GCE A LEVEL TOPIC: (A2) CHEMICAL KINETICS PLEASE DOWNLOAD BECAUSE THERE ARE MANY ANIMATIONS THAT HIDE SOME OF THE CONTENTS (THE ANIMATIONS DO NOT PLAY DURING THE PREVIEW)

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Reaction Kinetics (A2)
Prepared by: Faiz Abdullah

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AS recap
In As level, you should know what is:
 Rate of reaction
 Collision theory
 Boltzmann distribution of energy
 Catalyst
 Activation Energy
 Effect of temperature, concentration and catalyst on rate of
reaction

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For A2,
we are dealing with
DATA MANIPULATION

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Order of Reaction
What do you mean by order?

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Measuring rate of reactions
Usually, we are looking at HOW FAST THE CONCENTRATION
OF REACTANTS IS FALLING AT ONE TIME.
Suppose we have:
A(aq)+B(g)  Products
We can measure rate of reaction with:
-The decreasing concentration of A in mol/dm3 in 1 min.
-The decreasing volume of gas B in 1min

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Orders of reaction
Suppose of you have: A + B  products
From the experiment: you found out that….
When [A] doubles, rate doubles.
Rate of reaction is proportional to [A]
Therefore, order with respect to A is 1
When [A] doubles, rate increases four times
Rate of reaction is proportional to [A]2
Therefore, order of reaction w.r.t. A is 2
When [A] doubles, rate of reaction does not change
Rate of reaction does not depend on [A]
Therefore, order w.r.t A is 0
WARNING:
YOU CANNOT DEDUCE THE ORDER OF
REACTION JUST BY LOOKING AT THE
EQUATION!!!!
ORDERS OF REACTIONS ARE ALWAYS FOUND
BY DOING EXPERIMENTS

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Rate equation
Suppose A + 2B + C  products
From experiments, we found out that:
[A] doubles, rate of reaction doubles
Therefore, order w.r.t A is 1
[B] doubles, rate of reaction increases
by 4
Therefore, order w.r.t B is 2
[C] doubles, rate of reaction does not change
Therefore, order w.r.t C is 0
Rate Equation:
Rate = k [A] [B]2
Where k is rate constant

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Why is C in the chemical equation
but not in the rate equation????
Some reactions occur in multiple steps:
Step 1: A + 2B  2C + D
Step 2: C + D  products
Final : A + 2B + C  products
SOME STEPS ARE SLOW AND SOME ARE FASTER
SLOW
FAST
THIS STEP IS THE
RATE-DETERMINING
STEP
OVERALL REACTION RATE
DEPENDS ON THE SLOW STEP
WHEN you measure rate of reaction, what you are actually
Measuring is the rate of the determining step!!!

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Rate constant, k
Rate = k [A] [B]
Rate constant is constant (does not change value) only when
concentrations of reactants are changing.
RATE CONSTANT CHANGES WHEN:
1. TEMPERATURE CHANGES
2. ADDING CATALYST

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DEDUCING ORDER BY INTIAL
RATES METHOD
Run Initial [A]/mol Initial [B]/mol Initial rate/mols-1
1 1.00 1.00 1.25 x 10-2
2 1.00 2.00 2.5 x 10-2
3 2.00 2.00 2.5 x 10-2
Can you find the rate equation? Can you find k?
Make [A] constant,
[B] x 2, rate x 2
Order w.r.t B is 1
Make [B] constant,
[A] x 2, rate same
Order w.r.t A is 0
Rate = k [B]
k=1.25 x 10-2 s-1

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Deducing order from graphs
First- order reaction
Zero-order reaction

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Rate concentration graph

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Graphs Summary

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Half-life of first-order reaction
Half life  time taken to get half of the final concentration

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Half-life equation
Half-life (in seconds) can be used to find k, rate constant:
T1/2 = ln (2) / k

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EXPERIMENTAL TECHNIQUES
FOR STUDYING RATES
① Sampling followed by titration
② Using a colorimeter
③ Measurement of gas evolved

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CATALYST
① Homogenous: catalyst same phase as the reactants
② Heterogeneous: catalyst and reactants different phases

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CATALYSIS
In AS, you need to know FOUR SPECIFIC EXAMPLES

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1) HABER PROCESS
N2 + 3H2  2NH3
Catalyst: Iron
Note: Transition metals are good at acting as catalysts because
their atoms have unfilled d-orbitals.
Gases are adsorbed on to the surface of the metal, forming weak
bonds.
① Formation of bonds with the metal surface weakens the
bonds within the gas molecules
② The orientation of the adsorbed molecules may be favorable
for the reaction
THIS IS A
HETEROGENEOUS
SYSTEM

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2) Catalytic converters in vehicle
exhausts

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 Catalytic converters in vehicle exhausts aim to remove a
number of pollutant gases from vehicle exhausts.
 Pollutants: Nitrogen oxides, carbon monoxides.
 Inside the ceramic honeycomb, it has a very thin coat of:
o Platinum and palladium: oxidize CO and unburnt hydrocarbons
o Platinum and rhodium: reduce NOx to N2

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3) Nitrogen oxides in the
atmosphere
 Studies on acid rain have concluded that in the atmosphere the
presence of oxides of nitrogen, particularly NO2, increases rate
of oxidation of SO2  SO3.
 NO2 remains unchanged and is thought to form a weak
intermediate with SO2.

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4) The role of Fe2+ in I-/S2O8
2-
reaction
Step 1: S2O8
2- + 2Fe2+  2 SO4
2- + 2Fe3+
Step 2: 2Fe3+ + 2I-  2Fe2+ + I2
Overall reaction: S2O8
2- + 2I-  2Fe2+ + I2
Fe2+ does not change overall
Although there are two steps,
Ea is lowered overall