DF - Kinetics *02 Flashcards
catalyst
increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy. the catalyst is chemically unchanged at the end of the reaction
catalysis
speeding up a chemical reaction by using a catalyst
catalyst poison
catalysts can be poisoned so they don’t work anymore
- carbon monoxide, CO, poisons the solid iron catalyst used in the Haber process
- lead poisons the catalyst in catalytic converters which was a problem when lead was added to petrols
heterogeneous catalysts often get poisoned because the poison clings to the catalyst’s surface more strongly than the reactant does. so the catalyst is prevented from getting involved in the reaction its meant to be speeding up
heterogeneous
heterogeneous catalysts are in a different physical state from the reactants
eg. catalyst solid, the reactants are gases or liquids
iron in Haber process and Lead in catalytic converters
model to explain function of heterogeneous catalyst
soild heterogeneous catalysts can provide a surface for a reaction to take place on.
- ADSORPTION - reactant molecules arrive at the surface and bond with the solid catalyst.
- the bond between the reactant’s atoms are weakened and break up. This forms radicals, the radicals then get together and form new molecules
- DESORPTION - the new molecules are then detached from the catalyst
(need to draw this!)
explain the needed strength of adsorption
mustn’t be too strong or the catalyst wont let go of the atoms after the reaction BUT strong enough to weaken bonds between the reactant molecules or that he new molecules can form
cracking
- short-chain hydrocarbons more useful than long ones (5-10 used in petrol)
- long-chain alkenes can be cracked (broken up) into smaller hydrocarbons
- the products of cracking are random as the same molecule can give different cracking products
use of catalysts in cracking
without catalysts, the cracking process requires extremely high temp. and pressure which makes it expensive
By passing hydrocarbon vapour over a heated solid catalyst cracking can take place at much lower temperatures (around 450c) and pressures, saving a lot of money