Kinetics

• collision theory


  reactions can only occur when collisions take place between particles having sufficient energy
• Activation energy, Ea


  the minimum energy required to start a reaction by breaking of bonds
• why do most collisions not lead to a reaction?


  • not all particles have energy greater than Ea
  • not all collisions occur with molecules in the correct orientation
• Rate of reaction


  defined as the change in concentration of a substance in a given time
• units of rate


  moldm-3, s-1
• As a reaction proceeds, the rate decreases as the concentration of reactant falls. The rate at a particular time is equal to the gradient at the time

  
• to calculate gradient


  draw a tangent to curve and then do change in y/ change in x
• concentration(aq)


  increasing the conc. increases the rate

  more particles per unit volume

  more successful collisions per second
• pressure(g)


  increasing the pressure increases the rate

  more particles per unit volume

  more successful collisions per second
• surface area(s)


  increasing the surface area of solids increases the rate

  more particles are available to react at surface

  more successful collisions per second
• temperature


  increasing the temperature increases the rate

  many more particles have more/greater energy

  more successful collisions per second
• catalysts


  a catalyst increases the rate of reaction without being used up

  provides an alternative reaction pathway with a lower Ea

  more particles have energy greater than Ea

  more successful collisions per second. It is not changed in chemical composition or amount
• The maxwell-Boltzmann distribution 


  • when the particles within a particular sample collide, energy is exchange between them, so the particles have a spread of energies
  • the maxwell-Boltzmann distribution shows the distribution of energies of the particles within a sample

  
• Important features of the maxwell-boltzmann distribution


  • the area under the curve equals the total number of particles
  • no particles have zero energy (line goes through (0,0) )
  • there is not a maximum energy a particle can have (line does not touch x axis at high E)
  • the peak of the curve represents the most probable energy of a particle (Emp)
  • Ea is much higher than Emp
  • only a small proportion of particles have energy greater than Ea
• How would temperature affect the maxwell-Boltzmann distribution?


  The peak (emp) is further to right and lower than original. More particles have higher energy (area is same). Many more particles have energy > Ea. So more successful collisions per second

  
• Why does a small temp increase lead to a large increase in rate?

  Small temp changes causes a large increase in particles with energy greater than Ea.

  So many more successful collisions per second
• energy profile diagram- catalysed route 


  In the presence of a catalyst, the activation energy is lowered. But Delta H is not changed.

  
• Maxwell-Boltzmann distribution- catalyst


  The shaded area is now bigger, so more particles have energy greater than Ea so more successful collisions per second