Chemical Ideas 10.3
The Effect of
Concentration on Rate
rate Listen: [ rāt ]
n.
A quantity measured with respect
to another measured quantity.
speed = rate of
change of distance
m/s
inflation = rate of
change of prices
%/year
when taking about rate you MUST be
clear about units being used
rate of
reaction
rate at which
products are
converted to
reactants
0.0001 mol O2
formed
0.0002 mol H2O
formed
0.0002 mol H2O2
used up
EACH SECOND!!!
Measuring the rate of a reaction.
measure the change in amount of a
reactant or product in a certain time
1. Decide on a property of reactant or
product that you can measure.
2. Measure the change in property over
a certain time
change of property
3. Find the rate
time
concentration of H2O2
at start / mol dm3
Initial rate /
(cm3 of
O2(g))s-1
0.40
0.51
0.32
0.41
0.24
0.32
0.16
0.21
0.08
0.10
This graph shows us that rate is directly
proportional to the concentration of
hydrogen peroxide
rate =
constant
[H2]O2(aq)]
rate
 [H2Ox2(aq)
The concentration of the enzyme
catalase also affects the rate of the
reaction …
rate = constant x [catalase]
We can combine the two equations to
get …
rate = constant x [H2O2(aq)] x [catalase]
rate = k [H2O2(aq)] [catalase]
rate = k [H2O2(aq)] [catalase]
• This is the rate equation for the
reaction
• the constant k is called the rate
constant
• k varies with temperature, therefore
you must always state the
temperature at which measurements
are made.
Order of a Reaction
• For a reaction in which A & B are
reactants …
–A + B  products
• The general rate equation is…
– rate = k [A]m [B]n
• m and n are powers to which the
concentration must be raised. usually
have values of 0, 1 or 2.
• m & n are called the order of the
reaction
decomposition of hydrogen peroxide
rate = k [H2O2(aq)] [catalase]
• The reaction is first order with respect to
H2O2
• The reaction is also first order with
respect to catalase.
• The overall order of a reaction is given by
(m + n).
• the reaction is overall second order
• For the reaction 2Br (g)  Br2 (g)
• Rate equation is
– rate = k [Br]2
• S2O82-(aq) + 2I- (aq)  SO42- (aq) + I2 (aq)
– rate = k [S2O82-(aq) ] [I- (aq) ]
you cannot predict the rate equation
for a reaction from it’s balanced
equation
BrO3-(aq) + 5Br-(aq) + 6H+ (aq) 
5H2O
(l)
+ 3Br2 (aq)
rate = k [BrO3-] [Br-] [H+]2
you cannot predict the rate equation
for a reaction from it’s balanced
equation
Half Lifes
Chemical Ideas 10.3
(again)
Knowing how concentration
affects rate can tell us
something about the way
reactions occur.
Remember …
The rate of any reaction can be expressed in
terms of the concentrations of its
reactants
rate = k [A]x [B]y [C]z
x,y & z are the order of the reaction with
respect to that reaction.
If they =1 the number is not shown
decomposition of hydrogen peroxide
rate = k [H2O2(aq)] [catalase]
• The reaction is first order with respect to
H2O2
• The reaction is also first order with
respect to catalase.
• The overall order of a reaction is given by
(m + n).
• the reaction is overall second order
half-lives (t ½ )
Reactions which are first order
will show a curve that is
identical to radioactive decay!
50
Time taken for half of a
reactant to get used up
in the reaction
40
30
20
10
0
0
10
20
30
40
50
t ime / s
60
70
80
90
10
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
10
t ime / s
For a first order reaction the halflife is always constant no matter
what the starting amount!
zero order &
second order
reactions do not
have this feature
Finding the Order of a Reaction
you cannot predict the rate equation for a
reaction from it’s balanced equation
• To find out the order of a reaction it is
necessary to carry out practical
experiments.
• The data can then be used to determine
the order of the reaction.
Progress Curve Method
50
40
30
20
10
0
0
10
20
30
40
50
t ime / s
60
70
80
90
100
• Rate is calculated by drawing tangents to the curve at
various points
• Can then find the order with respect to a
reactant/product
• tedious & inaccurate (unless using a PC?)
Initial Rates Method – drawing tangents
• most used
• Several
experimental
runs are
completed (as
in activity
EP6.3).
• Initial rate is calculated by drawing tangents at
the origin.
• We then plot initial rate against concentration
producing graphs
first order
zero order
rate
rate
[A]
[A]
second order
second order
rate
rate
[A]
[A]2
Initial Rates Method – reciprocal of time
• Measuring how long to produce a small
fixed amount of one of the products.
• Time taken is called the reaction time.
• Rate is high – reaction time small
• Rate low – reaction time large.
• Average rate  1/t .
• Graph of 1/t against concentration.
half-lives method
• You can use the progress curve to
determine half-lives for the reaction.
• If they are constant then the reaction is
first order.
rate equations & rate
mechanisms
when we know the rate
equation we can link it to the
reaction mechanism.
We can then work out the rate
determining step.
CH3
CH3
C
+
Br
OHfirst order w.r.t.
(CH3)3CBr
CH3
zero order w.r.t.
OH-
CH3
CH3
C
CH3
OH
+
Br-
rate = k[(CH3)3CBr]
CH3
CH3
C
CH3
Br
CH3
step one
C+ + Br-
CH3
slow
CH3
CH3
CH3
C+ +
CH3
CH3
OH-
step two
Ξfast
CH3
C
CH3
OH
mechanism of enzyme catalysed reactions …
• when the substrate concentration is low for
the reaction
– rate = k[E][S] ([E] is concentration of enzyme)
• we can deduce from this that the rate
determining step involves one enzyme
molecule & one substrate molecule.
• Following steps are faster.
• Substrate concentration high then
– rate = k [E]
WHY? – why are some steps
slow & others fast?
• One reason = different steps have different
activation enthalpies.
– Large activation enthalpy, only a small number of
molecules pass over it each second so rate of
reaction is slow.
– Small activation enthalpy, greater proportion of
molecules can pass each second, hence a faster
rate.