Reaction Rates and Catalysts: What Makes Reactions Faster?

Diamond is thermodynamically unstable at the Earth’s surface. Given enough time, it should turn into graphite. It doesn’t matter: the conversion is so slow that your great-great-grandchildren’s diamonds will still be diamonds. Meanwhile, the fuel-air explosion in a car engine is over in about a thousandth of a second.

Whether a reaction can happen and how fast it happens are separate questions. The second one is the subject of chemical kinetics.

Collision theory

For two particles to react, three things must happen:

1. They must collide.
2. They must collide with enough energy — at least the activation energy (Eₐ).
3. They must collide in the right orientation, so that the atoms that need to bond actually meet.

Most collisions fail one of these tests and the particles simply bounce apart. The rate of reaction depends on how many successful collisions happen per second. Every factor that speeds up a reaction works by increasing that number.

1. Temperature

Heating is the most powerful lever. Higher temperature makes particles move faster, so they collide more often — but, more importantly, a much larger fraction of collisions have energy above Eₐ.

As a rough rule of thumb, many reactions around room temperature go about twice as fast for every 10 °C rise. That’s why food keeps longer in a fridge: the reactions that spoil it slow dramatically. It’s also why cooking works, and why fevers affect body chemistry.

2. Concentration

More particles in the same volume means more collisions per second. Pure oxygen makes things burn far more fiercely than air, which is only about 21% oxygen — the reason oxygen cylinders are kept well away from flames.

3. Pressure (for gases)

Raising the pressure of a gas squeezes the same particles into a smaller space, which is really just increasing concentration.

4. Surface area

A reaction involving a solid can only happen at its surface. Cut the solid into smaller pieces and far more particles are exposed.

A lump of coal burns steadily; coal dust suspended in air can explode. Flour, sawdust and even custard powder can cause dust explosions in factories for the same reason. Powdered medicines dissolve and act faster than tablets for the same reason too.

5. Catalysts

A catalyst speeds up a reaction without being used up. It works by providing an alternative reaction pathway with a lower activation energy. With a lower barrier, many more collisions have enough energy to succeed, even at the same temperature.

Key facts about catalysts:

• They’re not consumed — the same catalyst can be used again and again (though in practice catalysts can be “poisoned” by impurities).
• They don’t change ΔH — the energy of reactants and products is the same.
• They don’t change the position of equilibrium — they speed up the forward and reverse reactions equally, so equilibrium is simply reached faster. See Le Chatelier’s principle.

Catalysts all around you

• Catalytic converters in cars use platinum, palladium and rhodium spread thinly on a ceramic honeycomb (huge surface area) to turn carbon monoxide, unburned fuel and nitrogen oxides into CO₂, water and nitrogen.
• The Haber process uses an iron catalyst to make ammonia for fertilizer.
• Sulfuric acid is made with a vanadium(V) oxide catalyst in the contact process.
• Enzymes are biological catalysts — proteins that speed up specific reactions in living things, often by factors of millions or more. Catalase, found in potatoes and liver, breaks down hydrogen peroxide so fast it foams.

Many important industrial catalysts are transition metals or their compounds, because their partly filled d orbitals let them bond temporarily with reacting molecules.

Enzymes are fussy

Enzymes work within narrow conditions. Heat them too much and their precise 3D shape unravels (they’re “denatured”), and the reaction slows instead of speeding up. That’s why most human enzymes work best near body temperature, 37 °C, and why a high fever is dangerous.

Measuring rate

Rate is the change in amount of reactant or product per unit time. You can follow it by measuring:

• volume of gas given off (with a gas syringe),
• mass lost as gas escapes (on a balance),
• colour change (with a colorimeter),
• time for a precipitate to hide a mark under the flask.

Quick answers

Does a catalyst make more product? No. It makes the same product faster. The amount at equilibrium is unchanged.

Why does temperature have such a big effect? Because only the most energetic collisions succeed, and a small temperature increase greatly increases the number of those.

Is a catalyst the same as a reactant? No. It may take part in intermediate steps, but it’s regenerated by the end.

Related

Activation energy and heat changes are explained further in exothermic vs. endothermic reactions.