Collision Theory

*CHEMICAL
KINETICS

Collision theory and rates of reaction *One of the main considerations when

examining chemical reaction is whether it will take place fast enough for it to be useful.

*In other words, there’s no point carrying

out a reaction if it takes 150 years to form the product!

*Many chemical reactions occur very quickly. *Studying Reaction

Rates

*Defined as the change in

concentration of reactants of products per unit time.

*Units: mol dm¯³ s¯¹

*Rate of Reaction

* , the change in concentration, can be measured by monitoring a property that will change when the reactants are converted into products:

1.
2.
3.
4.

Change in pH (for acid base reaction)
Change in conductivity (for reaction involving electrolyte) Change in mass or volume (for reaction involving solids or gas)
Change in colour (for reaction involving transition metals or other coloured compound)

*Experimental

measurements of reaction rates *The rate of reaction can be expressed in 3 ways: 1.
2.
3.

Average rate
Instantaneous rate
Initial rate

AVERAGE
RATE
*
*Average rate is a measure of the change in concentration of reactant or product in a given time interval, t.

*Where;
1. = change in concentration of reactant or product
2. = time interval over which the change in concentration was measured.

*.For gases;
*.Where;
1. = change in volume of the gas produced or consumed during the reaction

Volume, V, of carbon dioxide evolved at time t in the reaction between calcium carbonate and hydrochloric acid t/s 0.
0

10.
0

20.
0

30.
0

40.
0

50.
0

60.
0

70.
0

80.
0

90.
0

10
0.0

V(C
O₂)
cm³

0.
0

19.
0

30.
0

37.
5

45.
0

50.
0

52.
0

53.
0

53.
0

53.
0

53.
0

*

*
Instantaneous Rate

INITIAL
RATE
*
*When , the instantaneous rate = the initial rate.

*In order to deduce the initial rate of reaction at t =

0 s, a tangent is drawn to the curve at 0 s, and the gradient of the tangential line gives the initial rate.

Plot of volume of carbon dioxide evolved