Use collision theory to explain how concentration, temperature, surface area and pressure change reaction rate.

What this dot point is asking

You must explain rate in terms of collisions between particles, and predict how each experimental variable changes that rate.

What rate means

The rate of reaction is how quickly reactants are used up or products are formed per unit time. We can follow it by measuring a change such as gas volume produced, mass lost, colour intensity, or conductivity, and plotting that quantity against time. The gradient of the curve gives the rate, which is usually fastest at the start and slows as reactants are consumed.

Collision theory

For a reaction to occur, particles must collide. Not every collision leads to reaction. A successful (effective) collision must satisfy two conditions.

• The particles must collide with energy equal to or greater than the activation energy.
• The particles must collide in the correct orientation so that bonds can break and form.

The Maxwell-Boltzmann distribution

At any temperature the particles in a sample have a spread of kinetic energies, shown by the Maxwell-Boltzmann distribution. Only the particles to the right of the activation energy on this curve can react. Raising the temperature shifts the whole distribution to higher energies and, crucially, greatly increases the fraction of particles above $E_a$. This is why a modest temperature rise produces a large rate increase.

How each factor changes the rate

Concentration: increasing the concentration of a dissolved reactant packs more particles into the same volume, so collisions become more frequent and the rate rises.

Pressure (gases): increasing pressure on a gaseous reaction squeezes particles closer together, which is effectively the same as raising concentration, so the collision frequency and rate increase.

Surface area: breaking a solid into smaller pieces or a powder exposes more particles at the surface where collisions with the other reactant can occur, so the rate rises. A powdered solid reacts far faster than a single lump of the same mass.

Temperature: raising the temperature makes particles move faster, so collisions are both more frequent and, far more importantly, more energetic. The fraction of collisions that exceed $E_a$ rises sharply.

In the exam, link every rate change back to either the frequency of collisions or the proportion that exceeds the activation energy, and use the Maxwell-Boltzmann curve to justify temperature effects.