reaction kinetics

activation energy

minimum amount of energy required for reactant particles to overcome the energy barrier in order for effective collisions to occur

transition state theory

reactant bonds are partially broken and new bonds between reactant molecules are partially formed

rate of reaction

change in concentration of a reactant or product per unit time (frequency of effective collisions)

factors affecting rate of reaction

1. temperature

2. catalyst

3. concentration of reactant (pressure for gas)

4. physical state of reactant

5. light

rate of reaction (temp)

as temperature increases, the average kinetic energy of reactant particles increase

the number of particles with energy more than activation energy increases

frequency of effective collision in the reaction increases

catalyst

substance that provides an alternative pathway with a lower activation energy that remains chemically unchanged after the reaction

rate of reaction (catalyst)

with catalyst, activation energy is lowered (new activation energy level)

the number of reactant particles with energy more than activation energy increases

frequency of effective collisions in the reaction increases

rate of reaction (conc.)

as concentration increases, no. of particles per unit volume increases

frequency of effective collisions in the reaction increases

rate of reaction (physical state)

reactants in solid states only can react on its surface

larger surface area exposed increases the frequency of effective collisions

rate of reaction (light)

photochemical reactions take place rapidly in presence of light

average kinetic energy increases when light energy is absorbed, leading to larger proportion of particles having energy higher than activation energy

frequency of effective collisions in the reaction increases

types of rate

1. instantaneous (t=x)

2. initial (t=0)

3. average

rate law (definition + formula)

rate of reaction is the concentration of reactants raised to the appropriate power

experimentally determined

rate = k[A]ᵐ[B]ⁿ

order of reaction

power to which concentration of a reactant is raised to in the rate reaction

overall order of reaction

sum of the individual order of each reactant

rate constant (k)

constant that related the concentration of the reactants

*no fixed units

k is temperature and catalyst dependent, not concentration dependent

zero order reaction

zero order reaction wrt reactant A is independent of [A]

rate equation= k[A]⁰

k: mol dm⁻³ s⁻¹

first order reaction

first order reaction wrt to A is rate is directly proportional to concentration of A

rate equation = k[A]

k: s⁻¹

has a constant half-life

half life (t ₁ₗ₂)

time taken for the concentration of the reactant A to decrease by half of its original value

t₁ₗ₂= ln2/k

second order reaction

second order reaction wrt A is a reaction where rate of reaction is directly proportional to [A]²

rate = k[A]²

k: mol⁻¹ dm³ s⁻¹

half life is not constant

pseudo order reaction

pseudo order reaction occurs when one of the reactant is in large excess concentration/presence of a catalyst

this is because the change in concentration of one reactant is negligible and remains relatively constant

types of experiment

1. continuous experiments

2. discontinuous experiments

continuous experiment

one experiment is conducted and change in concentration of reactant or product is measure at regular time intervals

1. chemical method

2. physical method

chemical method

sampling (quenching) and titration

ways to quench

1. addition of a base

2. sudden introduction of a large amount of water

3. submerging the reaction into ice bath to achieve rapid cooling

physical method (cont.)

1. change in gas volume (gas formed)

2. change in gas pressure (change in no. of moles)

3. change in electrical conductivity (change in ions)

4. change in colour intensity (coloured substances)

discontinuous experiments

several experiments are conducted while keeping the same procedure by varying initial concentration of a reactant while keeping the other variables constant

relative rate is proportional to 1/time take for a fixed amount of product to form

1. physical method

physical method (disc.)

1. sulfur clock (formation of solid)

2. iodine clock (formation of coloured compounds)

reaction intermediate

products produced during the reaction mechanism then used up at the end

rate determining step

the “slow” step that has higher activation energy than other steps

homogenous catalysts

catalyst in the same state as the reactants and is uniformly mixed

it provides a lower Ea by forming an intermediate which is later used up

the homogenous catalyst is chemically changed but will be regenerated at the end of the reaction

heterogenous catalysts

catalyst is in a different state from the reactant

provides an alternative pathway with lower Ea by means of adsorption, weakening the chemical bonds in reactant particles, lowering the Ea

1. adsorption

2. reaction at the surface

3. desorption

haber process

manufactures NH₃ at 450⁰C, finely divided iron (catalyst)

catalytic removal of oxides

removes oxides of nitrogen, CO and unburnt hydrocarbon by oxidising or reducing them

catalyst: Pt, Pd and Rh

converts them to N₂, CO₂ and H₂O

enzymatic catalyst

enzymes are biological catalysts that speed up or alter the rate of reaction in living systems while remaining chemically unchanged at the end of the reaction

properties of enzyme

1. remain chemically unchanged

2. required in small quantites

3. affected by temperature, pH, substrate concentration and enzyme concentration

4. highly specific

autocatalyst

the product is a catalyst