Catalysts (A-level only) — AQA A-Level Chemistry
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Catalysts (A-level only) explained
A catalyst provides an alternative route with a lower activation energy and is chemically unchanged at the end of the reaction.
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Transition metals are unusually good at this for one reason: variable oxidation states let them accept electrons from one reactant and pass them to another, changing state temporarily and then changing back. Heterogeneous examples, where the catalyst is in a different phase, include iron in the Haber process, vanadium(V) oxide in the Contact process, nickel in hydrogenation, and platinum with rhodium in catalytic converters. Homogeneous examples, where catalyst and reactants share a phase, include Fe²⁺ catalysing the reaction between S₂O₈²⁻ and I⁻, and Mn²⁺ autocatalysing the reaction between MnO₄⁻ and C₂O₄²⁻.
A heterogeneous catalyst is in a different phase from the reactants and the reaction occurs at active sites on the surface.
Heterogeneous means the catalyst is in a different phase from the reactants, typically a solid catalyst with gaseous or aqueous reactants. The mark scheme accepts a different phase from the reactants, or from the reactants and products, but not from the products alone. The mechanism has three stages. Reactant molecules are adsorbed onto active sites on the surface, which weakens their bonds, holds them in the right orientation and raises their concentration at the surface. They then react on the surface by a route of lower activation energy. Finally the products are desorbed, freeing the site for more reactant. Adsorption must be of the right strength: too weak and too few molecules stick, too strong and the products never leave.
The use of a support medium to maximise the surface area of a heterogeneous catalyst and minimise the cost.
Heterogeneous catalysis happens only on the surface, so metal buried inside a lump is wasted. The fix is to spread a thin layer of the metal over an inert support with a very large surface area. A catalytic converter uses a ceramic honeycomb coated with platinum, palladium and rhodium; the honeycomb contributes nothing chemically but gives an enormous area over which the gases can meet the metal. Two benefits follow. The number of active sites per gram of metal rises sharply, so the reaction is faster for the same mass of catalyst. And because far less of an expensive metal is needed, the cost falls, which is the reason converters are affordable at all given the price of rhodium.
Fe is used as a heterogeneous catalyst in the Haber process.
The Haber process makes ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), an exothermic reversible reaction run at roughly 450 °C and about 200 atm over solid iron. Because the reactants are gases and the catalyst is a solid, this is heterogeneous catalysis. Nitrogen and hydrogen are adsorbed onto active sites on the iron surface, the strong N≡N triple bond is weakened, the molecules react, and ammonia is desorbed. The catalyst raises the rate only. It does not shift the position of equilibrium, so it does not increase the yield; it allows an acceptable yield to be reached quickly at a lower temperature than would otherwise be needed. Sulfur impurities poison the iron, so the gases are purified first.
Heterogeneous catalysts can become poisoned by impurities that block the active sites and consequently have reduced efficiency; this has a cost implication.
Poisoning is adsorption that does not reverse. An impurity binds to the active sites so strongly that it never desorbs, so those sites are permanently out of use. With fewer sites available the rate falls, less product is made per unit time, and eventually the catalyst has to be regenerated or replaced. Sulfur compounds poison the iron in the Haber process, which is why the nitrogen and hydrogen feed is purified first, and lead poisons the platinum and rhodium in a catalytic converter, which is why cars fitted with converters must run on unleaded petrol. The cost runs in both directions: purifying the feedstock costs money, and so does buying replacement catalyst and shutting the plant down while it is changed.
A homogeneous catalyst is in the same phase as the reactants.
Homogeneous means one phase: catalyst and reactants mixed together, in practice almost always all in aqueous solution. There is no surface and there are no active sites, so the mechanism is entirely different from heterogeneous catalysis. Instead the catalyst reacts with one of the reactants to form an intermediate species, which then reacts on to give the product and regenerate the catalyst. Two standard examples both involve transition metal ions in solution. Fe²⁺ catalyses the otherwise very slow reaction between peroxodisulfate, S₂O₈²⁻, and iodide, I⁻. Mn²⁺ catalyses the reaction between MnO₄⁻ and C₂O₄²⁻, and since Mn²⁺ is itself a product of that reaction it is an autocatalyst.
When catalysts and reactants are in the same phase, the reaction proceeds through an intermediate species.
In a homogeneous mechanism the catalyst is consumed in the first step and regenerated in the second, and the species it forms in between is the intermediate. The manganate(VII) and ethanedioate titration is the standard case. The uncatalysed reaction is slow because both reactants are negative ions, MnO₄⁻ and C₂O₄²⁻, which repel each other, so the activation energy is high. Once a little Mn²⁺ has formed it opens a new route: 4Mn²⁺ + MnO₄⁻ + 8H⁺ → 5Mn³⁺ + 4H₂O, then 2Mn³⁺ + C₂O₄²⁻ → 2Mn²⁺ + 2CO₂. Both steps now involve oppositely charged ions attracting, so both have lower activation energies, and the Mn²⁺ returns at the end. Since Mn²⁺ is a product, this is autocatalysis.
Your focus
- Define a catalyst in terms of an alternative route of lower activation energy and being chemically unchanged.
- Classify named industrial and laboratory catalysts as heterogeneous or homogeneous, stating the phases involved.
- Explain why variable oxidation states make transition metals effective catalysts.
Show all 21 objectives
- State the meaning of heterogeneous in terms of the phases of the catalyst and the reactants.
- Describe the adsorption, reaction and desorption stages that occur at an active site.
- Explain why adsorption that is too strong or too weak reduces a catalyst's effectiveness.
- Explain how a support medium increases the number of active sites available per gram of catalyst.
- State how using a support reduces the cost of a catalyst and give an example of one in use.
- Distinguish the role of the catalyst from that of the inert support in a catalytic converter.
- Write the equilibrium equation for the Haber process and state the conditions used.
- Explain why the iron catalyst changes the rate but not the equilibrium yield of ammonia.
- Justify the classification of this catalysis as heterogeneous by naming the phases present.
- Explain catalyst poisoning in terms of impurities blocking active sites on the surface.
- Name a poison for a specified industrial catalyst and state the precaution taken against it.
- Describe the economic consequences of poisoning for an industrial process.
- Classify a catalysed reaction as homogeneous from the state symbols in its equation, as when Mn²⁺(aq) catalyses MnO₄⁻(aq) with C₂O₄²⁻(aq).
- Name a homogeneous catalyst for a specified reaction and write the species involved.
- Define an autocatalyst as a product that catalyses its own reaction and give an example.
- Write the two step equations for a homogeneous catalytic mechanism, showing the intermediate and the regenerated catalyst.
- Explain why an uncatalysed reaction between two anions is slow and why the catalysed route is faster.
- Interpret a rate curve that steepens as a reaction proceeds as evidence of autocatalysis.
Catalysts (A-level only) exam tips
Marking Points
- one mark for defining a catalyst as providing an alternative route of lower activation energy
- one mark for stating that the catalyst is chemically unchanged at the end of the reaction
- one mark for classifying a named example correctly as heterogeneous or homogeneous
- one mark for linking catalytic ability to the metal's variable oxidation states
- one mark for stating that the catalyst is in a different phase from the reactants
- one mark for adsorption of reactants onto active sites on the surface
- one mark for weakening of bonds, correct orientation or increased surface concentration as the reason the rate rises
- one mark for desorption of the products, which frees the active site
- one mark for stating that the support increases the surface area of the catalyst
- one mark for linking greater surface area to more active sites and so a faster rate
- one mark for stating that less of the expensive metal is needed, so reducing cost
- one mark for a named example, such as the ceramic honeycomb in a catalytic converter
- one mark for the balanced equation N₂ + 3H₂ ⇌ 2NH₃ written with an equilibrium sign
- one mark for identifying iron as the catalyst and the catalysis as heterogeneous, with a solid catalyst and gaseous reactants
- one mark for stating that the catalyst increases the rate but does not change the position of equilibrium or the yield
- one mark for typical conditions of about 450 °C and about 200 atm
- one mark for stating that the impurity is adsorbed onto and blocks the active sites
- one mark for the consequence, fewer sites available so a slower rate and less product
- one mark for a named poison such as sulfur in the Haber process or lead in a catalytic converter
- one mark for a cost consequence, such as purifying the feedstock or replacing the catalyst
- one mark for stating that the catalyst is in the same phase as the reactants
- one mark for identifying the phase concerned, usually all species in aqueous solution
- one mark for a named homogeneous example such as Fe²⁺ with S₂O₈²⁻ and I⁻
- one mark for defining an autocatalyst as a product of the reaction that catalyses that same reaction
- one mark for defining an autocatalyst as a product of the reaction that catalyses that reaction
- one mark for explaining the slow start by repulsion between two negatively charged reactant ions and a high activation energy
- one mark for each correct step equation, 4Mn²⁺ + MnO₄⁻ + 8H⁺ → 5Mn³⁺ + 4H₂O and 2Mn³⁺ + C₂O₄²⁻ → 2Mn²⁺ + 2CO₂
- one mark for attraction between the negative reactant ion and the positive catalyst ion as the reason the new route is faster
Examiner Tips
- 💡Write both halves of the definition: an alternative route, and a lower activation energy.
- 💡Name the phases of catalyst and reactants when classifying, rather than just asserting the label.
- 💡If asked why transition metals catalyse reactions, go straight to variable oxidation states.
- 💡Say different phase from the reactants; adding and products is safe, but products alone is not.
- 💡Spell adsorption with a d and use it consistently throughout the answer.
- 💡Give the three stages in order: adsorption, reaction, desorption.
- 💡Give both reasons, surface area and cost, because questions usually credit each separately.
- 💡Name a real support such as a ceramic honeycomb rather than saying something to hold it.
- 💡Say the support is inert if asked whether it takes part in the reaction.
- 💡Separate rate from yield in every equilibrium answer, because a catalyst affects only the first.
- 💡Quote the conditions with units and note that the temperature is a compromise.
- 💡Say which species is the solid and which are gases when justifying the classification.
- 💡Use the phrase blocks the active sites, because that is the wording the mark scheme uses.
- 💡Name a specific poison and the specific process it affects.
- 💡Answer the cost part separately, since it usually carries its own mark.
- 💡State the phase of every species when you classify, not just that of the catalyst.
- 💡Use the word product in any definition of an autocatalyst.
- 💡Keep adsorption language out of homogeneous answers entirely.
- 💡Write both steps and check that the catalyst appears on the left of one and the right of the other.
- 💡Use oppositely charged ions attract for the rate explanation and save lower activation energy for the general point.
- 💡Balance each step for atoms and for charge separately.
Common Mistakes
- saying a catalyst lowers the activation energy of the reaction rather than offering a route with a lower one
- claiming the catalyst takes no part in the reaction at all, rather than being used and regenerated
- stating that a catalyst shifts the position of an equilibrium instead of reaching it faster
- classifying vanadium(V) oxide in the Contact process as homogeneous because the gases pass over it
- defining heterogeneous as a different phase from the products, which the mark scheme rejects
- writing absorbed rather than adsorbed, so describing molecules soaking into the bulk solid
- saying the catalyst dissolves in the reaction mixture, which would make it homogeneous
- omitting desorption, leaving a surface that would soon be permanently blocked
- saying the support itself catalyses the reaction, when it is chemically inert
- explaining the cost saving by the support being cheap rather than by less metal being used
- confusing the support medium with a promoter, or with the catalyst being poisoned
- claiming a larger surface area lowers the activation energy, when it increases the number of sites
- claiming the iron catalyst increases the yield of ammonia
- writing the equation with a single arrow, so losing the reversible nature of the process
- naming the catalyst as iron(III) oxide in the reaction itself rather than iron
- describing the process as homogeneous because everything happens inside one reactor
- saying the poison reacts with and uses up the catalyst rather than blocking its surface
- claiming poisoning raises the activation energy rather than reducing the number of usable sites
- giving it stops working as the cost implication, without naming purification, replacement or downtime
- suggesting the catalyst can simply be washed clean, when the adsorption is effectively irreversible
- describing active sites or adsorption in a homogeneous mechanism, where no surface exists
- defining an autocatalyst as a reactant that catalyses the reaction, which the mark scheme specifically rejects
- calling a reaction homogeneous just because it takes place in a single vessel
- saying the catalyst dissolves and is therefore used up
- calling Mn²⁺ a reactant rather than a product when defining the autocatalyst, which the mark scheme rejects
- offering the catalyst lowers the activation energy as the explanation for the faster intermediate route, when the mark is for attraction between oppositely charged ions
- leaving the step equations unbalanced for charge, most often by omitting the H⁺ ions
- writing one overall equation instead of the two separate steps that show the intermediate