What Changes the Rate of a Reaction? (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about what changes the rate of a reaction.

These four topics sit together because every one of them changes the rate through the same two routes: how often particles collide, and what proportion of those collisions carries at least the activation energy.

The rate of reaction is a measured quantity: the speed at which a chemical reaction takes place, in moles per decimetre cubed per second. Once you can measure it, the question is what changes it. Temperature, concentration, pressure and a catalyst all do, and they all act through one of two routes. Either they change how frequently the particles collide, or they change what proportion of those collisions carries at least the activation energy.

We start with how the rate of reaction is measured from a concentration-time graph. Then temperature, which acts on both routes at once. Then concentration and pressure, which act on collision frequency. Then catalysts, which act on the activation energy.

The rate of reaction is the speed at which a chemical reaction takes place, and it has units of moles per decimetre cubed per second, or moles per decimetre cubed per minute.

It is not fixed for the whole reaction. As the reaction proceeds the reactants are used up and changed into products, so the concentration of the reactants falls while the concentration of the products rises, and the rate changes as they do.

You get the rate from a concentration-time graph. At the start of the reaction the curve looks almost linear, so the initial rate can be found by treating that section as a straight line, or by drawing a tangent at the start of the curve. Later on the curve becomes shallower, and that shallowing is the rate decreasing with time. To find the rate at a particular concentration, you draw a tangent at that point on the graph.

That measured rate is what every factor in the rest of this video acts on.

Increasing the temperature of the reaction mixture increases the rate of reaction, and it does it in two ways at once.

At higher temperatures the particles are moving faster, so they collide more frequently.

Also at higher temperatures a higher proportion of the molecules have energy equal to or greater than the activation energy. A higher proportion of the collisions are therefore successful, and successful collisions become more frequent.

Temperature is the only factor in this video that acts on both routes: it raises the collision frequency, and it raises the proportion of molecules with enough energy to react.

The more concentrated a solution is, the greater the number of particles in a given volume of solvent. More particles in the same volume means an increased collision frequency, and so an increased rate of reaction.

An increase in pressure does the same thing for reactions that involve gases. When the pressure is increased the molecules have less space in which they can move, so the same number of particles occupies a smaller volume. The collision frequency increases, the number of effective collisions increases with it, and the rate of reaction increases.

Both concentration and pressure act through collision frequency.

A catalyst increases the rate of a reaction by providing the reactants with an alternative reaction pathway which is lower in activation energy than the uncatalysed reaction.

A Maxwell-Boltzmann distribution shows what that does. By lowering the activation energy, a greater proportion of the molecules in the reaction mixture have the activation energy or higher, and therefore have sufficient energy for an effective collision. The area under the curve beyond the activation energy is much larger with the catalyst present than without it.

Catalysts divide into two types. A homogeneous catalyst is in the same phase as the reactants, for example where the reactants and the catalyst are all in solution. A heterogeneous catalyst is in a different phase to the reactants, for example where the reactants are gases and the catalyst used is a solid.

A catalyst acts on the second route: it lowers the activation energy, so a greater proportion of molecules have sufficient energy to react.

When you calculate the rate of reaction, you can use either the increase in the concentration of the products or the decrease in the concentration of the reactants. Either one will give you the rate.

An increase in temperature raises the collision frequency and raises the proportion of molecules with energy equal to or greater than the activation energy. An increase in concentration and pressure raise the collision frequency. A catalyst lowers the activation energy, so a greater proportion of molecules have sufficient energy for an effective collision.

Four factors, and the same two routes underneath them all: how often the particles collide, and what proportion of those collisions carries at least the activation energy. That is what the rate you measure from a concentration-time graph is telling you about.

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