Post on 06-Feb-2016
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Slide 1 of 61
14-4 Zero-Order Reactions
A → products
Rrxn = k [A]0
Rrxn = k
[k] = mol L-1 s-1
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Integrated Rate Law
- dt= kd[A] [A]0
[A]t
0
t
-[A]t + [A]0 = kt
[A]t = [A]0 - kt
Δt
-Δ[A]
dt= k
-d[A]Move to the
infinitesimal= k
And integrate from 0 to time t
Slide 3 of 61
14-5 First-Order Reactions
H2O2(aq) → H2O(l) + ½ O2(g)
= -k[H2O2] d[H2O2 ]
dt
= - k dt[H2O2]
d[H2O2 ][A]0
[A]t
0
t
= -ktln[A]t
[A]0
ln[A]t = -kt + ln[A]0
[k] = s-1
Slide 4 of 61
First-Order Reactions
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Half-Life
t½ is the time taken for one-half of a reactant to be consumed.
= -ktln[A]t
[A]0
= -kt½ ln½[A]0
[A]0
- ln 2 = -kt½
t½ = ln 2
k
0.693
k=
Slide 6 of 61
Half-Life
ButOOBut(g) → 2 CH3CO(g) + C2H4(g)
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Some Typical First-Order Processes
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14-6 Second-Order Reactions
Rate law where sum of exponents m + n +… = 2.
A → products
dt= - kd[A]
[A]2[A]0
[A]t
0
t
= kt +1
[A]0[A]t
1
dt= -k[A]2
d[A][k] = M-1 s-1 = L mol-1 s-1
Slide 9 of 61
Second-Order Reaction
Slide 10 of 61
Testing for a Rate Law
Plot [A] vs t. Plot ln[A] vs t. Plot 1/[A] vs t.
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14-7 Reaction Kinetics: A Summary
Calculate the rate of a reaction from a known rate law using:
Determine the instantaneous rate of the reaction by:
Rate of reaction = k [A]m[B]n ….
Finding the slope of the tangent line of [A] vs t or,
Evaluate –Δ[A]/Δt, with a short Δt interval.
Slide 12 of 61
Summary of Kinetics
Determine the order of reaction by:
Using the method of initial rates.
Find the graph that yields a straight line.
Test for the half-life to find first order reactions.
Substitute data into integrated rate laws to find the rate law that gives a consistent value of k.
Slide 13 of 61
Summary of Kinetics
Find the rate constant k by:
Find reactant concentrations or times for certain conditions using the integrated rate law after determining k.
Determining the slope of a straight line graph.
Evaluating k with the integrated rate law.
Measuring the half life of first-order reactions.
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Worked Examples Follow:
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CRS Questions Follow:
Slide 21 of 61
In the diagram to the right is a plot of the concentrations of all reactants and products as a function of time for a particular reaction. ln
[X
]
t
Which of the following statements is correct?
5. Cannot tell with the information given.
1. The reaction is second order.
2. The blue curve represents the time dependence on a particular product.
3. The rate constant for the reaction could have units of s-1.
4. The product represented by the green line is produced almost twice as fast as the product represented by the red line.
Slide 22 of 61
In the diagram to the right is a plot of the concentrations of all reactants and products as a function of time for a particular reaction. ln
[X
]
t
Which of the following statements is correct?
5. Cannot tell with the information given.
1. The reaction is second order.
2. The blue curve represents the time dependence on a particular product.
3. The rate constant for the reaction could have units of s-1.
4. The product represented by the green line is produced almost twice as fast as the product represented by the red line.
Slide 23 of 61
Reaction A has a rate constant which is equal to 16.2 L mol-1 s-1. A plot relating the concentrations of the reactants and products with time is plotted for reaction B to the right.
ln [
X]
t
In this question, two reactions are considered.
1. Reaction A is first order and Reaction B is second order.
2. Reaction A is second order and Reaction B is first order.
3. Both reactions are first order.
4. Both reactions are second order.
Slide 24 of 61
Reaction A has a rate constant which is equal to 16.2 L mol-1 s-1. A plot relating the concentrations of the reactants and products with time is plotted for reaction B to the right.
ln [
X]
t
In this question, two reactions are considered.
1. Reaction A is first order and Reaction B is second order.
2. Reaction A is second order and Reaction B is first order.
3. Both reactions are first order.
4. Both reactions are second order.
Slide 25 of 61
time / s0 50 100 150 200 250 300
[N2O
5]
0.0
0.1
0.2
0.3
0.4
0.5
81.7 s
81.7 s
81.7 s
2 5 2 4 22N O 2N O + OSome data for the decomposition of dinitrogen pentoxide are plotted to the right. Which of the following statements is correct?
1. The rate constant is 81.7 s.
3. The units of the rate constant are s.
4. The reaction is first order in N2O5.
5. The reaction is third order in N2O5.
2. The rate constant is1
.81.7 s
Slide 26 of 61
time / s0 50 100 150 200 250 300
[N2O
5]
0.0
0.1
0.2
0.3
0.4
0.5
81.7 s
81.7 s
81.7 s
2 5 2 4 22N O 2N O + OSome data for the decomposition of dinitrogen pentoxide are plotted to the right. Which of the following statements is correct?
1. The rate constant is 81.7 s.
3. The units of the rate constant are s.
4. The reaction is first order in N2O5.
5. The reaction is third order in N2O5.
2. The rate constant is1
.81.7 s