Reaction Kinetics

Collision Theory

1. Particles must collide in the correct orientation

2. Particles must collide with sufficient minimum energy (activation energy) to bring about necessary reorganisation of bonds in colliding particles

Rate of Reaction

1. Effective collision: when particles collide at the correct orientation and sufficient energy

2. Greater the frequency of effective collisions, greater the rate of reaction

Factors affecting rate

1. Concentration / pressure —> frequency of effective collisions

2. Surface area —> frequency of effective collisions

3. Temperature —> greater KE, greater fraction of particles with sufficient KE above EA

4. Catalyst —> alternative pathway with lower EA, greater fraction of particles with sufficient KE above EA

Boltzmann Curve — higher temp

Boltzmann Curve — catalyst present

Heterogeneous catalyst

Usually solid catalyst and gaseous reactants

Brings ALL reactant particles closer to each other and weakens all bonds, hence providing alternative pathway of lower activation energy

One step reaction

Mode of Action of heterogenous catalyst

Adsorption: of reactants particles onto surface of catalyst, weak IMF formed between reactant and catalyst

Reaction: reactants brought closer to each other and existing interactions in reactant molecules are weakened

Desorption: products leave catalyst, catalyst regenerated

Homogenous catalyst

Catalyst and reactants in the same phase

Two step reaction

Autocatalyst

Biological catalyst

Enzymes have specific active site that is complementary to reactant molecules in shape and charge

Rate equation

rate = k[A]^x[B]^y

Order of reaction

Zero: rate is not affected by reactant

First: rate is directly proportional to the concentration of reactant

Second: rate is directly proportional to the SQUARE of concentration of reactant

Factors affecting k (rate constant)

Concentration: no change

Temp: increases

Addition of catalyst: increases

Initial rate method

Multiple experiments

Collect time taken in each experiment

Rate-concentration graph to be plotted

Continuous method

Single experiment

Collect concentration remaining in reaction

Concentration-time graph to be plotted

Physical experiments to find rate equation

1. Gas collection

2. Gravimetric

3. Pressure change

4. pH change

5. Electrical conductivity

6. Colorimetry

Titrimetric method — continuous

• Titration result (concentration of reactant or product) are plotted against time

• Analysis of shape and half-life of concentration-time graph enables one to find the overall order of reaction

• Order wrt A can be investigated if all other reactants is present in large excess

Clock reaction — initial rate

Appearance of colour: can be simplified to rate = 1/(time taken)

Disappearance of colour: cannot be simplified to rate = 1/(time taken)

Deducing order of reaction (initial rate table)

Comparing Exp 1 and 2 where [X] is the same,

When [Y] of E2 is twice of [O2] of E1, rate of E2 is twice of rate of E1, therefore order wrt to O2 must be zero/one/two

Deducing order of reaction (initial rate graph)

Rate / Concentration — Look at shape

Deducing order of reaction (continuous graph)

Concentration-time graph

Shape: if linear, zero order

Half life: if at least 2 half lives are equal, first order. Otherwise, second order

For 1st order, t_1/2= ln2 / k

Initial + Continuous methods

A + B (large excess) —> C

Continuous:

[B] is assumed to be constant, rate eqn= k’[A]^m —> find order of reaction wrt A. Observe shape and use half life to determine order of reaction wrt A

Initial rate:

Keep [A] constant, change [B] (but still in large excess). Compare time taken for both reactions to reach same [A] left, use inspection method to determine order of reaction wrt B