A catalyst is a substance that alters the rate of a chemical reaction but is not consumed in the reaction
Heterogeneous catalysis involves reactants in different physical states i.e. liquid reacting with solid e.g. MnO2 on H2O2
Homogeneous catalysis involves reactants in same physical states i.e. both in aqueous solution e.g. potassium iodide on H2O2
Enzymes are homogeneous biological catalysts e.g. amylase on starch
Autocatalysis occurs when the product of a reaction increases the reaction rate i.e. reaction makes its own catalysts e.g. reduction of manganate (VII) ions with Fe 2+
Activation Energy
Activation energy is the minimum energy required by particles colliding to cause a reaction
Exothermic reactions give out heat [Energy of: products < reactants ]
Endothermic reactions take in heat [Energy of: products > reactants ]
Average kinetic energy of particles is directly proportional to the temperature – greater the energy, greater the speed, greater the reaction rate. This means:
- the number of collisions per second increases
- each collision is more energetic and a higher proportion of collisions has the necessary activation energy
The second factor is more significant
Pollution and Catalytic Converters
Engines produce harmful CO, NO, NO2 and hydrocarbons.
Catalytic converters (e.g. palladium and platinum) speed up reactions to reduce harmful emissions
e.g. 2CO + O2 >>> 2CO2
2CO + 2NO >>> 2CO2 + N2
This is an example of heterogenous catalysis.
