title: lesson 3 rate law and reaction order learning objectives: – know that rate law can only be...
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Title: Lesson 3 Rate Law and Reaction Order
Learning Objectives:– Know that rate law can only be derived from experimental
data
– Understand the concept of reaction order
– Identify reaction order from appropriate graphs
– Complete an experiment to determine the order of a reaction with respect to the concentration of acid.
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Recap On the same axes, sketch the Maxwell-
Boltzmann distribution for a lower and a higher temperature, and use this to explain why increasing the temperature increases the rate of reaction.
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Recap Excess magnesium, was added
to a beaker of aqueous hydrochloric acid. A graph of the mass of the beaker and contents was plotted against time (line 1).
What change in the experiment could give line 2?A. The same mass of magnesium in
smaller piecesB. The same volume of a more
concentrated solution of hydrochloric acid
C. A lower temperatureD. A more accurate instrument to
measure the time
M ass
Tim e
2
1
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Recap Which quantities in the enthalpy level
diagram are altered by the use of a catalyst?
A. I and II only B. I and III only C. II and III only D. I, II and III
II
III
IE n th a lpy
Tim e
Which statement is true about using sulfuric acid as a catalyst in the following reaction?
CH3–CO–CH3(aq) + I2(aq) CH3–CO–CH2–I(aq) + HI(aq)
I. The catalyst increases the rate of reaction.II. The catalyst lowers the activation energy for the
reaction.III. The catalyst has been consumed at the end of the
chemical reaction.
A. I and II onlyB. I and III onlyC. II and III
onlyD. I, II and III
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
In this reaction, the concentration of butyl chloride, C4H9Cl, was measured at various times, t.
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
The average rate of the reaction over each interval is the change in concentration divided by the change in time:
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
The average rate decreases as the reaction proceeds.
What do you notice about the average rate?
Why? As the reaction goes forward, there are fewer collisions between reactant molecules.
ExampleCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
Given the following data, what is the average rate of the following reaction over the time interval from 54.0 min to 215.0 min?
CH3OH (aq) + HCl (aq) → CH3Cl (aq) + H2O (l)
Time (min) [HCl] (M)0.0 1.85
54.0 1.58107.0 1.36215.0 1.02
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
Given: [HCl]54 min = 1.58 M [HCl]215 min = 1.02 M
Find: avg. rate of disappearance of HCl
Avg. rate = - D [HCl]D t
= - (1.02 M - 1.58 M) 215 min - 54 min
= 0.0035M / min
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?• A plot of concentration vs. time for this reaction yields a curve like this.
• The slope of a line tangent to the curve at any point is the instantaneous rate at that time.
C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)
Finding the rateCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
Rate laws for the reaction must be determined experimentally.
Measure the instantaneous reaction rate at the start of the reaction (i.e. at t = 0) for various concentrations of reactants.
You CANNOT determine the rate law for the reaction by looking at the coefficients in the balanced chemical equation!
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Now look at this example... An oxidised buckminsterfullerene, C60O3 decomposes into C60O,
releasing O2:
The reaction can be measured by change of absorbance of light of a certain wavelength.
Absorption ∝ [C60O3]Remember: Rate is expressed as a positive value!
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Rate calculated as a function of time:
Rate of reaction plotted against the absorbance of C60O3:
Rate decreases over time, slowing as the concentration of C60O3
decreases.
This mirrors the absorbance graph on the previous slide!
Rate must be related to concentration at each time
The straight line graph of rate against absorbance confirms:
Reaction rate ∝ [C60O3]
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Rate expression or Rate law Reaction rate ∝ [C60O3]
This proportional relationship is converted into an equation by introducing a constant.
Reaction rate = k[C60O3] k = rate constant
This expression is a first order expression because the concentration is raised to the power one.
In general, the rate is proportional to the product of the concentrations of the reactants, each raised to a power.m and n, are known as
the orders of the reaction with respect to reactants A and B.
Overall order is the SUM of the individual orders.
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The table below gives some examples of some reaction equations.
There is no predictable relationship between the co-efficients in the equation and the values for the order of reaction with respect to the reactants.
ORDERS OF REACTION CAN ONLY BE OBTAINED BY EXPERIMENTAL DATA!
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Solutions
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What is reaction order? Reaction order describes how changes to the
concentration of reactants affect the rate of a reaction Assuming temperature and pressure are fixed0th Order
(0o)
Changing the concentration does not
affect the rate
[R] doubled rate same
[R] halved rate same[R] trebled rate same
1st Order (1o)
[R] doubled rate doubled
[R] halved rate halved
[R] trebled rate trebled
2nd Order (2o)
[R] doubled rate quadrupled
[R] halved rate quartered
[R] trebled rate x 9
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For example:
The reaction is 0th order w.r.t reactant A Comparing Runs 2 and 3:
[A] doubles but [B] remains fixed Rate unchanged
The reaction is 1st order w.r.t reactant B Comparing Runs 1 and 2:
[B] doubles but [A] remains fixed Rate doubles
Overall the reaction is 1st order
Run # Initial [A] ([A]0)
Initial [B] ([B]0)
Initial Rate (v0)
1 1.00 M 1.00 M 1.25 x 10-2 M/s
2 1.00 M 2.00 M 2.5 x 10-2 M/s
3 2.00 M 2.00 M 2.5 x 10-2 M/s
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Another example:Experiment Initial [NO] /
mol dm–3
Initial [H2] /mol dm–3
Initial rate /mol (N2) dm–3
s–1
1 0.100 0.100 2.53×10–6
2 0.100 0.200 5.05×10–6
3 0.200 0.100 1.01×10–5
4 0.300 0.100 2.28×10–5
The reaction is 1st order w.r.t reactant H2
Comparing Runs 1 and 2: [H2] doubles but [NO] remains fixed Rate doubles
The reaction is 2nd order w.r.t reactant NO Comparing Runs 1 and 3:
[NO] doubles but [H2] remains fixed Rate quadruples
Overall the reaction is 3rd order (1st order + 2nd order = 3rd order)
First Order ReactionsCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
Expt [A] (M) Rate (M/s)
1 0.50 1.00
2 1.00 2.00
3 2.00 4.00
x2 x2
x2 x2
As [A] doubles, the rate doubles
[A] rate
• First Order Reaction– Overall reaction order = 1– Rate = k[A]
Second Order ReactionsCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
Expt Initial [A] (M) Initial [B] (M ) Rate (mol dm-3 s-1)
1 0.1 0.2 1.6 x 10-2
2 0.1 0.4 3.2 x 10-2
3 0.2 0.2 6.4 x 10-2
x1 x2 x2
[A] stays the same [B] doubles
x2 x1 x4
the rate doubles [B] rate
[A] doubles [B] stays the same
the rate is x4 [A]2 rate
Second Order ReactionsCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
[A] doubles [B] stays the same
[A] stays the same [B] doubles
the rate doubles [B] rate
the rate is x4 [A]2 rate
What is the rate equation for this reaction?
Rate = k[A]2 [B]
The reaction is second order in respect of A and first order in respect of B. The overall reaction order is 3.
Initial [X]/M Initial [Y]/M Initial [Z] / M Initial rate/ mol dm-3 s-1
0.10 0.10 0.10 2.40 x 10-3
0.10 0.10 0.30 7.20 x 10-3
0.05 0.10 0.10 2.40 x 10-3
0.10 0.40 0.10 3.84 x 10-2
Second Order ReactionsCh 1.1 A2
How
do
you
find
reac
tion
rate
s?x1 x3 x3
[Z] triples [X] &[Y] stay the same
X0.5 x1 x1
the rate trebles [Z] rate
[X] halves [Y] & [Z] stay the same
the rate is the same
[X]0 rate
NEx1
[Y] quadruples [X] & [Z] stay the same
the rate goes up by 16 (ie 42 ) [Y]2 rate
x1 x4 x1 x1
Second Order ReactionsCh 1.1 A2
How
do
you
find
reac
tion
rate
s?
What is the rate equation for this reaction?
Rate = k[Y]2 [Z]
The reaction is second order in respect of Y and first order in respect of Z. The overall reaction order is 3.
[X] halves [Y] & [Z] stay the same
[Z] triples [X] &[Y] stay the same
the rate trebles [Z] rate
the rate is the same
[X]0 rate
[Y] quadruples [X] & [Z] stay the same
the rate goes up by 16 (ie 42 ) [Y]2 rate
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Determination of the order of reactionInitial rates method This involves carrying out separate experiments
with different starting concentrations of A, with other reactants held constant effect on [A] can be observed. This can then be repeated for reactant B.
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Solutions
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Graphical representation of reaction kinetics Zero order reaction Concentration of reactant A does not affect
the reaction
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Graphical representation of reaction kinetics First-order reaction Rate is directly proportional to the
concentration A
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Graphical representation of reaction kinetics Second-order reaction Rate is directly proportional to the square of
concentration A
Note: The concentration – time graph is steeper at the start and levels off more (when compared to first-order graph)
Parabola shape – characteristic of the square function
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Summary
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Rate-Concentration Graphs0th Order
No effectGradient 0
1st Order
Direct proportionGradient positive and constant
2nd Order
Squared relationshipGradient positive and increasing
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0th Order
Half-life decreases
Concentration-Time Graphs1st Order
Half-life constant
2nd Order
Half-life increases
t1/2 t1/
2
t1/2 t1/2 t1/2t1/
2
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Constant half life is a feature of only first order reactions
Constant half life can be used to establish that a reaction is first order w.r.t that reactant.
The shorter the half life, the faster the reaction.
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Rate Graphs in Practice
In the experiment you will follow the progress of a reaction using a data logger with pH probe
Follow the instructions here.
This will collect so much data that the only realistic way to analyse it will be by spreadsheet. There is an example here.
Information about R2 values can be found here:https://www.youtube.com/watch?v=kiCeJHwpYDQ
How to do line equations here:https://www.youtube.com/watch?v=Ogx7CJ1JD9k
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Review
The order of a reaction tells us the effect on the rate of changing the concentration of the reactants.
Order can be determined by: Directly comparing experimental data The gradient of a rate-concentration graph The shape of a concentration-time graph