Catalysts Flashcards
What does homogenous mean?
A homogeneous catalyst is in the same phase as the reactants.
What does heterogeneous mean?
A heterogeneous catalyst is in a different state to the reactants.
What is a catalyst?
A substance that speeds up a reaction without being used up. It provides an alternate reaction pathway lowering activation energy?
How does a heterogenous catalyst work?
Reactants adsorbed onto active sites. This weakens bonds, brings molecules closer and provides a more favourable orientation. The reaction then takes place and products are desorbed.
Why are transition metals usually good catalysts?
They have variable oxidation states.
Describe how V2O5 catalysed the contact process:
V2O5 + SO2 —> V2O4 + SO3
V2O4 + 1/2O2 —> V2O5
How do homogeneous catalysts work?
First, catalyst reacts with reactants to form intermediate. Intermediate then reacts faster to produce product than original reactant (and regenerates catalyst).
What is autocatalysis?
One of the products of the reaction is a catalyst for the reaction. These reactions start slowly (at an uncatalysed rate) then as the concentration of product builds up the reaction is catalysed and the rate increases.
Describe how 2MnO4-+16H+ +5C2O42- → 2Mn2+ +8H2O+10CO2 shows autocatalysis:
Mn2+ acts as a catalyst:
4Mn2+ +MnO4– + 8H+ → 5Mn3+ +4H2O
2Mn3+ + C2O42– → 2CO2 + 2Mn2+
How can heterogenous catalysts be poisoned?
Heterogeneous catalysts can become poisoned by
impurities that block the active sites and consequently have reduced efficiency; this has a cost implication.
How are vanadium species in oxidation states IV, III, and II formed?
Vanadium species in oxidation states IV, III, and II are formed by the reduction of vanadate(V) ions by zinc in an acidic solution.
How does pH affect the oxidation state of a transition metal?
In general, it is easier to oxidise a transition metal in alkaline conditions and reduce a transition metal in acidic conditions.
How to distinguish between aldehydes and ketones?
The reduction of [Ag(NH3)2+ (Tollens’ reagent) to metallic silver is used to distinguish between aldehydes and ketones. The Ag is reduced when in presence of aldehydes to form silver mirror.
