Ch 3 · Homogeneous and heterogeneous catalysts · AS & A Level Chemistry 9701
Topic 3 of 3 in Reaction kinetics — AS & A Level Chemistry 9701 — 48 lessons.
Introduction
Catalysts make many industrial and laboratory reactions economically viable by speeding them up without being used up themselves. Cambridge 9701 requires you to explain this effect at the molecular level, using the Boltzmann distribution, and to represent it correctly on a reaction pathway diagram, distinguishing what a catalyst changes from what it leaves unchanged.
Key ideas — Catalyst and catalysis
A catalyst is a substance that increases the rate of a chemical reaction without itself being used up in the overall process; it can be recovered chemically unchanged at the end of the reaction. Catalysis is the process by which a catalyst increases the rate of reaction. A homogeneous catalyst is in the same physical state as the reactants, for example aqueous mathrmFe^2+ catalysing the reaction between aqueous mathrmS_2O_8^2- and I^- ions. A heterogeneous catalyst is in a different physical state from the reactants, typically a solid catalysing a reaction between gases, for example the iron catalyst in the Haber process; reaction occurs at active sites on the solid's surface, so increasing the surface area of a heterogeneous catalyst increases its effectiveness.
🧑🏫 Teacher's explanation
Imagine you are trying to climb over a very steep hill to get to the other side. This hill represents the 'activation energy'—the minimum energy required for a reaction to occur. A catalyst is like a secret tunnel through that hill. It doesn't change where you are starting or where you are going, but it provides a much easier, lower-energy path. Because the path is easier, more people (or in chemistry, more particles) can make it through in a shorter amount of time.
📖 Definitions
🔍 Worked example — Easy
A reaction has an activation energy of 80 kJ mol^-1 without a catalyst. When a catalyst is added, the activation energy decreases to 50 kJ mol^-1. Calculate the decrease in activation energy.
🔍 Worked example — Standard
In the Haber process, N_2(g) + 3H_2(g) leftharpoons 2NH_3(g), a solid iron catalyst is used. If the surface area of the iron is doubled, what happens to the rate of reaction?
🔍 Worked example — Exam-style
Compare the effect of a catalyst on the enthalpy change (Δ H) of a reaction where the energy of reactants is 150 kJ mol^-1 and the energy of products is 100 kJ mol^-1.
⚠️ Common errors
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🎯 Exam question
(a) Name the type of catalysis occurring. [1] (b) Explain, in terms of the reaction mechanism and the Boltzmann distribution, why the rate of reaction increases when this catalyst is added. [3] [4 marks]
Key ideas — Catalysts and a lower activation energy pathway
A catalyst increases the rate of reaction by providing an alternative reaction mechanism (pathway) of lower activation energy than the uncatalysed route. The catalyst is typically involved in an intermediate step, for example by temporarily bonding to a reactant or by changing oxidation state and back again, so that the overall sequence of steps requires less energy to reach the transition state than the single uncatalysed step. Crucially, a catalyst does not alter the overall enthalpy change of the reaction, and does not appear as a reactant or product in the overall chemical equation, since it is fully regenerated by the end of the mechanism.
🧑🏫 Teacher's explanation
Imagine you are trying to get to the other side of a steep, jagged mountain. You could climb straight over the peak, but it takes a massive amount of energy and time. A catalyst is like a tunnel through the mountain. You still end up at the same destination (the same products), and the height of the mountain (the enthalpy change) hasn't changed, but the path through the tunnel requires much less effort to traverse.
📖 Definitions
🔍 Worked example — Easy
A reaction has an activation energy of 80 kJ mol^-1 without a catalyst. When a catalyst is added, the activation energy decreases to 50 kJ mol^-1. State how this affects the rate of reaction.
🔍 Worked example — Standard
Calculate the number of moles of H_2 required to react completely with 32 g of O_2 in the reaction: 2H_2 + O_2 arrow 2H_2O.
🔍 Worked example — Exam-style
A reaction pathway diagram shows the reactants at 50 kJ mol^-1 and the transition state at 150 kJ mol^-1. The products are at 20 kJ mol^-1. Calculate the activation energy (E_a) and the enthalpy change (Δ H) for the uncatalysed reaction.
⚠️ Common errors
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🎯 Exam question
Key ideas — The catalytic effect and the Boltzmann distribution
On a Boltzmann distribution diagram, adding a catalyst does not alter the distribution curve itself, since the temperature and hence the spread of molecular energies is unchanged. Instead, the catalyst lowers the activation energy needed, so the vertical line marking E_a moves to the left, to a lower energy value. Since the area under the curve beyond this new, lower activation energy is larger than the area beyond the original, higher activation energy, a greater fraction of molecules now possess enough energy to react on collision, so the frequency of effective collisions, and hence the rate, increases.
🧑🏫 Teacher's explanation
Imagine you are trying to push a heavy boulder over a steep hill. The height of that hill is the 'activation energy'. If you find a tunnel through the hill, the journey becomes much easier and faster, even though the hill itself hasn't changed. In chemistry, a catalyst is like that tunnel; it provides a different, easier path for the reaction to take.
📖 Definitions
🔍 Worked example — Easy
A reaction has an activation energy of 50 kJ mol^-1. If a catalyst is added, the new activation energy is 35 kJ mol^-1. State how the Boltzmann distribution curve changes.
🔍 Worked example — Standard
In a sample of gas at 300 K, the fraction of molecules with energy ≥ E_a is 0.02. After adding a catalyst, the fraction of molecules with energy ≥ E_a(new) is 0.15. Calculate the factor by which the frequency of successful collisions has increased.
🔍 Worked example — Exam-style
A reaction pathway diagram shows a peak at 100 kJ mol^-1 without a catalyst. With a catalyst, the peak is at 75 kJ mol^-1. Calculate the reduction in activation energy in kJ mol^-1.
⚠️ Common errors
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🎯 Exam question
Key ideas — Reaction pathway diagrams with and without a catalyst
On a reaction pathway diagram, the reactants and products are drawn at the same enthalpy levels for both routes, so the overall Δ H is identical whether or not a catalyst is used. The uncatalysed route is drawn as a single peak reaching a high activation energy; the catalysed route is drawn as a lower peak (or, where the mechanism has multiple steps, a sequence of smaller peaks) reaching a lower maximum enthalpy, showing the reduced activation energy of the alternative pathway.
🧑🏫 Teacher's explanation
Imagine you are trying to push a heavy boulder over a steep hill to reach a valley on the other side. The height of that hill represents the activation energy (E_a). If you find a tunnel through the hill, the journey becomes much easier because you don't have to climb as high, even though you end up at the same valley.
📖 Definitions
🔍 Worked example — Easy
A reaction has an activation energy of 80 kJ mol^-1 without a catalyst and 50 kJ mol^-1 with a catalyst. Calculate the reduction in activation energy.
🔍 Worked example — Standard
For the reaction N_2 + 3H_2 arrow 2NH_3, the enthalpy change Δ H is -92 kJ mol^-1. If a catalyst is added, what is the new Δ H?
🔍 Worked example — Exam-style
In a reaction pathway diagram, the energy of reactants is 150 kJ mol^-1, the energy of products is 100 kJ mol^-1, and the peak energy (without catalyst) is 250 kJ mol^-1. Calculate the activation energy (E_a) and the enthalpy change (Δ H).
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State how the activation energy of the reaction changes. [1] (b) State how the enthalpy change Δ H changes. [1] (c) Explain, in terms of the Boltzmann distribution, why the rate of reaction increases when the catalyst is added. [2] [4 marks]
Worked example
Question. Sketch and label a reaction pathway diagram for an exothermic reaction, showing both the uncatalysed pathway and the pathway in the presence of an effective catalyst, and use it to explain why the catalysed reaction is faster. [4]
🔍 Worked example — Easy
Calculate the change in enthalpy (Δ H) for the reaction 2H_2 + O_2 arrow 2H_2O if the energy of the reactants is 484 kJ/mol and the energy of the products is 482 kJ/mol.
🔍 Worked example — Standard
A reaction has an activation energy of 100 kJ/mol without a catalyst. When a catalyst is added, the activation energy decreases to 75 kJ/mol. Calculate the reduction in activation energy.
🔍 Worked example — Exam-style
In a reaction pathway diagram, the energy of reactants is 500 kJ/mol, the peak energy (transition state) is 750 kJ/mol, and the energy of products is 400 kJ/mol. Calculate the activation energy (E_a) and the enthalpy change (Δ H).
⚠️ Common errors
Common mistakes
Practice
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