GCE A LEVEL TOPIC: (A2) CHEMICAL KINETICS
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2. +
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
5. +
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
6. +
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
7. +
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
8. +
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!!!
9. +
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
10. +
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
19. +
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
21. +
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
22. +
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.
23. +
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