1. 11.2 Reaction Rate and Concentration
YOU ARE EXPECTED TO BE ABLE TO:
Define the initial rate of a reaction as the instantaneous rate at the start of the reaction.
Write a rate law expression for a given reaction given experimental results that show the variation of initial rate with concentration.
Define the concept of order of reaction and determine the order of a given reaction from initial rate vs concentration data
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2. Initial Rate of Reaction
The initial rate of reaction is the instantaneous rate measured at the start of the reaction
The initial rate can be used as a benchmark
Measurements of the initial rate for a reaction at different starting concentrations can be used to determine the effect of concentration on rate
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3. The Rate Expression or Rate Law
In general:
Reactions occur as a result of collisions
The higher the concentration of molecules, the greater the rate
In some reactions, the rate is directly proportional to the concentration
In other cases, the rate may increase much more quickly than the concentration
The relationship between rate and concentration is known at the rate law or rate expression
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4. Determining the Rate Law Expression
The rate law MUST be determined from experiment – it is not related to the stoichiometry of the equation for the reaction
Examine the concentration vs time graph for N2O5
Determine instantaneous rates at different times and concentrations of N2O5
Plot instantaneous rate vs [N2O5 ]
This is a straight line, (equation y = mx + b)
Rate = k[N2O5] ; this is the rate law expression; k is the rate constant
P305 fig 11.3
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5. Using Initial Rates to determine the rate Law
Measurements of the initial rate of reaction at different initial concentrations can be used to determine the rate law expression
If a graph of initial concentration vs initial rate is a straight line, then rate is directly proportional to concentration:
rate = k [reactant]
If a graph is a curve, then rate = k[reactant]m
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6. Order of Reaction The rate law expression is of the form:
Rate = k[A]m
The power to which the concentration is raised is called the order of reaction
If m = 0, then “zero order”
If m = 1, then “first order”
If m = 2, then “second order”
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7. Calculating the Order of Reaction when there is One Reactant
If m is not equal to 1, the order of the reaction can be determined by comparing two rates at two different concentrations:
Rate1 = k[A]1m
Rate2 = k[A]2m
Rate1 = k[A]1m = [A]1 m
Rate2 k[A]2m [A]2
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8. Example 11.1 CH3CHO(g)  CH4(g) + CO(g) [ ] 0.10M 0.20M 0.30M 0.40M
Rate (mol/L.s)
Choose the first two concentrations
Rate2 = = 4.0
Rate
[CH3CHO]2 = = 2.0
[CH3CHO]
4.0 = (2.0)m ; m = 2
Rate = k[CH3CHO]2 The reaction is second order
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9. Calculate the Rate Constant
Rate = k[CH3CHO]2
Choose one set of data
0.085mol/L.s = k x (0.10 mol/L)2
k = 0.085mol/L.s = 8.5 L/mol.s
(0.10 mol/L)2
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10. Order of Reaction with More Than One Reactant
Most reactions involve >1 reactant
aA + bB  products
General rate law expression is:
rate = k[A]m x [B]n
m is “order with respect to A
n is order with respect to B
Overall order is m + n
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11. Determining the Order
Hold concentration of one reactant constant and vary other - measure initial rate
Repeat, holding concentration of other reactant constant
See Example 11.2, page 308
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12. Determining the Order of Reaction Graphically
As an alternative to calculating the order of reaction by comparing two pieces of data, a graphical process can be used
Plot rate vs concentration
If the result is a straight line, there is a first order reaction, slope = k
If the result is a curve, it is necessary to construct a graph of (natural) log rate vs (natural) log concentration to determine m
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13. Why Use a Logarithm? If rate = k[A]m Then ln rate = ln k + m ln [a]
This will be a straight line with slope m
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14. Zero Order Reactions
In rare cases, the rate of reaction is independent of the concentration of reactant(s)
In these cases
Rate = k[A]0
A plot of rate vs [A] would be a horizontal straight line
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