Presentation of HL content for T16 Chemical kinetics.

1. T6/16- Chemical kinetics
Picture and some content from AS boardworks ©

2. Lesson outline….
• Lesson 1- Topic 6 review.
• Lesson 2- All about the rate equation.
• Lesson 3- Determining a rate equation
experimentally(P).
• Lesson 4- Evaluation of the iodination of propanone.
• Lesson 5- Evaluating proposed reaction mechanisms.
• Lesson 6- The Arrhenius equation.
• Lesson 7- Determining Ea experimentally (P).
• Lesson 8- exam question review☺
*Note: P= Practical☺

3. 16.1- Rate expression and
reaction mechanism

4. Lesson 1- Topic 6 review.
Level 4: State the main principles of collision
theory.
Level 7: Explain how an increase in
temperature affects the activation energy
(Ea)of a chemical reaction.
Level 5/6: Describe at least two ways of
determining the rate of a chemical reaction.

5. Topic 6 review questions- see Pearson questions
(MCQ and short answer)

6. Starter

8. Measuring rates

10. Homework task….
• Introduction to ‘kognity’
• Complete the SL review task on topic 6:
https://kristin.kognity.com/schoolstaff/app/che
mistry-hl-2016/assignment/44231/

11. Lesson 2- All about the rate equation.
Level 4: Label all aspects of a rate
equation/expression.
Level 7: Explain the difference between a
zero, first and second order reaction using
graphs, text and numbers.
Level 5/6: Deduce the overall order of a
reaction from given information.

12. The rate equation/expression
The rate equation is an equation that relates the
concentrations of substances involved in a reaction to the
rate of the reaction. For the reaction A + B C
the rate of reaction depends on the concentrations of A and B
([A] and [B]) and various constants in the following way:
Rate = k[A]m [B]n
 k is the rate constant (units depend of values of m and n)
 m is the order of reaction with respect to A
 n is the order of reaction with respect to B.
m + n = overall order of the reaction

13. Determining the rate equation
The rate equation can be determined by completing a
series of experiments varying the concentrations of each of
the reactants.
A similar set of experiments can be carried out keeping [A]
constant and varying [B] to determine how changing [B]
affects the rate of reaction.
To determine how [A] affects the rate, several different
experiments can be carried out in which [B] is kept constant
and [A] is changed. The data can then be used to work out the
relationship between rate and [A].
A + B C
*See iodine and propanone investigation in lesson 3.

14. Orders of reaction

15. Two very useful graphs to remember…
Taken from Pearson 2nd edition page 293

16. The effect of temperature on k
When temperature increases, rate of reaction increases. This is
because the rate constant, k, increases with temperature.
556
575
629
666
700
781
4.45 × 10–5
1.37 × 10–4
2.52 × 10–3
1.41 × 10–2
6.43 × 10–2
1.34
temp. (K)
k(moldm3s–1)
temperature (K)
rate constant, k
(moldm3 s–1)
Note: the rate constant k is written as a lower case letter to distinguish
it from K for kelvin or Kc for equilibrium constant.

17. Check for learning….
See homework questions from Pearson on
reaction order and the rate equation.

18. Lesson 3- Determining the rate expression
experimentally (P).
Level 4: Measure the rate of a reaction in
response to changing [reactant].
Level 7: Explain why measuring time alone is
not an indication of the rate of a reaction.
Level 5/6: Calculate the values of x, y and z
(reactant orders) and determine the rate
expression.

19. starter

20. Aim: to determine the overall order of
a reaction experimentally.
• See method from inthinking- I2 + propanone.
Mixture [CH3COCH3]
mmol.dm-3
[I2]/
mmol.dm-3
[H+]/
mmol.dm-3
Time for
colour change
(s)
A 400 1.00 400
B 800 1.00 400
C 1200 1.00 400
D 400 0.50 400
E 400 0.25 400
F 400 1.00 800
G 400 1.00 1200
*The points where the concentration of a reactant has been changed are highlighted in green.

21. Lesson 4- Evaluation of the iodination of
propanone ( ‘acetone’).
Level 4: State the overall order of this
chemical reaction.
Level 7: Evaluate some of the assumptions
which have been made in this practical.
Level 5/6: Calculate the rate constant for this
reaction based on your experiment.

22. starter
The literature value for the rate constant of this
particular reaction is given as:
K= 3.76 x 10-5
How does your rate constant compare to this value?

23. Interpreting your results….
Your experimental data may be used to answer the
following questions:
1. Calculate the order with respect to propanone.
2. Calculate the order with respect to Iodine.
3. Calculate the order with respect to H+.
4. Deduce the rate equation for this reaction.
5. *Calculate a value for the rate constant (k)
6. Derive the units of the rate constant.

24. Aim: to determine the overall order of
a reaction experimentally.
• See method from inthinking- I2 + propanone.
[CH3COCH3]
mmol.dm-3
[I2]/
mmol.dm-3
[H+]/
mmol.dm-3
Time for
colour
change (s)
Rate=
∆ [I2]/time
(mmol.dm-3.s-1)
Rate
constant k
(units?)
a. 400 1.00 400 49.42 0.020
b. 800 1.00 400 24.58 0.041
c. 1200 1.00 400 17.94 0.056
d. 400 0.50 400 26.11 0.019
e. 400 0.25 400 15.36 0.016
f. 400 1.00 800 27.54 0.036
g. 400 1.00 1200 15.48 0.065
*Note: we’re assuming here that all of the I2 is used up and so the change in concentration is
initial- 0 ☺ e.g. for mixture a, it would be 1.00- 0.00 = 1.00 mol.dm-3

25. Assumptions made in this
experiment….
• we’re assuming here that all of the I2 is used up and so the
change in concentration is initial- 0 ☺ e.g. for mixture a, it
would be 1.00- 0.00 = 1.00 mol.dm-3
• Assuming that we are detecting the colour change at the right
time.
• Assume that the temperature is constant (remember the
value of k is affected by temperature)

26. Lesson 5- Evaluating proposed reaction
mechanisms.
Level 4: Define what the rate determining
step of a reaction mechanism is.
Level 7: Explain why a reactant which has a
zero order effect on the rate does not
appear in the rate expression.
Level 5/6: Deduce the rate expression of a reaction
based on a proposed reaction mechanism.

27. More about the rate determining step

28. The rate equation and mechanisms

29. Potential energy profile for a two step
reaction (from Pearson HL pg 297)

30. What do exam questions look like?
(taken from Pearson 2nd ed HL pg 300)

31. A question with a twist! (Pearson page 299)
Q. Determine the rate expression and thus, the overall reaction order.
Q. Determine the rate expression and the overall rate of this reaction.
Initially it looks like rate= k [N2O2] [O2]
But, because the production of the N2O2 is reliant on the reaction between the
two moleucules of NO, the [N2O2] is replaced by [NO]2 in the expression to give:
Rate = k [NO]2[O2] with an overall reaction order of 3 Not 2!
* Remember- golden rule, if a species appears before or in the rate determining
step then it must be accounted for in the rate expression!

32. More practice!
1. Questions on kognity ☺

33. 16.2- Activation energy

34. Lesson 6- The Arrhenius equation.
Level 4: Recall that the rate constant, k, is
temperature dependent.
Level 7: Calculate the activation energy of a
reaction from values of the rate constant
at only 2 temperatures.
Level 5/6: Calculate the activation energy (Ea) of a
reaction over a range of temperatures using the
Arrhenius equation.

35. Starter- Savante Arrhenius

36. The Arrhenius Equation
•Collision Theory: A bimolecular reaction occurs
when two correctly oriented molecules collide
with sufficient energy.
•Activation Energy (Ea): The potential energy
barrier that must be surmounted before
reactants can be converted to products.

37. 1. Arrhenius considered the activation energy
of a reaction (Ea)

38. 2. Arrhenius considered the frequency of collisions
with correct orientation (A) (‘the frequency factor’)

39. 3. Arrhenius considered the fact that temperature affects the rate
constant of a reaction, where k is proportional to temperature (T)
556
575
629
666
700
781
4.45 × 10–5
1.37 × 10–4
2.52 × 10–3
1.41 × 10–2
6.43 × 10–2
1.34
temp. (K)
k(moldm3s–1)
temperature (K)
rate constant, k
(moldm3 s–1)
Note: the rate constant k is written as a lower case letter (in italic) to
distinguish it from K for kelvin or Kc for equilibrium constant.

40. The Arrhenius Equation
(*see section 1 of d.book)
•This relationship is summarized by the
Arrhenius equation.
•Taking natural logs and rearranging, we get:
lnk = - Ea
R




1
T



+ lnA
k = Ae
- Ea
RT







41. 1. Using the Arrhenius Equation
Temp
(1/T)K-1
k
(M-1 s-1)
0.0018 3.52e-7
0.0016 3.02e-5
0.0015 2.19e-4
0.0014 1.16e-3
0.0012 3.95e-2
* remember- convert temperature readings to Kelvin (add 273)

42. Using the Arrhenius Equation
(graphing method)
The second-order rate constant for the decomposition of
nitrous oxide (N2O) into nitrogen molecule and oxygen
atom has been measured at different temperatures:
Determine graphically
the activation energy
for the reaction.
k (M -1
s-1
) t (°C )
1.87x10-3
600
0.0113 650
0.0569 700
0.244 750
The second-order rate constant for the decomposition of
nitrous oxide (N2O) into nitrogen and oxygen has been
measured at different temperatures:
Determine graphically
the activation energy
for the reaction.
Note: use logger pro☺
K mol.dm-3.s-1 t (°C )
1.87x10-3
600
0.0113 650
0.0569 700
0.244 750

43. 2. Using the Arrhenius Equation
(simultaneous equations method ‘2-point method’)
see section 1 of d.book






-=
122
1 11
ln
TTR
E
k
k a
Pearson pg 303

44. The answer!
Ensure you can carry out this calculation on your calculator!

45. Check for learning…. (taken from Pearson page 304)

46. Lesson 7- Determining Ea experimentally (P).
Level 4: Recall that the slope of an Arrhenius
plot is equal to Ea/R.
Level 7: Determine the experimental error in
your calculated Ea value.
Level 5/6: Determine the Ea of a reaction
from experimental data.

47. Aim: to determine the Ea of a reaction from
experiment and graphical technique.
See practical from inthinking.
Link to shared results doc.:
https://docs.google.com/spreadsheets/d/1RhlIt-
a1H3Py0yi8tM9mqxvXdN8Q1Uq_agPfVwPg0Ng/edit#gid
=0

48. Lesson 8
Review of topic- IB exam questions☺
*See practice questions from Pearson.

49. Starter- if slope = -Ea/R, what was the activation energy for this
reaction?

50. Past paper questions
• See question set from Pearson (~1.5 hours
work☺)