Chemistry · Analytical and physical chemistry · Year 2

Catalysis and enzymes

Potato, platinum or iron — which decomposes H₂O₂ fastest, and why?

The situation

At Navoiyazot, ammonia is synthesised in columns packed with an iron catalyst; under every petrol-driven car runs a “honeycomb” coated with platinum, palladium and rhodium; and in a potato cell the enzyme catalase breaks down poisonous hydrogen peroxide. If in the laboratory we put all three into H₂O₂, all of them fizz with oxygen, but at different rates. And in the Petri dish in the corner, coloured spirals are turning on their own. How does a catalyst speed up a reaction, why are some thousands and others millions of times more effective, and what “poisons” it?

Topic

Catalysis: homogeneous, heterogeneous and enzymatic. Three catalysts side by side: iron catalyst granules in the Navoiyazot ammonia column, a car's catalytic converter (Pt/Pd/Rh honeycomb) and potato…

Key ideas

Activation energy
Heterogeneous catalysis
Michaelis–Menten

Formulas

\frac{k_{k a t}}{k} = e^{(E_{a} - E_{a, k a t}) / R T}

If the activation energy falls, the rate rises exponentially (with $A$ unchanged): for H₂O₂, 75 → 56 kJ/mol (I⁻) — about 2000 times at 25 °C.

H_{2} O_{2} + I^{-} \overset{}{\to} H_{2} O + IO^{-}, H_{2} O_{2} + IO^{-} \overset{}{\to} H_{2} O + O_{2} ↑ + I^{-}

Homogeneous catalysis: iodide is regenerated through the intermediate IO⁻; $v = k [H_{2} O_{2}] [I^{-}]$ .

r = \frac{k K_{A} K_{B} p_{A} p_{B}}{{(1 + K_{A} p_{A} + K_{B} p_{B})}^{2}}

Langmuir–Hinshelwood: both substances adsorb on the same surface and react; if $p_{A}$ grows too large, A “occupies” the surface and reduces the rate.

v = \frac{V_{m a x} [S]}{K_{M} + [S]}, \frac{1}{v} = \frac{K_{M}}{V_{m a x}} \cdot \frac{1}{[S]} + \frac{1}{V_{m a x}}

The Michaelis–Menten and Lineweaver–Burk equations: at $[S] = K_{M}$ , $v = V_{m a x} / 2$ .

J_{i} = \sum_{k} L_{i k} X_{k}, L_{i k} = L_{k i}

Onsager relations: near equilibrium the fluxes depend linearly on the forces and the kinetic coefficients are symmetric.

Theory

Catalysis and its types

A catalyst speeds up a reaction while remaining chemically unchanged at the end: it takes the reaction along a different path (mechanism) with a lower activation energy. In the decomposition of H₂O₂, $E_{a}$ ≈ 75 kJ/mol; with iodide ≈ 56, with platinum ≈ 49, with catalase ≈ 23 kJ/mol (other measurements give 8–23); if $A$ is unchanged, a decrease of 19 kJ/mol raises the rate about 2000 times at 25 °C. A catalyst speeds up the forward and reverse reactions by the same factor, so it changes neither the heat effect nor the position of equilibrium — it only brings the system to equilibrium sooner (equilibrium is shifted by concentration, pressure and temperature — the “Reversible reactions and equilibrium” experiment). If the catalyst and the reagents are in one phase, it is homogeneous; if the catalyst is a solid and the reaction takes place on its surface, it is heterogeneous; biological catalysts are enzymes. If a product of the reaction is itself the catalyst, that is autocatalysis. An inhibitor slows a reaction down, a promoter raises the activity of the catalyst, and a catalyst poison blocks the active sites. The simple effect of concentration, temperature and a catalyst at school level is in the “Rate of reaction and catalyst” experiment.

Homogeneous catalysis and autocatalysis (laboratory)

In the presence of iodide, H₂O₂ decomposes in two steps: the slow $H_{2} O_{2} + I^{-} \overset{}{\to} H_{2} O + IO^{-}$ (rate-limiting) and the fast $H_{2} O_{2} + IO^{-} \overset{}{\to} H_{2} O + O_{2} + I^{-}$ ; the rate is $v = k [H_{2} O_{2}] [I^{-}]$ , $k$ ≈ 1.2·10⁻² L/(mol·s) (25 °C). Experiment: 10 mL of 3 % H₂O₂ and 10 mL of KI solution are poured into a conical flask, the flask is connected to a gas syringe, and the volume of O₂ released is recorded against time; the initial rate is the slope at the start of the $V (t)$ graph. If [I⁻] doubles, the rate doubles; if the temperature rises by 10 °C, the rate increases about 2 times; $k$ and $E_{a}$ are found from these measurements. Autocatalysis: $2 MnO_{4}^{-} + 5 H_{2} C_{2} O_{4} + 6 H^{+} \overset{}{\to} 2 Mn^{2 +} + 10 CO_{2} + 8 H_{2} O$ — even at 70 °C the solution at first hardly decolourises for about a minute (induction period); then the Mn²⁺ formed speeds up the reaction and the pink colour suddenly disappears: the kinetic curve is S-shaped. If MnSO₄ is added beforehand, the induction period disappears. An acid (H⁺) is a homogeneous catalyst for the hydrolysis of esters (“Chemical kinetics and Arrhenius”). Safety: at school only 3 % H₂O₂ is used; KMnO₄ stains skin and clothes, oxalic acid is toxic; take care with the hot water bath and wear goggles.

Oscillating reactions and non-equilibrium thermodynamics

The Belousov–Zhabotinsky reaction: oxidation of malonic acid by bromate in an acidic medium; the catalyst is Ce³⁺/Ce⁴⁺ or ferroin (an Fe²⁺ complex, red) ↔ ferriin (Fe³⁺, blue). In a stirred beaker the colour alternates red ↔ blue with a period of tens of seconds; in an unstirred thin layer waves and spirals appear (autocatalytic excitation + diffusion). This spontaneous self-organisation of an open system far from equilibrium is a dissipative structure (I. Prigogine); the general science was named synergetics by H. Haken. In the region near equilibrium the local equilibrium hypothesis holds and fluxes depend linearly on forces: $J_{i} = \sum L_{i k} X_{k}$ , with the Onsager relations $L_{i k} = L_{k i}$ (for example, thermodiffusion and the diffusion thermal effect are linked by a single coefficient — “Thermoelectric phenomena”). Curie's principle: in an isotropic medium, fluxes and forces of different tensor rank (for example, a scalar chemical reaction and a vector heat flux) do not couple. Prigogine's theorem: in the linear region, in a stationary state the rate of entropy production is a minimum; oscillations and spirals are possible only in the nonlinear region far from equilibrium. The spiral in the Petri dish is a calculation using the Barkley model (shown in the colour of ferroin).

Heterogeneous catalysis: steps, theories, industry

Steps: diffusion of the reagents to the surface → adsorption → reaction on the surface → desorption of the product → diffusion. Physical adsorption (van der Waals, 5–40 kJ/mol, reversible, multilayer) and chemical adsorption (chemical bond, 40–400 kJ/mol, monolayer) — for catalysis the chemical kind is the important one. The Langmuir–Hinshelwood mechanism: CO and O both adsorb on Pt and react; if there is a lot of CO, it occupies the surface and leaves no room for O₂ — the rate passes through a maximum. That is why a cold catalytic converter “does not work” and suddenly starts working after a certain light-off temperature (50 % conversion, ≈ 200–300 °C). Theories: multiplet (Balandin — the geometry of the atoms of the active site matches the bonds of the molecule), active ensembles (Kobozev), electronic theory (Volkenstein — the electrons of semiconductor and metal), chain theory (Semenov — chains started by surface radicals). Promoters: structural (Al₂O₃ keeps the iron grains from sintering), electronic (K₂O). Poisons: S, As, Pb, CO — they occupy the active site. Industry: ammonia (Fe + K₂O, Al₂O₃; 400–500 °C, 15–30 MPa), SO₂ → SO₃ (V₂O₅ + K₂SO₄, 420–600 °C, cooling between the beds), NH₃ → NO (Pt–Rh gauze, ≈ 900 °C), catalytic cracking of oil (zeolites), polyethylene (Ziegler–Natta). Contact units are columns with beds of catalyst; in a chemical-technological system the reactor, heat exchanger and separators work together.

Enzymes and enzyme kinetics

Enzymes are biological catalysts of protein nature; they are highly selective and work under mild conditions (37 °C, pH ≈ 7). The active site consists of a substrate-binding part and a catalytic part; a regulating substance attaches to the allosteric site and changes the activity. Many enzymes need a cofactor (a metal ion: the haem iron in catalase) or a coenzyme (NAD⁺, FAD — derivatives of vitamins). Isoenzymes are different forms speeding up the same reaction; multienzyme systems form metabolic chains. Classes (EC): oxidoreductases (catalase), transferases, hydrolases (amylase, pepsin), lyases, isomerases, ligases. Michaelis–Menten: $E + S \overset{}{⇌} ES \overset{}{\to} E + P$ , and from the quasi-steady-state approximation $v = V_{m a x} [S] / (K_{M} + [S])$ ; $V_{m a x} = k_{k a t} {[E]}_{0}$ , and for catalase $k_{k a t}$ ≈ 4·10⁷ s⁻¹ — one of the fastest enzymes. The Lineweaver–Burk plot of $1 / v$ against $1 / [S]$ is a straight line: the intercept is $1 / V_{m a x}$ and the slope is $K_{M} / V_{m a x}$ . Inhibitors: competitive ones compete for the active site ( $K_{M}$ increases, $V_{m a x}$ is unchanged, and a large amount of substrate overcomes them); non-competitive ones bind elsewhere ( $V_{m a x}$ decreases; heavy-metal ions Cu²⁺, Hg²⁺); irreversible ones destroy the enzyme completely. Activators (Cl⁻ for amylase). There is an optimum of temperature and pH: at high temperature the protein is denatured. Applications: protease and amylases in washing powders, bread and beer (amylase), cheese (the enzyme rennet), medical preparations.

Lab work, safety and the model

Detecting catalase in food: a piece of potato or a sample of milk is put into H₂O₂ — oxygen bubbles (their volume in a gas syringe) show that catalase is present; in boiled potato and boiled milk the catalase is denatured. Properties of enzymes: by varying [H₂O₂], the initial rate is found from the volume of O₂ in 10 s, and $K_{M}$ and $V_{m a x}$ from the Lineweaver–Burk plot. Effect of temperature and pH: the rates are compared using a water bath (0–80 °C) and buffer solutions (pH 2–12). Enzymatic hydrolysis of starch and protein: starch with salivary amylase at 37 °C; every minute one drop is taken into iodine solution: blue → violet → reddish-brown (dextrins) → yellow (the achromic point, the starch is used up); protein (egg white) with pepsin at pH ≈ 2 — the cloudy solution becomes clear. Safety: only 3 % H₂O₂, with goggles; iodine stains skin; wear gloves for the HCl used with pepsin; do not taste the samples used in the test. Model: $k$ , $E_{a}$ , $K_{M}$ , activities and temperature/pH profiles are model numbers of the same order as in the literature; the LH lattice is a schematic stochastic model, the converter is effective first-order kinetics ( $E_{a}$ = 80 kJ/mol); equilibrium conversions are calculated for an ideal gas with constant ΔH, ΔS (approximate); Belousov–Zhabotinsky — the Oregonator (beaker) and the Barkley model (Petri dish); the number of bubbles in the three cylinders is qualitative.

Did you know?

Boris Belousov discovered the oscillating reaction in the early 1950s, but journals rejected his paper because “this cannot happen”; Anatoly Zhabotinsky studied it again in the 1960s and made the phenomenon widely known.
Karl Ziegler and Giulio Natta shared the 1963 Nobel Prize in Chemistry for polymerisation catalysts; with their catalysts, millions of tonnes of polyethylene and polypropylene are produced every year today.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Optical methods of analysis — Year 2
Extraction and chromatography — Year 2
Spectra: determining structure — Year 2
Alloys: phase diagrams — Year 2
Chemical kinetics and Arrhenius — Year 2
Colloids and adsorption — Year 2
Computational chemistry and docking — Year 2