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Catalysis And Enzymes

Learn and apply Catalysis And Enzymes in the published Chemistry course sequence.

  • GCE A-Level H1 Chemistry 8873-2027
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H1 Heterogeneous Catalysis and Enzymes: Orientation

H1 connects catalysts to a lower-energy pathway, then applies that idea to catalytic NOx removal and to the specificity and condition sensitivity of enzymes.

H1 8873 scope
  • Explain catalytic NOx removal as the prescribed heterogeneous example and enzyme lock-and-key behaviour.
  • Homogeneous catalysis and extended H2 catalyst examples are not required.

Definitions (Must Know)

  • A catalyst increases reaction rate and is regenerated by the end of the reaction.
  • Heterogeneous catalysis uses a catalyst in a different phase from the reactants.
  • An active site is a region of a catalyst or enzyme where reacting species bind.
  • An enzyme is a biological catalyst with a specific three-dimensional active site.
  • Denaturation is disruption of the enzyme’s effective shape so the substrate no longer binds suitably.

Detailed Explanations

A. Energy explanation

The catalyst changes the pathway, not the reactant and product energy levels. It therefore changes neither Δ H nor the equilibrium position. Its lower barrier gives a larger rate constant at the same temperature.

B. Catalytic NOx removal

Gaseous pollutants adsorb on a solid catalyst surface. Bonds weaken, adsorbed species react, and products such as N₂ and CO₂ desorb, freeing sites. One representative equation is 2CO + 2NO → 2CO₂ + N₂.

C. Enzyme specificity

Only a substrate with suitable shape and interactions forms a productive enzyme–substrate complex: the lock-and-key model.

D. Temperature and pH

Rising temperature initially increases collision frequency. Excessive heat disrupts bonding that maintains the active-site shape. Unsuitable pH changes ionisation and interactions at the active site, reducing binding or catalytic effectiveness.

Worked Examples

Modelled example 1

A catalyst and the Boltzmann distribution

Core

Problem

Explain why a catalyst increases reaction rate at fixed temperature using a Maxwell–Boltzmann distribution.
Study the worked solution

  1. Preserve the distribution

    Method

    State that the particle-energy distribution is unchanged.

    Reason

    Temperature has not changed.

    Working

    Same Maxwell–Boltzmann curve.

  2. Lower the pathway barrier

    Method

    State that the alternative pathway has lower activation energy.

    Reason

    The catalyst changes the pathway rather than reactant or product energy levels.

    Working

    The Eₐ threshold moves left.

  3. Link fraction to rate

    Method

    Identify the larger area at or above the lower Eₐ.

    Reason

    More collisions can meet the energy requirement, so the rate constant and rate increase.

    Working

    Successful fraction ↑ ⇒ k↑ ⇒ rate ↑.

Guided practice 2

Enzyme temperature profile

About 7 min

Problem

Why does enzyme reaction rate rise and then fall as temperature increases?

Try this before viewing the solution

Hints

Hint 1: before optimum
Increasing kinetic energy raises collision frequency and productive enzyme–substrate encounters.
Hint 2: after optimum
Excessive heat disrupts interactions maintaining the active-site shape.
View solution step by step

  1. Explain the rise

    Method

    Link warming to greater kinetic energy and collision frequency.

    Reason

    More enzyme–substrate encounters form productive complexes per unit time.

    Working

    Rate rises toward an optimum.

  2. Explain the fall

    Method

    State that excessive temperature disrupts the active-site structure.

    Reason

    Fewer substrate molecules bind with suitable shape and interactions.

    Working

    Denaturation causes the sharp post-optimum rate fall.

Common misconception 3

Correct a low-temperature denaturation claim

Find and correct the mistake

Learner claim

An enzyme reaction is slow at 10 °C but returns to its earlier rate when rewarmed to 30 °C. A learner says the enzyme was denatured at 10 °C. Correct the claim using the evidence.

Classify the low-temperature effect

The reversible slowdown at 10 °C is mainly due to

View solution step by step

  1. Use the reversibility evidence

    Method

    Note that activity returns on warming.

    Reason

    Recovery shows the active-site structure was not permanently disrupted.

    Working

    The enzyme was not denatured at 10 °C.

  2. Give the kinetic explanation

    Method

    State that particles have lower kinetic energy at 10 °C.

    Reason

    Collision frequency and productive enzyme–substrate encounters are reduced.

    Working

    Lower temperature slows the reaction reversibly.

Examiner practice 4

Explain catalytic NOx removal

5 marks

Problem

A solid catalytic converter promotes 2CO + 2NO → 2CO₂ + N₂. Explain the heterogeneous catalytic process at the particle level. [5 marks]

Try this before viewing the solution

View solution step by step

  1. Adsorb reactants

    1 mark

    Method

    State that CO and NO adsorb on active sites of the solid surface.

    Reason

    Adsorption brings the gaseous reactants into the catalytic pathway.

    Working

    CO(g) and NO(g) occupy surface sites.

  2. Weaken and rearrange bonds

    2 marks

    Method

    State that adsorption weakens existing bonds and allows new bonds to form.

    Reason

    The surface provides an alternative pathway of lower activation energy.

    Working

    Adsorbed species react to form CO₂ and N₂.

  3. Desorb products

    1 mark

    Method

    State that carbon dioxide and nitrogen leave the surface.

    Reason

    Desorption releases the gaseous products.

    Working

    CO₂(g) and N₂(g) desorb.

  4. Regenerate sites

    1 mark

    Method

    State that desorption frees active sites for another cycle.

    Reason

    The solid catalyst is not consumed overall.

    Working

    Available sites can adsorb further pollutant molecules.

Challenge 5

Explain an enzyme pH profile

Minimal support

Problem

An enzyme has its highest rate near pH 7 but a much lower rate at pH 2, even though temperature and substrate concentration are unchanged. Explain the lower rate without saying that acid merely makes particles move more slowly.

Try this before viewing the solution

Hints

Hint 1: focus on charge and structure
Changing pH changes ionisation of groups involved in active-site shape and substrate interactions.
Hint 2: link to complexes
Ask whether the substrate can still bind with the required shape and charge complementarity.
View solution step by step

  1. Change active-site interactions

    Method

    State that low pH changes protonation and charge of groups in or supporting the active site.

    Reason

    Those changes can disrupt ionic and hydrogen-bonding interactions that maintain shape or bind substrate.

    Working

    Active-site charge/shape becomes less suitable at pH 2.

  2. Link to rate

    Method

    State that fewer productive enzyme–substrate complexes form.

    Reason

    The substrate has poorer shape or interaction complementarity at the altered active site.

    Working

    Productive binding ↓ ⇒ reaction rate ↓.

Mind Stretchers

Attempt each unfamiliar application before opening the hint, then compare your reasoning with the solution.

Mind stretcher 1: A blocked surfaceExtension

Question. Why can a small amount of strongly adsorbed impurity reduce catalytic-converter performance?

Show Hint

Track the number of available active sites.

Show Answer

The impurity occupies active sites, reducing pollutant adsorption and the number of surface reactions per second.

Mind stretcher 2: Separating temperature effectsExtension

Question. An enzyme is slower at 10 °C than 30 °C but recovers when rewarmed. Is it denatured at 10 °C?

Show Hint

Denaturation is a structural loss; low temperature mainly affects kinetic energy.

Show Answer

No. Low temperature reduces collision frequency and successful encounters but usually does not permanently disrupt the active site, so activity can recover on warming.