Effect of a catalyst on a reaction
What are catalysts?
A catalyst is a substance which speeds up a reaction, but is chemically unchanged at the end of the
reaction. When the reaction has finished, you will have exactly the same mass of catalyst as you had
at the beginning.
Some common examples :
Use of acids as catalysts in organic chemistry : esterification and its reverse (the acid
catalysed hydrolysis of esters).
Experimental part: Oxidation of iodide ions I- by dichromate ions Cr2O7
2-
1. Experiment 1 : action of iodide ions on dichromate ions
Iodide ions should be spontaneously oxidised to brown iodine (I2) by dichromate ions to give chrome
III ions (Cr3+ ).
An experiment proves that acidic aqueous solutions containing dichromate and iodide ions are mixed
with no reaction.
Actually, the reaction is not taking place !
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2. Experiment 2 :Action of iodide ions on dichromate ions in the presence of iron II ions
The addition of a solution of iron(II) ion in the previous mixture induces the rapid formation of
brown iodine I2.
A reasonable hypothesis for the mechanism is that dichromate oxidises iron(II) to iron(III), and
that iron(III) then oxidises iodide to iodine I2.
An identification test proves the presence of a large amount of iron II ions in the beaker at the end
of the reaction .
3. Experiment 3 : Action of iron III ions on iodide ions
The two colourless solutions are mixed.
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Questions
Experiment 1 :
 Write the half-equation for iodine/iodide.
 The half-equation for dichromate/chrome III is : Cr2O72- + 14 H+ + 6e- = 2Cr3+ + 7H2O
Write the equation of the reaction between iodide and dichromate ions in acid aqueous
medium.
 Did dichromate ions actually react with iodide ions ?
 How can you explain this ?
Experiment 2 :

Why do we have to add iron II ions to make the reaction start ? What else could be done to
obtain the same result ?

Are Fe 2+ ions behaving as a reactant ?

What role can iron II ions play in iodide reaction with dichromate ?
Experiment 3



Did the iron III ions react with iodide ions ?
Is the hypothesis valid (experiment 2)
Iron (v), known as ferrate, is a powerful oxidant. Try to explain what happens during
experiment 2.
General questions

Among the following safety devices, what would you choose to make experiments 1, 2 and 3 ?
Fume cupboard
Gloves
Safety glasses
Explain why.

Give an example of catalyst you studied in chemistry or biology.
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