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3. When the molecules are adsorbed onto the surface they can migrate across the surface of the catalyst
4. The adsorption of reactant molecules on a catalyst surface effectively increases the concentration of
the molecules
·In the third step the products are desorbed from the surface of the solid catalyst and diffuse away
The Efficiency of a Catalyst
·The strength of adsorption on, and desorption from, the catalyst's surface plays an important role in the
efficiency of a catalyst
·If the adsorption is too weak then the reactant will not stay on the surface of the catalyst long enough for
the reaction to occur
·The active sites are only regenerated if the product desorbs from the surface, if the adsorption is too
strong, desorption cannot occur quickly enough and thus the rate of migration of more reactant molecules
over the surface of the catalyst is reduced
Tungsten (W) and Silver (Ag)
·Not often used as catalysts
·Tungsten- forms very strong bonds with some molecules meaning the product isn't released quick enough
to increase the overall reaction rate
·Silver- forms only very weak bonds with molecules, so it is unable to hold onto the gas long enough for
the reaction to take place. It is however used in the formation of epoxyethane as it slows down the
reaction, reducing the chance of epoxyethane reacting explosively
Nickel (Ni) and Platinum (Pt)
·Good catalysts
·Achieve a good balance between adsorption of reactants and desorption of products
Surface Area
·By increasing the surface area of the catalyst the number of molecules that can react at the same time
increases resulting in the rate of reaction increasing
·A support medium is often used to make the area of the catalyst as large as possible
·In catalytic converters a ceramic lattice coated with a thin layer of platinum and rhodium is used, this
provides maximum surface area for minimum cost
Catalytic Poisoning
·During a reaction, reactants are adsorbed onto active sites on the surfaces of heterogeneous catalysts
·Impurities in the reaction mixture may also bind to the catalyst's surfaces and block the reactants from
being adsorbed- catalytic poisoning
·Reduces the surface area of the catalyst that is available to the reactants, slowing down the reaction
·Increases the cost of the process because less product is being made at any one time
Lead poisoning in catalytic converters
·Lead can coat the surface of the catalyst in catalytic converters, cars must only be run on unleaded petrol
Sulphur poisoning in the iron catalysts of the Haber Process
·Hydrogen is produced from methane which is obtained from natural gas which contains impurities
·One impurity is sulphur which if left will be adsorbed to form iron sulphide preventing catalysis…read more

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Homogenous Catalysts
·In the same physical state as the reactants
·Usually an aqueous catalyst for a reaction between two aqueous solutions
·The reactants combine with the catalyst to make an intermediate species, which then reacts to form
the products and reform the catalyst
The enthalpy profile for a homogeneously catalysed
reaction has two humps in it, corresponding to the two
reactions. The activation energy needed for the two
reactions is lower than that needed to make the products
directly from the reactants.
Fe2= catalysing the reaction between S2O82- and I-
·REDOX reaction between iodide ions and peroxodisulphate ions:
S2O82- (aq) + 2 I- (aq) I2 (aq) + 2SO42-(aq)
·This reaction takes place very slowly because both ions are negatively charged, meaning they will repel
each other
·The addition of Fe2= increases the rate of reaction because at each stage there is now a positive and a
negative ion, so there is no repulsion
Stage 1
The Fe2= ions are oxidised to Fe3= ions by the S2O82- ions
S2O82- (aq) + 2Fe2+(aq) Fe3= (aq) + 2SO42-(aq)
Stage 2
The newly formed intermediate Fe3= ions now easily oxidise I- ions to iodine, and the catalyst is
2 Fe3= (aq) + 2 I- (aq) I2 (aq) + 2Fe2+(aq)
·You can test for iodine by adding starch solution, it will turn blue /black if iodine is present
Mn2+ autocatalysing the reaction between MnO4- and C2O42-
· This is an autocatalysis reaction because Mn2+ the product of the reaction acts as a catalyst for the
·As the reaction progresses and the amount of product increases, there reaction rate increases
2 MnO4- (aq) + 16H+(aq) + 5 C2 O42- (aq) 2 Mn2+ (aq) + 8H2O(l) + 10CO2 (g)
· At the start of the reaction there isn't any Mn2+ to catalyse it, so at first the reaction rate is very slow
and the activation energy is very high
Step 1
Once a little bit of the Mn2+ catalyst has been made it react with the MnO4- ions to make Mn3+ ions
4 Mn2+ (aq) + MnO4- (aq) + 8H+(aq) 5Mn3+(aq) + 4H2O(l)
Step 2
The Mn3+ ions are the intermediate. They then react with the C2O42- ions to make CO2 and reform the
Mn2+ catalyst
2Mn3+(aq) + C2 O42- (aq) 2Mn2+(aq) + 2CO2 (g)…read more


:) PurpleJaguar (: - Team GR

So helpful! thanks :)

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