Catalysts Flashcards
What is a catalyst
Speeds up the rate of a chemical reaction without being used up itself, provides an alternative reaction pathway with a lower activation energy
What is a heterogeneous catalyst
One that is in a different phase from the reactants, usually solid while the reactants are gases/in solution
What is a phase
One of the three states of matter: solid, liquid or gas which chemical reactions often have more than one of
What are intermediates
Particles which do not appear in the balanced equation but which are formed during one step of a reaction then used up in the next step
What is the mechanism of a reaction
A description of how a reaction takes place showing step by step the bonds which break and the new bonds which form as reactants turn into products
Why is there a dip in the energy profile for catalysed reactions
Represents the intermediate formed during the reaction which is less stable and therefore higher in energy than the reactants and products
How do solid catalysts work
Work by adsorbing molecules onto an active site on the surface of the catalyst which increase the proximity of molecules and weaken the covalent bonds in the molecules so reactions occur more easily and rate increased, products then desorbed, used in industry to provide a surface for reactions to occur in
Where does reaction happen on solid catalysts
Reactants adsorbed onto surface onto active sites
What is adsorption
Formation of weak bonds between reactants and the surface of the solid catalyst at active sites
Example of heterogenous catalyst
In the Haber process which uses a heterogenous solid iron catalyst to speed up reaction between hydrogen and nitrogen gas to form ammonia
Why do transition metals make good catalysts
As they have variable oxidation states meaning electrons can be transferred or gained to produce a reactive intermediate
What is the contact process
Uses a vanadium oxide catalyst to speed up conversion of sulfur dioxide to sulfur trioxide
Why is it vitally important to choose the correct metal catalyst
-as if adsorption too strong eg tungsten the products cannot desorb
-if adsorption too weak reactants not absorbed and reaction rate too slow or there is no reaction
Why is it generally necessary to purify the reactants before they are passed over the catalyst
As catalysts can be poisoned by impurities which block active sites meaning reacting molecules cannot bind
Effect of using leaded petrol on a catalytic converter
Will poison the catalytic converter meaning it is ineffective as it’s active sites will be blocked
