Catalysis

Catalysts “speed up the rate of a reaction without being used up in the process”

NB; If they are changed during the course of a reaction they must be regenerated when it is finished.

Catalysts work by providing an alternative reaction pathway with a lower activation

energy

Uncatalysed activation

energy

Catalysed activation energy

Demonstration

• 1) Make a solution of 0.2g of hydrated cobalt chloride in 5mls.

• 2) Make up a solution of 5g of Rochelle Salt in 60mls in a 250ml beaker.

• 3) Add 20mls of 20 volume hydrogen peroxide to the solution of Rochelle salt and heat to c75oC. Note the slow evolution of gas.

• 4) Add the cobalt chloride solution; note the colour changes and rate of reaction.

• 5) If a sample is removed and placed on ice it will keep its colour longer.

LHS before the addition of a catalyst.RHS after the addition of Co 2+

During the reaction pink Co2+ is oxidised to green Co3+.

After the reaction they revert to pink Co 2+.

• Eg; The reaction of hydrogen peroxide with sodium tartarate (Rochelle Salt);

• 5H2O2 + C4H4O62- → 4CO2 + 2OH- + 6H2O

• It has two steps;• 3H2O2 + C4H4O6

2-→2CO2+ 2HCO2- + 4H2O

• 2HCO2- + 2H2O2 → 2CO2 + 2OH- + 2H2O

• This is catalysed by Co2+ ions.

• Eg; The reaction between iodide ions and peroxodisulphate ions.

• 2I-(aq) + S2O8

2-(aq) → I2 (aq) + 2SO4

2-(aq)

• Is catalysed by aqueous iron (iii) ions.• First an electron is transferred from the

iodide ion, oxidising it to iodine and reducing Fe3+ to Fe2+.

• 2 Fe3+ + 2I- → 2 Fe2+ + I2• Then Fe2+ reacts with the

peroxodisulphate ion, transfering its electron to reduce it to sulphate ions, becoming oxidised and regenerating the Fe3+

• 2 Fe2+ + S2O82- → 2SO4

2- +2Fe3+

Types of catalyst

There are two types of catalyst;

1) Homogenous are in the same state as the reactants.

2) Heterogenous are in a different state to the reactants.

Homogenous catalysts• For reactions that take place in the aqueous

phase the catalyst must itself be in the aqueous phase.

• Eg; Esterification;• CH2CH3OH (aq) + CH3CO2H (aq)

Ethanol Ethanoic acid↓

• CH2CH3O(O)CCH3 (aq) + HOH• Ethyl ethanoate.

• The reaction is catalysed by sulphuric acid.• As it is in the same phase as the reactants it

is an homogenous catalyst.

Catalysis and the breakdown of the ozone.• Ozone is unstable and will break down;

• 2O3 (g) → 3O2 (g) • This reaction is catalysed by chlorine

radicals. (A radical is an atom or group of atoms with an unpaired electron.)

• UV causes chloroflouro carbons to break down, releasing chlorine radicals;

• Eg; CCl2F2 (g) → CClF2 (g) + Cl. (g)

• NB Chlorine radicals are gaseous, like the reactants, so this is homogenous catalysis.

• Breakdown occurs in two steps;• 1) Chlorine radicals attack ozone.

• Cl. (g) + O3 (g) → ClO. (g) + O2 (g)

• 2)The new radical formed breaks down, regenerating the catalyst.

• ClO. (g) + O (g) → Cl. (g + O2 (g)

• O Is formed in the stratosphere by the action of uv on oxygen and ozone molecules.

Heterogenous catalysis

• Eg; The Haber process• N2 (g) + 3H2 (g) → 2NH3 (g)• Is catalysed by solid metallic

iron.• The catalyst is in a different

phase from the reactants, so this is heterogenous catalysis.

1) Gaseous reactants are adsorbed onto the surface of the solid catalyst;

2) In the process the molecules are held closer together and their bonds are weakened, making them easier to break.

3) New bonds are formed and the products are deadsorbed.

Mechanism of heterogenous catalysis

Inorganic compounds will allow catalysis to take place all over their surface.

They are non-specific, catalysing a wide range of reactions.

Catalytic convertors• Catalytic convertors

catalyse reactions between the waste gases in car exhausts, reducing the impact of harmful emissions.

• The catalysts are platinum group metals coated on to an inert, “honeycomb” support material.

• Incomplete combustion of hydrocarbons in petrol results in the formation of poisonous carbon monoxide.

• C + ½O2 → CO• Nitrogen can also be combined with oxygen

to form oxides which can also have harmful effects.

• N2 + ½O2 → NO• Catalytic convertors will turn them into less

harmful compounds.

• Eg; 2CO + 2NO → 2CO2 + N2

Enzymes

Enzymes are biological catalysts, proteins with a complex structure.

Catalysis only takes place at one place, the active site.

As the active site has a specific shape enzymes only catalyse specific reactions.