Ib Chem - Kinetics Summary Sheet (Sl)

Kinetics

Wednesday, 10 August 2011

12:08 PM

A reaction occurs when two particles at the right orientation collide with each other with enough energy. Rate of reaction is concerned

with how quickly a reaction reaches a certain point (not to be confused with how far a reaction goes...i.e. equilibrium). It is measured in

mol dm-3 s-1 (i.e. concentration per sec)

Rate = Change in concentration or amount / change in time = (c2 - c1)/(t2 - t1)

= ∆[Concentration of reactants]/ ∆time or ∆[Concentration of products]/∆time

For the reaction aA + bB --> cC + dD

Reaction rate:

The Maxwell-Boltzmann distribution curve

To speed up the reaction, you need to increase the number of the very energetic particles - those with energies equal to or greater than

the activation energy. Increasing the temperature has exactly that effect - it changes the shape of the graph.

In the next diagram, the graph labelled T is at the original temperature. The graph labelled T+t is at a higher temperature.

If you now mark the position of the activation energy, you can see that although the curve hasn't moved very much overall, there has been

such a large increase in the number of the very energetic particles that many more now collide with enough energy to react.

Reivison Sheets Page 1

Remember that the area under a curve gives a count of the number of particles. On the last diagram, the area under the higher

temperature curve to the right of the activation energy looks to have at least doubled - therefore at least doubling the rate of the reaction.

Factors affecting reaction rate:

Concentration:

As the concentration of one or both reactants increase, the rate of reaction also increases as there is more of a chance of the reactants

colliding with each other. This explains why the greatest rate of reaction occurs when the two solutions are initially mixed. As the reaction

proceeds, there are less and less reactants colliding with each other....