3.2.2 (a) Collision Frequency and Rate of Reaction

Syllabus

(a) effects of concentration, including pressure of gases, on rates of reaction, in terms of collision-frequency

What does this mean?

At GCSE "collision-frequency" was the answer to almost all questions about rates of reaction.

And you'll still get marks for raising the point because clearly two molecules can't react unless they are actually touching.

And, since all molecules are moving, they won't touch until they collide.

We should also be aware that it is possible for collisions to be unsuccessful either because there is not enough energy for the reaction to proceed or because they simply collide in the wrong orientation.

Ignoring these factors for now, what affects the collision frequency?

Pressure

Gas particles are far apart.

So gases can be compressed into smaller spaces simply by pushing the particles closer together.

This increases the pressure of the gas.

The particles don't have any more or less kinetic energy just because they are in a smaller space so they are moving at the same average speed.

But because they are closer together they will collide more frequently.

The proportion of collisions that are successful will be no higher at high pressure but the sheer number of extra collisions per second means that there will be more successful collisions per second.

So the rate of reaction will increase.

Concentration

Exactly the same holds true about solutions.

The higher the concentration of a solution the closer together the molecules will be.

Increasing the concentration won't add any energy or change the speed of the molecules.

But if we were to add a solid to an acid solution there will be more collisions per second with the solid in high concentration simply because the particles are closer together.

If we added an alkaline solution for it to react with the acid particles and alkali particles would be closer together with high concentrations and so would collide more frequently and provide a faster reaction.

Temperature.

Increasing the temperature of a reaction mixture will increase the speed of the particles and this will increase the collision frequency and hence the rate of the reaction.

But this is not the most important factor.

Because increasing the energy of the particles also increases the proportion of successful collisions which is much more important to the rate of the reaction.

This will be explained in more depth when we study the Boltzman distribution.

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