Catalysts and Enzymes
Catalysts
A catalyst is a substance that increases the rate of a chemical reaction without being used up in the process. It is still present at the end of the reaction, chemically unchanged.
How Catalysts Work
Catalysts provide an alternative reaction pathway that has a lower activation energy (Ea). This means a greater proportion of reacting particles have sufficient energy to react, so more collisions are successful.
On an energy profile diagram, a catalysed reaction has a lower hump (activation energy barrier) than the uncatalysed reaction, but the overall energy change (ΔH) remains the same. The catalyst does not change how much energy is released or absorbed — it only makes the reaction happen faster.
Key Properties of Catalysts
- Not used up — they can be recovered at the end and reused
- Only a small amount is needed — because they are regenerated
- Specific — most catalysts only work for particular reactions
- Do not change the amount of product formed, only how quickly it forms
- They do not appear in the overall equation (but may appear in individual steps)
Industrial Catalysts
| Catalyst | Reaction | Process |
|---|---|---|
| Iron (Fe) | N₂ + 3H₂ ⇌ 2NH₃ | Haber process |
| Vanadium pentoxide (V₂O₅) | 2SO₂ + O₂ ⇌ 2SO₃ | Contact process |
| Manganese dioxide (MnO₂) | 2H₂O₂ → 2H₂O + O₂ | Decomposition of hydrogen peroxide |
| Platinum/palladium/rhodium | CO + NOx → CO₂ + N₂ | Catalytic converters |
| Nickel (Ni) | Hydrogenation of alkenes | Making margarine |
Advantages of Catalysts in Industry
- Increase rate → products made faster, increasing output
- Lower temperatures can be used → reduces energy costs
- Reduces environmental impact → less fuel burned to heat reactors
- Not consumed → long-lasting, reducing costs
Disadvantages of Catalysts
- Can be expensive (e.g. platinum, palladium)
- Can be poisoned — impurities in the reactants can coat the catalyst surface, stopping it from working (e.g. sulfur poisons the iron catalyst in the Haber process)
- Often only work for one specific reaction
- Must eventually be disposed of — some are toxic
Enzymes: Biological Catalysts
Enzymes are biological catalysts — large protein molecules that speed up reactions in living organisms. Every enzyme has a specific shape called the active site, which fits only particular substrate molecules.
How Enzymes Work: Lock and Key Model
1. The substrate (reactant molecule) fits into the enzyme's active site like a key fits a lock
2. An enzyme-substrate complex is formed
3. The reaction occurs (bond breaking or forming)
4. The product is released and the enzyme is unchanged, ready to catalyse another reaction
Factors Affecting Enzyme Activity
Temperature:
- Increasing temperature increases the rate (particles move faster, more collisions with the active site)
- Each enzyme has an optimum temperature (usually around 37°C for human enzymes)
- Above the optimum, the enzyme is denatured — the active site changes shape permanently and the substrate can no longer fit
- Denatured ≠ killed (enzymes are not alive), but the damage is irreversible
pH:
- Each enzyme has an optimum pH
- Too far from the optimum causes denaturation (the active site changes shape)
- Most human enzymes work best at pH 7 (neutral), but pepsin in the stomach works best at pH 2 (acidic)
Substrate concentration:
- Increasing substrate concentration increases rate (more substrate molecules to collide with active sites)
- Eventually, the rate levels off because all active sites are occupied (saturated) — adding more substrate makes no difference until a product is released
Enzymes in Industry
| Enzyme | Use |
|---|---|
| Protease | Biological washing powders (breaks down protein stains) |
| Lipase | Biological washing powders (breaks down fat stains), baby food |
| Carbohydrase (amylase) | Converting starch to sugar syrup |
| Isomerase | Converting glucose to fructose (sweeter, used in slimming foods) |
Advantages of Using Enzymes in Industry
- Work at low temperatures → lower energy costs
- Work at normal pressure → cheaper equipment
- Biodegradable → less environmental impact
- Highly specific → fewer unwanted by-products
Disadvantages
- Denatured by high temperatures or wrong pH — conditions must be carefully controlled
- Can be expensive to produce and purify
- Contamination of the enzyme can stop it working
- Slower than some chemical catalysts at high temperatures
Exam Tips
- Catalysts lower activation energy — always state this when explaining how they work
- Catalysts are NOT used up and do NOT change the overall energy change
- Know the difference between a catalyst being poisoned (coated so it cannot work) and an enzyme being denatured (active site changes shape)
- For enzyme questions, always link temperature/pH effects to the shape of the active site
- Never say an enzyme is "killed" — it is denatured