Catalysts (Leaving Cert Chemistry): Revision Notes

Catalysts

What are catalysts?

A catalyst is a substance that speeds up the rate of a chemical reaction without being permanently changed or consumed in the process. Think of a catalyst as a helpful friend who makes things happen faster but doesn't get worn out in the process.

General properties of catalysts

Catalysts share several key characteristics that make them so useful in chemistry:

Active participation with recovery

Catalysts take an active role in chemical reactions by providing alternative reaction pathways. However, they emerge from the reaction completely unchanged in their chemical composition. This means the same catalyst molecule can be used over and over again to speed up many reaction cycles.

Specificity

Catalysts are highly selective - they typically work for one particular reaction or a small group of very similar reactions. For example, the enzyme catalase found in liver cells specifically catalyses the breakdown of hydrogen peroxide into water and oxygen, but won't catalyse other types of reactions.

Small amounts needed

Only tiny quantities of catalyst are required to have a significant effect on reaction rates. This makes catalysts very economical to use in industrial processes.

Biological catalysts (enzymes)

Living organisms use special protein catalysts called enzymes. These biological catalysts are extremely specific and efficient. The enzyme catalase is so effective that it can break down hydrogen peroxide faster than almost any other known catalyst.

Types of catalysis

Chemists have identified three main types of catalysis based on the physical states of the reactants and catalyst.

Homogeneous catalysis

Homogeneous catalysis occurs when the catalyst and all reactants exist in the same phase (usually liquid or gas). The word "homogeneous" means "same throughout" - like how milk looks the same throughout after being mixed.

In homogeneous catalysis, there's no physical boundary between the catalyst and reactants since they're all mixed together uniformly.

Heterogeneous catalysis

Heterogeneous catalysis happens when the catalyst exists in a different physical phase from the reactants. This type of catalysis involves a clear boundary between the catalyst surface and the reactants.

Key features of heterogeneous catalysis:

• The catalyst is usually a solid
• Reactants are typically gases or liquids
• Reactions occur at the catalyst surface
• Active sites on the catalyst surface are where reactions take place

Mechanisms of catalysis

Scientists have developed two main theories to explain how catalysts actually work at the molecular level.

The intermediate formation theory

This theory suggests that catalysts work by forming temporary intermediate compounds with the reactants. The process happens in steps:

This theory works well for explaining homogeneous catalysis where molecules can mix freely and form these temporary intermediate compounds.

The surface adsorption theory

This theory explains heterogeneous catalysis by describing what happens at the catalyst surface:

The finely divided catalyst provides a large surface area with many active sites, making the process very efficient.

Autocatalysis

Autocatalysis is a special type of catalysis where one of the products of the reaction acts as a catalyst for the same reaction. It's like a reaction that gets faster as it progresses because it creates its own catalyst.

Catalyst poisoning

Unfortunately, catalysts can lose their effectiveness through a process called catalyst poisoning. This occurs when unwanted substances bind strongly to the catalyst's active sites, preventing the intended reaction from taking place.

Effects of poisoning

• Reduced catalyst efficiency
• Higher operating costs
• Need for catalyst replacement
• Environmental pollution if catalysts fail

Real-world applications: catalytic converters

One of the most important applications of heterogeneous catalysis is the catalytic converter found in vehicle exhaust systems.

Structure and function

Catalytic converters contain a ceramic honeycomb structure coated with precious metals including platinum, palladium, and rhodium. These metals serve as catalysts to convert harmful exhaust gases into less toxic substances.

Environmental benefits

Catalytic converters perform several crucial pollution control reactions:

Carbon monoxide conversion:

• $\text{CO} + \frac{1}{2}\text{O}_2 \rightarrow \text{CO}_2$
• Converts poisonous carbon monoxide to carbon dioxide

Nitrogen oxide reduction:

• $\text{NO} \rightarrow \frac{1}{2}\text{N}_2 + \frac{1}{2}\text{O}_2$
• Converts toxic nitrogen oxides to harmless nitrogen gas

Hydrocarbon oxidation:

• $\text{Hydrocarbons} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}$
• Converts smog-producing hydrocarbons to carbon dioxide and water

Catalyst maintenance

Catalytic converters require proper fuel quality to function effectively. Lead-free petrol is essential because lead compounds would poison the catalyst. Modern converters can last over 100,000 kilometres when properly maintained.

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