Rates of Reaction

The collision theory model

• For two reactant particles to react, they must:
  • Collide with each other
  • Collide with at least the minimum kinetic energy needed to break the existing bonds, called the activation energy (E_a)
  • Collide with the correct orientation so that bonds can rearrange
• A collision that meets all three conditions is a successful collision
• The rate of reaction is set by the number of successful collisions per second
• Anything that increases either the frequency of collisions or the proportion of collisions that have enough energy will speed the reaction up

How concentration changes the rate

• A higher concentration packs more reactant particles into the same volume
• More particles in a fixed volume means more collisions per second between the right kinds of particle
• More collisions → more successful collisions per second → faster reaction
• Doubling concentration roughly doubles the collision rate

How pressure changes the rate (for gases)

• Increasing the pressure of a gas means compressing the same number of particles into a smaller volume
• The result is the same as increasing concentration in solution — more particles per unit volume, more collisions per second, faster reaction

How temperature changes the rate

• Increasing the temperature has two effects, both of which speed the reaction up:
  • Particles move faster and so collide more often
  • More importantly, a larger fraction of particles now have kinetic energy at or above the activation energy, so a much larger proportion of collisions are successful
• The energy effect dominates: as a quick guide, the rate of many common reactions doubles for every 10 °C rise in temperature

How surface area changes the rate

• For a solid reacting with a liquid or gas, the reaction can only happen at the surface of the solid where the two phases meet
• Cutting the solid into smaller pieces exposes more of the underlying particles
• More exposed area = more collisions per second between the solid surface and the surrounding particles = faster reaction
• Same idea as crushing a sugar cube into powder — it dissolves much faster