chemical kinetics

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Reaction Kinetics (A2)Prepared by: Faiz Abdullah

+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 ReactionWhat do you mean by order?

+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

+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 timesRate 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 changeRate 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

+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

+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 THERATE-DETERMININGSTEP

OVERALL REACTION RATEDEPENDS ON THE SLOW STEP

WHEN you measure rate of reaction, what you are actuallyMeasuring is the rate of the determining step!!!

+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 CHANGES2. ADDING CATALYST

+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 2Order w.r.t B is 1

Make [B] constant,

[A] x 2, rate sameOrder w.r.t A is 0

Rate = k [B]

k=1.25 x 10-2 s-1

+Deducing order from graphs

First- order reaction

Zero-order reaction

+Rate concentration graph

+Graphs Summary

+Half-life of first-order reaction

Half life time taken to get half of the final concentration

+Half-life equation

Half-life (in seconds) can be used to find k, rate constant:

T1/2 = ln (2) / k

ONLY FOR FIRST –

ORDER REACTIONS

+EXPERIMENTAL TECHNIQUES FOR STUDYING RATES

① Sampling followed by titration

② Using a colorimeter

③ Measurement of gas evolved

+CATALYST

① Homogenous: catalyst same phase as the reactants

② Heterogeneous: catalyst and reactants different phases

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CATALYSISIn AS, you need to know FOUR SPECIFIC EXAMPLES

+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 AHETEROGENEOUSSYSTEM

+2) Catalytic converters in vehicle exhausts

+ 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

+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.

HOMOGENEOUS SYSTEM

+4) The role of Fe2+ in I-/S2O8

2- reaction

Step 1: S2O82- + 2Fe2+ 2 SO4

2- + 2Fe3+

Step 2: 2Fe3+ + 2I- 2Fe2+ + I2

Overall reaction: S2O82- + 2I- 2Fe2+ + I2

Fe2+ does not change overall

Although there are two steps,Ea is lowered overall