chemical ideas 10.3 the effect of concentration on rate
TRANSCRIPT
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Chemical Ideas 10.3
The Effect of Concentration on Rate
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rate Listen: [ rāt ]n.
A quantity measured with respect to another measured quantity.
speed = rate of change of distance
inflation = rate of change of
prices
when taking about rate you MUST be clear about units being
used
m/s %/year
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rate of reaction
rate at which products are converted to
reactants
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0.0001 mol O2 formed
0.0002 mol H2O formed
0.0002 mol H2O2 used up
EACH SECOND!!!
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Measuring the rate of a reaction.
1. Decide on a property of reactant or product that you can measure.
2. Measure the change in property over a certain time
3. Find the rate
measure the change in amount of a reactant or product in a certain
time
change of property
time
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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
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This graph shows us that rate is directly proportional to the
concentration of hydrogen peroxide
rate [H2O2(aq)]rate = constant x
[H2O2(aq)]
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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]
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• 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.
rate = k [H2O2(aq)] [catalase]
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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
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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
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• For the reaction 2Br (g) Br2 (g)
• Rate equation is – rate = k [Br]2
• S2O82-
(aq) + 2I- (aq) SO4
2- (aq) + I2 (aq)
– rate = k [S2O82-
(aq) ] [I- (aq) ]
you cannot cannot predict the rate equation for a reaction from it’s balanced
equation
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BrO3-(aq) + 5Br-
(aq) + 6H+ (aq)
5H2O (l) + 3Br2 (aq)
rate = k [BrO3-] [Br-] [H+]2
you cannot cannot predict the rate equation for a reaction from it’s balanced
equation
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Half Lifes
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0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
160.0
180.0
200.0
220.0
240.0
0 20 40 60 80 100 120 140 160 180
time / s
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Chemical Ideas 10.3 (again)
Knowing how concentration affects rate can tell us something about the way reactions occur.
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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
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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
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half-lives (t ½ )Reactions which are first
order will show a curve that is identical to radioactive decay!
Time taken for half of a reactant to get used up
in the reaction
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For a first order reaction the half-life is always constant no matter what the starting amount!
zero order & second order reactions do not have this
feature
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Finding the Order of a Reaction
• 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.
you cannot cannot predict the rate equation for a reaction from it’s balanced equation
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Progress Curve Method
• 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?)
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Initial Rates Method – drawing tangents
• most used• Several
experimental runs are completed (as in activity EP6.3).
0.00
0.10
0.20
0.30
0.40
0.50
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
concentr ation
• Initial rate is calculated by drawing tangents at the origin.
• We then plot initial rate against concentration
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producing graphs
[A]
[A] [A]2
[A]
rate
raterate
rate
zero order first order
second order second order
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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.
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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.
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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.
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CH3
CH3
CH3
BrC + OH-
CH3
CH3
CH3
Br-C OH +
first order w.r.t. (CH3)3CBr
zero order w.r.t. OH-
rate = k[(CH3)3CBr]
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CH3
CH3
CH3
BrC +
OH-CH3
CH3
CH3
C OH+
step one CH3
CH3
CH3
C+ Br-
CH3
CH3
CH3
C+step two
Ξfast
slow
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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]
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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.