CI 10.5 How do catalysts work?
Collision theory and enthalpy profiles can help us to understand
how catalysts work.
Bond breaking is endothermic (requires an energy input to
overcome the activation enthalpy barrier). Bonds first stretch and
then break, reactants are made and new bonds instantly form. If
few molecules have enough energy to overcome this activation
enthalpy, then the reaction will be slow.
Catalysts provide an alternative reaction pathway for breaking and
remaking bonds that has a lower activation enthalpy.
Activation enthalpy
uncatalysed reaction
E
n
t
h
a
l
p
y
Activation enthalpy
catalysed reaction
H
Reactants
Products
Progress of reaction
Catalysts and equilibrium
Catalysts only affect the RATE and not the position of
equilibrium in a reversible reaction. The composition of the
reaction mixture remains unchanged.
Homogeneous catalysts
A homogeneous catalyst forms an intermediate compound before
breaking down to the final product and reforming the catalyst
again. This is why the enthalpy profile shows TWO humps.
Enthalpy
Intermediate compound
Final product
H
Reactants
Products
Progress of reaction
CFCs act as homogeneous catalysts in the stratosphere, breaking
down ozone. Cl atoms catalyse the reaction, forming the
intermediate ClO:
Cl + O3  O2 + ClO  intermediate
ClO + O  Cl + O2
O3 + O  O2 + O2  overall change
A single Cl atom can catalyse the reaction of many ozone
molecules through a catalytic cycle.
Industry uses mostly heterogeneous catalysts. However
homogeneous catalysts can be more specific and controllable.
Eg. Methanol Rhodium (aq)
ethanoic acid
Conversion is 99% with soluble rhodium compounds.
 Do problems for 10.5 p.243 questions 1 and 2.
SL: Other ways ozone is removed
Radicals such as hydroxyl and nitrogen monoxide can destroy
ozone, as well as chlorine and bromine.
In general:
X + O3  XO + O2
XO + O  X + O2
Overall reaction:
O + O3  O2 + O 2
Hydroxyl radicals (HO) form in the stratosphere when water
molecules react with oxygen. The reaction with ozone is:
HO + O3  HO2 + O2
HO2 + O  HO + O2
The reformed HO radicals can react with more ozone, in a
catalytic cycle.
Nitrogen monoxide (NO) forms nitrogen dioxide and dioxygen
when it reacts with ozone. NO and NO2 are relatively stable
radicals which can be collected in ordinary ways.
 do assignment 8, p.71
a) Write an equation to show the formation of HO radicals from
O atoms and water.
H2O(g) + O(g)  HO + HO
b) Write equations to show how nitrogen monoxide can destroy
ozone in a catalytic cycle.
NO + O3  NO2 + O2
NO2 + O  NO + O2
SL A4: The CFC story
In the early 1970s there was concern about jet aircraft releasing
NO in their exhausts. Levels were not significant at the time. In
1974 CFCs became a concern…
 read about it p.71-74 and do assignments 9 and 10.