OCR AS Chemistry F332: How Catalysts Work

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How Catalysts Work
In a chemical reaction, existing bonds in the reactants must first stretch and break.
Then new bonds can form as the reactants are converted to products.
Bond breaking is an endothermic process.
A pair of reacting molecules must have enough energy between them to collide with
an energy greater than the activation energy before reaction can occur.
If the energy barrier is very high then relatively few pairs of molecules will have enough
energy to react when they collide ­ so the reaction is slow.
Catalysts speed up reactions by providing an alternative reaction pathway for the
breaking and remaking of bonds that has a lower activation enthalpy.
Now that the energy barrier is lower, more pairs of molecules can react when they
collide ­ this means that the reaction proceeds more quickly.
Note that the enthalpy change, H, is the same for the catalysed and uncatalysed
reactions.
The effect of a catalyst on the enthalpy profile for a reaction

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Catalysts and Equilibrium
Catalysts do not affect the position of equilibrium in a reversible reaction.
They alter the rate at which the equilibrium is attained, but not the composition of
the equilibrium mixture.
Homogeneous Catalysts
Homogeneous catalysts normally work by forming an intermediate compound with
the reactants. That is why the enthalpy profile for the catalysed reaction above has two
humps ­ one for each step.
The intermediate compound then breaks down to give the product and reform
the catalyst.…read more

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