๐ In-Depth Theory
What is a Catalyst?
A CATALYST is a substance that INCREASES THE RATE of a reaction WITHOUT being used up or permanently changed itself.
Key properties:
Increases rate โ provides a reaction pathway with LOWER ACTIVATION ENERGY.
Not used up โ the catalyst is the same at the end of the reaction as at the start.
Present in small quantities โ a small amount can catalyse a large quantity of reactants.
SPECIFIC โ each catalyst only works for particular reactions (or types of reaction).
How catalysts work:
They provide an ALTERNATIVE REACTION PATHWAY with a lower activation energy.
A greater proportion of collisions now have energy โฅ the lower activation energy โ faster rate.
The overall energy change (ฮH) is not affected โ same reactants and products, same energy difference.
Types of Catalyst
HETEROGENEOUS CATALYSTS:
In a DIFFERENT PHASE from the reactants.
Usually a solid catalyst with gaseous or liquid reactants.
Reaction occurs on the SURFACE of the catalyst.
Example: IRON catalyst in the Haber process (Nโ + 3Hโ โ 2NHโ) โ solid iron, gaseous reactants.
Example: PLATINUM/RHODIUM in catalytic converters โ converts CO, NO and hydrocarbons to COโ, Nโ and HโO.
Example: NICKEL in margarine production โ hydrogenation of vegetable oils.
HOMOGENEOUS CATALYSTS:
In the SAME PHASE as the reactants (usually both dissolved in solution).
Example: hydrogen peroxide decomposition โ Feยฒโบ ions in solution catalyse it.
BIOLOGICAL CATALYSTS (ENZYMES):
Protein molecules that catalyse specific biochemical reactions.
Example: amylase โ catalyses starch hydrolysis. Catalase โ decomposes HโOโ.
Highly specific โ one enzyme, one reaction.
Sensitive to temperature and pH โ denature above ~40ยฐC.
Industrial Importance of Catalysts
Catalysts are enormously important in industry:
HABER PROCESS (making ammonia for fertilisers):
Nโ + 3Hโ โ 2NHโ โ iron catalyst.
Without catalyst: reaction too slow to be economical at practical temperatures.
With iron catalyst: reaction is fast enough at ~450ยฐC.
CONTACT PROCESS (making sulfuric acid):
2SOโ + Oโ โ 2SOโ โ vanadium pentoxide (VโOโ
) catalyst.
CATALYTIC CONVERTERS in cars:
Platinum and rhodium catalyse conversion of toxic exhaust gases.
ECONOMIC AND ENVIRONMENTAL BENEFITS of catalysts:
Reactions can be run at LOWER TEMPERATURES โ less energy needed โ lower cost.
FEWER BY-PRODUCTS in many cases โ cleaner processes.
Smaller reactors needed โ lower capital cost.
Catalysts can be RECOVERED and REUSED โ economical.
Disadvantage: catalysts can be POISONED โ impurities in the reaction mixture bond to the catalyst surface and block active sites.
โ ๏ธ Common Mistake
A catalyst is NOT used up โ it is the same at the END of the reaction as at the START. It does NOT change the products or the overall energy change (ฮH). It ONLY provides an alternative pathway with LOWER ACTIVATION ENERGY โ making the reaction faster, not different.