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5.11Kinetics

Catalysis — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 10 parts Lesson 14 of 14 2 quizzes

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Lesson summary

5.11 Catalysis — AP CHEMISTRY notes: Explain how catalysts change the reaction mechanism, lower activation energy and act through binding, covalent bonding or surfaces.Catalyst — a substance that increases the rate of a reaction without being consumed overallIt provides an altern…

  • Effect on k
  • Overall ΔH
  • Equilibrium
  • Net concentration
  • Consumed and regenerated
  • Intermediates

Section 1 of 10What a catalyst does

Catalyst
  • Catalyst a substance that increases the rate of a reaction without being consumed overall
  • It provides an alternative mechanism with a lower activation energy.
  • It may also orient reactants better, increasing the frequency factor A.
Effect on k
  • Effect on k a catalyst changes k (through Ea or A) and so changes the rate

Section 2 of 10What a catalyst does not change

Overall ΔH
  • Overall ΔH the energy change of the reaction is the same
Equilibrium
  • Equilibrium the forward and reverse reactions are speeded equally, so equilibrium is reached sooner but its position is unchanged
Yield
  • Yield the amount of product at equilibrium is unchanged

Section 3 of 10Catalysts in mechanisms

Net concentration
  • Net concentration the catalyst concentration is constant overall
Consumed and regenerated
  • Consumed and regenerated it is a reactant in one step and a product in a later step
  • It is often consumed in the rate-determining step.
Intermediates
  • Intermediates a catalysed path may add a step with a new intermediate such as A−catalyst

Section 4 of 10Types of catalyst

TypeDescriptionExample
HomogeneousSame phase as the reactantsAqueous acid in ester hydrolysis
HeterogeneousDifferent phase from the reactantsV2O5 in the contact process, Fe in the Haber process

Section 5 of 10Binding catalysts: enzymes

Enzyme
  • Enzyme a biological catalyst, usually a large protein with an active site
  • The substrate binds by hydrogen bonds, dipole-dipole attractions and dispersion forces.
Binding effect
TypeDescription
Binding effect orients reactants favourably or lowers Ea and forms an enzyme-substrate intermediate
Lock-and-key modelthe substrate fits a fixed active site.
Induced-fit modelthe active site changes shape to fit the substrate.

Section 6 of 10Covalent catalysis

Acid-base catalysis
  • Acid-base catalysis a reactant or intermediate gains or loses a proton
  • This creates a new, more reactive intermediate and new elementary steps.
Surface catalysis
  • Surface catalysis a reactant or intermediate bonds to the solid surface
  • Gold catalyses N2O decomposition by forming a weak O-Au bond that weakens the N-O bond.

Section 7 of 10Surface catalysis steps

1
Reactants diffuse to the surface and are adsorbed.
2
Adsorption weakens bonds in the reactants.
3
Reaction occurs between bound species.
4
Products desorb from the surface.

Section 8 of 10More from the specification

1Alternative reaction pathways via catalysis
  • Alternative reaction pathways via catalysis introducing solid catalysts lowers activation energy barriers, providing an industrial means to accelerate chemical transformations.
2Overall order
  • The sum of the orders with respect to each reactant.
3Successful collision
  • A collision with the correct orientation and enough energy to react.
4Gradient
  • The change in y divided by the change in x on a graph.
5Half-life
  • The time for the concentration of a reactant to halve.

Section 9 of 10Key terms

TermMeaning
CatalystSubstance that speeds up a reaction by an alternative lower-Ea path and is regenerated.
Homogeneous catalystCatalyst in the same phase as the reactants.
Heterogeneous catalystCatalyst in a different phase from the reactants.
EnzymeBiological catalyst with an active site.
AdsorptionCollection of a substance on a surface.
Active siteRegion of an enzyme where the substrate binds.

Section 10 of 10Exam technique

1
Saying a catalyst is not involved in the reaction — it takes part in the mechanism but is regenerated.
2
Saying a catalyst shifts equilibrium — it speeds both directions equally and leaves the position unchanged.
3
Saying a catalyst changes ΔH — ΔH depends only on reactants and products.
4
Saying a catalyst gives the molecules more kinetic energy — it lowers the barrier, not the energy of the particles.

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