9.3/9.4 - Collision Theory

• collision theory

  for a reaction to happen particles must collide in the right direction and with enough kinetic energy for bonds to break (activation energy)
• activation energy

  minimum amount of kinetic energy required for bonds to break and reaction to happen
• reactions with low activation energy

  need less energy to break the bonds (usually heat energy) so happen more easily
• kinetic theory assumptions about ideal gases

  made of molecules that are in constant random motion in straight lines

  gas pressure is due to collisions between molecules and walls of the container which are elastic - no loss in kinetic energy

  volume occupied by molecules themselves is negligible relative to volume of the container

  there are no intermolecular forces between gas molecules

  average kinetic energy of molecules is proportional to their temperature in Kelvin
• when do real gases approach ideal gas behaviour and why?

  at high temperatures molecules move so fast that intermolecular forces can be ignored

  at low pressures the volumes are so big that space taken up by molecules can be ignored
• when do real gases deviate from ideal gas behaviour and why?

  High pressure or low temperature; due to intermolecular forces and molecular volume.

  high pressure: low volume so can't ignore space occupied by molecules

  low temperature: as gases liquify there are intermolecular forces present
• Maxwell-Boltzmann distribution

  graph showing the distribution of molecular kinetic energy for gas particles at a certain temperature

  shows they are moving at different speeds as they have different amounts of kinetic energy
• effect of surface area on rate of reaction

  the higher the surface area the faster the rate of reaction because there is a higher chance of collisions
• effect of concentration on rate of reaction

  the higher the concentration the more particles in a given volume so more collisions so faster rate of reaction
• effect of pressure on rate of reaction

  higher pressure increases rate of reaction as there are more particles in a given volume so more chances of collisions
• effect of temperature on rate of reaction

  the higher the temperature the more molecules have enough energy to react when they collide so faster rate of reaction
• effect of catalysts on rate of reaction

  increase rate of reaction by providing an alternative route for the reaction with a lower activation energy

  one way of doing this is by forming an intermediate which breaks down to form the products, releasing the catalyst