Catalysts and Enzymes

The helpers that are never used up

Your saliva starts digesting the starch in a piece of bread within seconds, at body temperature. Without help, starch and water would take an extremely long time to react. The helper is an enzyme called amylase. Car exhaust passes over a few grams of platinum, palladium and rhodium and comes out cleaner; ammonia for fertilizer is made over an iron catalyst. Catalysts speed up reactions without being used up, and they make possible a large share of the chemical industry and every process in your cells.

How a catalyst works

A catalyst is a substance that increases the rate of a reaction without being consumed. It works by providing an alternative reaction pathway (a different mechanism) with a lower activation energy.

• With a lower barrier, a much larger fraction of collisions have enough energy to react at the same temperature, so the rate increases.
• The catalyst takes part in the reaction, but it is regenerated at the end, so it is not used up and does not appear in the overall equation (it is often written above the arrow).
• A catalyst does not change ΔH: the reactants and products have the same energies with or without it.
• It lowers the barrier for the reverse reaction by the same amount, so it speeds up both directions. It helps a reaction reach equilibrium sooner but does not change the position of equilibrium (see the Chemical Equilibrium unit).

Homogeneous and heterogeneous catalysts

Homogeneous catalysts are in the same phase as the reactants, usually dissolved in the same solution or mixed in the same gas.

• Iodide ions catalyse the decomposition of hydrogen peroxide in solution: 2H₂O₂(aq) → 2H₂O(l) + O₂(g).
• Chlorine atoms from CFCs catalyse the destruction of ozone in the stratosphere, which is why CFCs were phased out under the Montreal Protocol.

Heterogeneous catalysts are in a different phase, usually a solid with gas or liquid reactants. Reactions happen on the solid's surface in four stages: reactant molecules adsorb (stick) to the surface, their bonds weaken, new bonds form, and products desorb (leave).

• Platinum, palladium and rhodium in catalytic converters.
• Iron in the Haber–Bosch process for ammonia.
• Nickel in the hydrogenation of vegetable oils to make margarine-type fats.

Heterogeneous catalysts are often used as fine powders or thin coatings on honeycomb supports, to give a huge surface area. They can be poisoned by substances that bind strongly to the surface and block it: lead from leaded gasoline ruined catalytic converters, which is one reason Canada banned leaded gasoline for cars (in 1990).

Enzymes: biological catalysts

Enzymes are protein molecules (made of folded chains of amino acids) that catalyse the reactions in living things. Each enzyme has an active site, a pocket whose shape and chemical groups fit a particular reactant, called the substrate.

1. The substrate binds to the active site, forming an enzyme-substrate complex. In the induced-fit model, the active site changes shape slightly to grip the substrate (an improvement on the older lock-and-key model).
2. The grip strains the substrate's bonds and holds groups in the right orientation, lowering the activation energy.
3. The products are released, and the enzyme is ready for another substrate molecule.

Enzymes are remarkably specific and fast. Catalase, the enzyme in potatoes and liver that decomposes hydrogen peroxide (you met it as an example of a catalyst in SCH3U), is one of the fastest enzymes known: one molecule can break down millions of H₂O₂ molecules every second.

Each enzyme works best at an optimum temperature (about 37 °C for most human enzymes) and optimum pH. Too much heat or the wrong pH changes the protein's shape (denaturation), and the active site no longer fits, which is why cooked potato barely makes hydrogen peroxide foam.

A Canadian contribution: enzyme kinetics

In 1913 the Canadian scientist Maud Menten, working in Berlin with Leonor Michaelis, published a mathematical description of how the rate of an enzyme-catalysed reaction depends on the concentration of substrate. The Michaelis–Menten equation shows that at low substrate concentration the rate increases almost in proportion to [S], but at high [S] it levels off: every enzyme molecule is busy, so adding more substrate cannot make it go faster (the enzyme is saturated). Biochemists and drug designers still use this equation. Menten later had a long career as a medical researcher and professor at the University of Pittsburgh.