Notesale: Turn your study into money

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Catalysts
Affect the rate of reaction without being
chemically changed themselves

Allow reactions to proceed at lower
temps and pressures thus saving
valuable resources

Used in the nitration
of benzene

Heterogeneous catalysts
Where the reactants/chemicals are in a
different state/phase to the catalyst
...
This increases the surface to
mass ratio

E
...
This is
called ‘poisoning‘
...


•

Finely divided catalyst may gradually be
lost from the support medium
...

EACH OF THE 2 STEPS HAS
A LOWER Ea, than the
uncatalysed single step so
reaction goes faster

Examples of heterogeneous catalysts
• Synthesis gas is made from methane
present in natural gas and steam
CH4(g) + H2O(g)

CO + 3H2(g)

Used to make methanol
CO(g) + 2H2(g)

CH3OH(g)

Catalyst is chromium oxide, Cr2O3

Today the most widely used catalyst for this
process is a mixture of Copper, zinc oxide
and aluminium oxide
...
Catalyst is Fe2+
Overall reaction?
S2O82-(aq) + 2I-(aq)

2SO42-(aq) + 2Fe3+(aq)

Catalysed reaction takes place in 2
steps
S2O82-(aq) + 2Fe2+(aq)
2Fe3+(aq) + 2I-(aq)

2SO42-(aq) + 2Fe3+(aq)
2Fe2+(aq) + I2aq)

Why is the uncatalysed reaction slower?
Takes place between 2 ions of the same
charge (both negative), which repel giving
a high activation energy
...


Autocatalysis
Reaction in which one of the products acts
as a catalyst
These reactions start
slowly at the uncatalysed
rate
...

From then on it behaves
like a normal reaction
gradually slowing down as
the reactants are used up
...
Once a little Mn2+ ions has
formed, it can react with MnO4- to form
Mn3+ as an intermediate species which then
reacts with ions to reform Mn2+

4Mn2+

(aq)

+ MnO4-(aq) + 8H+(aq)

5Mn3+ (aq) + 4H2O (l)

Manganate (VII)
ions

2Mn3+(aq) + C2O42-(aq)
Ethanedioate ions

2Mn2+ (aq) + 2CO2 (g)

N
...


Reason for low efficiency - adsorption
too strong
...
Not a good catalyst because
•reactants not brought together
•So no increase in reactant
concentration on catalyst surface
•reactants not held long enough for a
reaction to occur
•reactant bonds not weakened

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