Rates of Reaction & Collision Theory
What Controls How Fast a Reaction Happens?
Chemical reactions happen at different speeds. Some, like an explosion, are over in a fraction of a second. Others, like rusting, take months or years. The rate of reaction measures how quickly reactants are converted into products. Understanding what affects this rate is a key part of the Edexcel specification.
For a reaction to occur, particles must collide with sufficient energy and in the correct orientation. The minimum energy needed for a successful collision is called the activation energy.
Collision Theory
Collision theory explains how reactions happen at the particle level. Reactant particles are constantly moving and colliding with each other. However, not every collision leads to a reaction. Only collisions where particles have energy equal to or greater than the activation energy — and where the particles are orientated correctly — result in a successful reaction.
The rate of reaction increases when:
- The frequency of collisions increases (more collisions per second).
- The energy of collisions increases (a greater proportion of collisions exceed the activation energy).
Factors Affecting the Rate of Reaction
Four main factors change the rate of a reaction. Each one works by changing either the frequency or the energy of particle collisions.
Increasing the temperature makes particles move faster. This has two effects: particles collide more frequently, and each collision carries more energy. A greater proportion of collisions now exceed the activation energy, so the rate increases significantly.
Increasing the concentration of a solution means there are more reactant particles in the same volume. They are closer together and collide more frequently, increasing the rate. For reactions involving gases, increasing the pressure has the same effect — the particles are pushed closer together, increasing collision frequency.
Breaking a solid into smaller pieces increases its surface area to volume ratio. More particles on the surface are exposed to the other reactant, so collisions happen more often and the reaction is faster. Powdered marble chips react with acid much faster than large lumps of the same mass.
Catalysts
A catalyst is a substance that speeds up a chemical reaction without being used up. It is not chemically changed and has the same mass at the end of the reaction. It does not alter the products formed.
A catalyst works by providing an alternative reaction pathway that has a lower activation energy. This means a greater proportion of particle collisions have enough energy to react, so the rate increases — even though the temperature has not changed.
A catalyst speeds up a reaction by lowering the activation energy. It provides an alternative pathway. The catalyst is unchanged chemically and unchanged in mass at the end of the reaction. It does not change the products.
Enzymes are biological catalysts found in living organisms. The specification notes that enzymes are used in the production of alcoholic drinks, where yeast enzymes catalyse the fermentation of sugars into ethanol and carbon dioxide.
Measuring the Rate of Reaction
The rate of a reaction can be determined experimentally by measuring how quickly a product is formed or a reactant is used up. Two common methods are:
- Measuring gas production — collect gas in a gas syringe or measure loss of mass on a balance over time. For example, reacting hydrochloric acid with marble chips produces \(CO_2\) gas.
- Observing a colour change — in the reaction between sodium thiosulfate and hydrochloric acid, a yellow sulfur precipitate forms. The time taken for the solution to become opaque (a cross beneath the flask disappears) indicates the rate.
Graphs of mass, volume or concentration against time can be used to compare rates. A steeper gradient means a faster reaction. The gradient of the curve at any point gives the rate of reaction at that moment. As reactants are used up, the gradient decreases and the line levels off — the reaction is complete. Two curves on the same axes can show the effect of changing a variable (e.g. higher temperature gives a steeper initial gradient but the same final amount of product).
On a graph of volume of gas against time, the gradient (steepness) of the line represents the rate. A steeper line = faster rate. The line levels off when the reaction is complete and all the limiting reactant has been used up.