Here is a 1 page summary explaining the link between reaction order and mechanism: Reaction mechanisms involve multiple elementary steps where molecules interact to form products. However, the overall rate of reaction is determined by the slowest step, known as the rate-determining step (RDS). The order of the reaction provides clues about which elementary step is rate-determining. For example, consider the reaction: A + B → C If the reaction is first order with respect to [A], it suggests the RDS involves the collision and bonding of only A molecules. The formation of the A-B complex must be fast compared to this bond formation. However, if the reaction is second order with respect to [

1. • Find a friend, and each of you crumple up a piece of paper.
Sitting facing one another, with knees almost touching, toss
your pieces of paper together, trying to get them to hit
while still in the air. Do this 10 times - how many successful
"collisions" did you record?
• Now, grab another friend with their own wad of paper. This
time, a successful collision will only occur when all three
pieces of paper hit together simultaneously - just two out
of three won't do. Out of 10 tosses, how many are
successful?
 Add one more friend and repeat the exercise. Any
successful collisions, where all four pieces of paper collide
at the same instant?

2. HL Kinetics
Syllabus

3. 16.1 Rate Expression
• Understand
– Rate constant
– Overall order
– Order of reaction
• Use experimental data to deduce rate
expression.
• Solve problems using the rate expression.

4. KINETICS GIVES US CLUES TO A
REACTION MECHANISM
THIS IS ALL EXPERIMENTALLY
DETERMINED

5. DEFINITIONS
• Rate is how fast concentration of reactant is
falling ( or vice versa ) measured in
– mol dm-3 s-1
• Rate Constant (k) is only constant if T and Ea
stays constant. A catalyst or higher T would
increase k. Changes in concentration do not.

6. The Concentration Terms
• Where rate = k[A]a [B]b [C]c
• The powers are called the ‘order of reaction’.
– If no effect on rate regardless of concentration we call it zero
order.
– If we double concentration of [A], and the rate doubles we say
Rate = k[A]
– If we double concentration of [A], and rate is quadrupled we say
Rate = k[A]1
– If we double the concentration of [A}, and the rate is 8x faster
we say Rate = k [A]2
• Overall orders are sums of the individual orders.

7. Orders of Reaction
• xA + yB + zC……  products
• Rate = k [A]x [B]y [C]zthis is rate equation
• How do we measure rate of reaction ?
– How fast concentration of one of reactants is
falling 
– Units

8. Zero Order
• This means the concentration of that
substance does not influence the rate
• Rate = k[A]0

9. First Order
• Whatever you do to the concentration, you do
to the rate.
• Double conc = double rate
• Triple conc = triple rate
• Etc
• Rate = k[A]

10. Second Order
• Whatever you do to the concentration you
square the rate.
• Rate = k[A]2

11. Notes
• The coeffcients in balanced chemical equation
are not the orders !
• You cannot predict orders they must be found
experimentally.
• Orders give clues to mechanisms…….later 

12. Finding Orders from Rate Experiments
• Use blog to calculate experimental k and order


13. Finding Orders of Reaction Graphically
• The real world is not so straight forward 

14. Zero Order wrt [A]
• Rate = constant, i.e. independent of value of [A].
• If A is only substance taking part in the reaction
then Rate = k
• A horizontal straight line shows a zero order rxn.

15. First Order wrt [A]
• Rate = k[A]
• Rate constant is slope of the graph
• What are units ?

16. Second Order wrt [A]
• Rate = k[A]2
• Plot rate against concentration of A2

17. Finding value for Rate Constant k
• Rate is slope of the graph 

18. Recognising Order from Graphs
• http://www.chem.purdue.edu/gchelp/howtos
olveit/Kinetics/IntegratedRateLaws.html

19. Using Half Life
• For a first order reaction, half life is constant –
independent of initial concentration.
• ONLY FIRST ORDER REACTIONS HAVE
CONSTANT HALF LIVES.
• Sums 142

20. Activation Energy
• Describe qualitatively the relationship
between the rate constant (k) and
temperature (T).
• Simple but worth restating……..The rate of
reaction is affected by Temperature ( and
catalyst ).

21. Arrhenius Equation

22. Qualitative description
• T appears in Ea/RT
• If T increases then Ea/RT gets smaller
• This is a negative terms so as T increases
Ea/RT gets less negative
• -Ea/RT is a power so as T is raised then e-Ea/RT
gets larger.
• So the rate constant gets larger as T gets
larger.

23. Useful Arrhenius Equation 

24. So Why is This Useful ?

25. Huh ?
• For every rise in T of 10K there is a DOUBLING
of reaction rate.
• This is a very useful ( and oft tested ) rule of
thumb 

26. Example Calculations
• 146 – 7.
• Lab : determination of activation energy.

27. Reaction Mechanism
• Explain that reactions can occur by more than
one step and that thesloweststep determines
the rate of reaction
• This is called the RATE DETERMINING STEP.

28. The What ?
• http://www.chemguide.co.uk/physical/basicra
tes/ordermech.html
• Review using ChemGuide and produce a 1 side
A4 substituting your own examples to
illustrate the link between order and reaction
mechanism.