Collision theory and activation energyIB MYP Chemistry: Revision notes

Collision theory explains how reactions happen. For a reaction to occur:

1. The reacting particles must collide.
2. They must collide with at least a minimum amount of energy, the activation energy.

A collision that meets both conditions is a successful collision. Many collisions are not successful because the particles bounce apart without reacting.

The rate of reaction depends on the number of successful collisions per second.

Activation energy is the minimum energy that colliding particles need for a reaction to happen. It acts like an energy barrier. Particles with less than the activation energy bounce off each other; particles with at least that much energy react.

Different reactions have different activation energies. Reactions with a low activation energy are fast at room temperature, while those with a high activation energy are slow.

These factors change how often particles collide, not how much energy they have.

• Higher concentration: more particles in the same volume, so more frequent collisions.
• Higher pressure (gases): particles are closer together, so more frequent collisions.
• Larger surface area (solids): more particles are exposed, so more frequent collisions.

More frequent collisions means more successful collisions per second, so a faster rate. As the reaction goes on, reactants are used up, their concentration falls and the rate slows.

Raising the temperature has two effects:

• The particles move faster, so they collide more frequently.
• They have more energy, so a greater proportion of collisions have energy equal to or greater than the activation energy.

The second effect is the more important one, which is why a small temperature rise can speed up a reaction a lot. Both give more successful collisions per second.

A catalyst speeds up a reaction without being used up. It provides an alternative pathway with a lower activation energy, so a greater proportion of collisions are successful. It does not change the energy the particles have.

Industrial benefits: a faster rate, a reaction at lower temperature or pressure (saving energy and cost) and a catalyst that can be reused.

Examples: iron in the Haber process for ammonia; platinum in catalytic converters. Drawbacks include the cost of some metals and catalyst poisoning by impurities.

Enzymes are biological catalysts: proteins that speed up reactions in living cells, such as in digestion and in fermentation by yeast. Each enzyme is specific, because its active site fits only one substrate.

Enzymes work best at an optimum temperature, about 37 °C for human enzymes. At high temperatures they are denatured: the active site changes shape and the enzyme stops working. Enzymes are used in industry in brewing, baking and biological washing powders.