5.111 Lecture Summary #30 Monday, November 24, 2014
Reading for Today: 14.1-14.5 in 5th ed and 13.1-13.5 in 4th ed.
Reading for Lecture #31: 14.6, 17.7 in 5th ed and 13.6, 17.7 in 4th ed.
Topic: Introduction to Kinetics
I. Rates of Chemical Reactions
II. Rate Expressions and Rate Laws
Kinetics Versus Thermodynamics
When considering a chemical reaction, one must ask whether the reaction will go forward
spontaneously (thermodynamics), and the reaction will go
(kinetics).
Stable/unstable r efers to ( tendency to decompose)
Labile/nonlabile (inert) refers to the at which this tendency is realized
Rate is important. A chemical kinetics experiment measures the rate at which the
concentration of a substance taking part in a chemical reaction changes with time.
Factors affecting rates of chemical reactions
Let’s consider the oscillating clock reaction
To understand this reaction, one must consider thermodynamics, chemical equilibrium, acid-
base, oxidation-reaction, kinetics, and the influence of oxidation and liganded state to color.
The overall reaction is: IO3- + 2 H2O2 + CH2(CO2H)2 + H+ → ICH(CO2H)2 + 2 O2 + 3 H2O
Its mechanism involves multiple steps, including:
(a) IO3- + I- + 2 H2O2 + 2 H+ → 2 O2 + 3 H2O + I2 (spontaneous when [I2] is low)
(b) I2 + CH2(CO2H)2 → ICH(CO2H)2 + H+ + I- (spontaneous when [I2] is high)
Reaction (a): addition of IO3- and I- to hydrogen peroxide (H2O2) under acidic conditions,
turns a clear solution to amber (I- is clear and I2 is amber).
Reaction (b): addition of I2 (I2 is amber) to malonic acid (CH2(CO2H)2), generates a complex
that is blue. Thus, the color of I depends on both oxidation and liganded state.
Let’s think about the oxidation-reduction processes in Reaction (a):
I in IO3- is being to I 2; I- is being to I 2;
O in H2O2 is being to O2; O in H2O2 is being to H2O.
With a large (+) E°, H2O2 is
The reaction rate is also sensitive to temperature.
1
, I. Rates of Chemical Reactions
Measuring average reaction rates
Consider: NO2 (g) + CO (g) → NO (g) + CO2 (g)
[NO]
Can monitor the changes in concentration of NO
(M)
0.0288
average rate = change in concentration
change in time
0.0160
average rate =
average rate = 0.0288 - 0.0160 M =
150. - 50. sec
50. 150.
time (sec) average rate depends on time interval chosen
Measuring instantaneous reaction rates
Consider: NO2 (g) + CO (g) → NO (g) + CO2 (g)
Instantaneous rate = limit∆t →0 [NO]t + ∆t - [NO] t = d[NO]
∆t dt
As ∆t approaches 0, the rate becomes the
0.0326 slope of the line tangent to the curve at
time t.
0.0249
[NO] Instantaneous rate at t=150 sec is
(M)
0.0326 - 0.0249 M = 7.7 x 10-5 M s-1
200. -100. sec
Initial rate = Instantaneous rate at time
equals sec
100. 150. 200.
time (sec)
2
Reading for Today: 14.1-14.5 in 5th ed and 13.1-13.5 in 4th ed.
Reading for Lecture #31: 14.6, 17.7 in 5th ed and 13.6, 17.7 in 4th ed.
Topic: Introduction to Kinetics
I. Rates of Chemical Reactions
II. Rate Expressions and Rate Laws
Kinetics Versus Thermodynamics
When considering a chemical reaction, one must ask whether the reaction will go forward
spontaneously (thermodynamics), and the reaction will go
(kinetics).
Stable/unstable r efers to ( tendency to decompose)
Labile/nonlabile (inert) refers to the at which this tendency is realized
Rate is important. A chemical kinetics experiment measures the rate at which the
concentration of a substance taking part in a chemical reaction changes with time.
Factors affecting rates of chemical reactions
Let’s consider the oscillating clock reaction
To understand this reaction, one must consider thermodynamics, chemical equilibrium, acid-
base, oxidation-reaction, kinetics, and the influence of oxidation and liganded state to color.
The overall reaction is: IO3- + 2 H2O2 + CH2(CO2H)2 + H+ → ICH(CO2H)2 + 2 O2 + 3 H2O
Its mechanism involves multiple steps, including:
(a) IO3- + I- + 2 H2O2 + 2 H+ → 2 O2 + 3 H2O + I2 (spontaneous when [I2] is low)
(b) I2 + CH2(CO2H)2 → ICH(CO2H)2 + H+ + I- (spontaneous when [I2] is high)
Reaction (a): addition of IO3- and I- to hydrogen peroxide (H2O2) under acidic conditions,
turns a clear solution to amber (I- is clear and I2 is amber).
Reaction (b): addition of I2 (I2 is amber) to malonic acid (CH2(CO2H)2), generates a complex
that is blue. Thus, the color of I depends on both oxidation and liganded state.
Let’s think about the oxidation-reduction processes in Reaction (a):
I in IO3- is being to I 2; I- is being to I 2;
O in H2O2 is being to O2; O in H2O2 is being to H2O.
With a large (+) E°, H2O2 is
The reaction rate is also sensitive to temperature.
1
, I. Rates of Chemical Reactions
Measuring average reaction rates
Consider: NO2 (g) + CO (g) → NO (g) + CO2 (g)
[NO]
Can monitor the changes in concentration of NO
(M)
0.0288
average rate = change in concentration
change in time
0.0160
average rate =
average rate = 0.0288 - 0.0160 M =
150. - 50. sec
50. 150.
time (sec) average rate depends on time interval chosen
Measuring instantaneous reaction rates
Consider: NO2 (g) + CO (g) → NO (g) + CO2 (g)
Instantaneous rate = limit∆t →0 [NO]t + ∆t - [NO] t = d[NO]
∆t dt
As ∆t approaches 0, the rate becomes the
0.0326 slope of the line tangent to the curve at
time t.
0.0249
[NO] Instantaneous rate at t=150 sec is
(M)
0.0326 - 0.0249 M = 7.7 x 10-5 M s-1
200. -100. sec
Initial rate = Instantaneous rate at time
equals sec
100. 150. 200.
time (sec)
2
