\text{O}_8^{2-} \rightarrow 2\text{Fe}^{3+} + 2\text{SO}_4^{2-}

Step 2:

2\text{Fe}^{3+} + 2\text{I}^- \rightarrow 2\text{Fe}^{2+} + \text{I}_2

Explanation:

$\text{Fe}^{2+}$ and $\text{Fe}^{3+}$ facilitate electron transfer between $\text{I}^-$ and $\text{S}_2\text{O}_8^{2-}$ , speeding up the reaction.

Autocatalysis: Self-Catalysis by a Product

In autocatalysis, one of the reaction products acts as a catalyst.

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Example: $\text{Mn}^{2+}$ Autocatalysis

Reaction:

2\text{MnO}_4^- + 16\text{H}^+ + 5\text{C}_2\text{O}_4^{2-} \rightarrow 2\text{Mn}^{2+} + 8\text{H}_2\text{O} + 10\text{CO}_2

Initially slow due to repulsion between $\text{MnO}_4^-$ and $\text{C}_2\text{O}_4^{2-}$ , the reaction speeds up as $\text{Mn}^{2+}$ ions are produced:

Step 1:

4\text{Mn}^{2+} + \text{MnO}_4^- + 8\text{H}^+ \rightarrow 5\text{Mn}^{3+} + 4\text{H}_2\text{O}

Step 2:

2\text{Mn}^{3+} + \text{C}_2\text{O}_4^{2-} \rightarrow 2\text{CO}_2 + 2\text{Mn}^{2+}

Explanation:

The $\text{Mn}^{2+}$ product catalyzes its own formation by accelerating the reduction of $\text{MnO}_4^-$ and oxidation of $\text{C}_2\text{O}_4^{2-}$

This effect demonstrates autocatalysis, where the reaction rate increases as more $\text{Mn}^{2+}$ accumulates, helping to overcome initial ion repulsion.

Catalysis Simplified Revision Notes for A-Level AQA Chemistry

6.2.7 Catalysis

Catalysis by Transition Metals

Heterogeneous Catalysts

Example: The Contact Process

Catalyst Poisoning

Homogeneous Catalysts

Example: Catalysis by $\text{Fe}^{2+}$ Ions

Autocatalysis: Self-Catalysis by a Product

Example: $\text{Mn}^{2+}$ Autocatalysis

500K+ Students Use These Powerful Tools to Master Catalysis For their A-Level Exams.

Other Revision Notes related to Catalysis you should explore

Catalysis Simplified Revision Notes for A-Level AQA Chemistry

6.2.7 Catalysis

Catalysis by Transition Metals

Heterogeneous Catalysts

Example: The Contact Process

Catalyst Poisoning

Homogeneous Catalysts

Example: Catalysis by Fe2+\text{Fe}^{2+}Fe2+ Ions

Autocatalysis: Self-Catalysis by a Product

Example: Mn2+\text{Mn}^{2+}Mn2+ Autocatalysis

500K+ Students Use These Powerful Tools to Master Catalysis For their A-Level Exams.

Other Revision Notes related to Catalysis you should explore

Example: Catalysis by $\text{Fe}^{2+}$ Ions

Example: $\text{Mn}^{2+}$ Autocatalysis