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C
hem
F
actsheet
January 2003
Number 43
1
To succeed with this topic you need to:
•
Know and understand the AS material on kinetics 
♦
collision theory
♦
activation energy
♦
MaxwellBoltzman graph
♦
Factors affecting rates of reaction
♦
Catalysts and energy profiles covered in Factsheet No. 10 (Kinetics I)
After working through the Factsheet you will be able to:
•
understand the parts making up the rate equation (or expression);
•
be able to find the orders for a particular chemical reaction from the
data given for it;
•
be able to perform calculations based on the rate equation;
•
be able to work out the units for the rate constant,
k
.
Rate expressions (equations)
The
balanced chemical equation
(the stoichiometric
equation)
can tell you
nothing
about the rate expression.
The rate expression can only be worked out from the
data
provided
by reaction rate
experiments.
Let us consider a reaction of substances A, B and C where the equation is
A + B + 2C
→
2D + E
The
rate expression
(or equation)
works out to be
[ ] = concentration i.e. mol dm
3
'orders'
rate =
k
×
[A]
2
×
[B]
1
rate constant
units of mol dm
3
s
−
1
In words:
The reaction is 2
nd
order with respect to A and 1
st
order with respect to B.
Overall, the reaction is 3
rd
order (2 + 1)
Note the following points about this example:
(1) We would mormally write [B], not [B]
1
. The 1 is put in to make it clear
(2) C does not appear in the rate expression – it is ‘0 order’ and
if shown
would look like this [C]
0
(3) the expression would normally be shown as
rate
=
k
[A]
2
[B]
with the ‘
×
’ symbol removed.
Finding the orders from experimental data
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