Catalysts

save energy, fuel costs and can carry out reaction at lower temperatures

catalyst

substance that increases the rate of reaction
provides an alternative reaction mechanism with lower activation energy
remains unchanged overall / renegerated

homogeneous catalyst

catalyst that is in the same phase as the reactants

heterogeneous catalyst

catalyst that is in a different phase as the reactants

adsorption

reactants stick / adsorps to the surface of the catalyst * for heterogeneous only

Impurities adsorp to catalyst and reduce surface area available for reaction / prevent bond weakening in reactants

reaction

bonds in the reactants are weakened and the reaction occurs

desorption

the products formed are released / desorps from the catalyst surface

Impurities form strong bonds to surface / less likely to desorb from surface

contact process

industrial manufacture of sulfuric acid using Vanadium (V) oxide as a heterogeneous catalyst
- formation of sulphur dioxide from its elements
formation of sulphur trioxide:
- V2O5 + SO2 -> SO3 + V2O4
- V2O4 + 1/2 O2 -> V2O5

Why are transition metals good catalysts?

variable oxidation states
can gain or lose electrons easily

catalytic converters

heterogeneous catalysts
consist of an internal ceramic honeycomb structure coated with finely divided platinum or palladium metal
- allows gases to pass in then out to be expelled
- large surface area of contact
(tetraethyllead adsorps to the catalyst surface too strongly - less surface area available for reactants - prevents bond weakening + reaction)

Why do catalysed reactions involve 2 steps

one step involves the reaction with the catalyst, one step involves the regeneration of the catalyst

cobalt (II) ion catalyst

catalyses the oxidation of tartrate ions (C4H4O6 2-) by hydrogen peroxide (H2O2)
effervescence observed on addition of catalyst (showing reaction has occurred as CO2 produced)
colour changes from pink to green back to pink (showing catalyst regenerated)

Fe 2/3+ catalyst

catalyses the redox reaction - oxidation of I- ions and reduction of peroxodisulfate ions (S2O8 2-)
reaction is slow without catalyst as involves collision between 2 negatively charged ions - high activation energy to overcome repulsion
S2O8 2- + 2Fe 2+ -> 2SO4 2- + 2Fe3+
2I - + 2Fe 3+ -> I2 + 2Fe 2+
order of steps is reversed for Fe 3+

autocatalysis

when one of the products of a reaction acts as a catalyst for the reaction
MnO4 - is reduced to Mn2+ in acid solution which acts as an autocatalyst
ethanedioate ions (C2O4 2-) oxidised to carbon dioxide