9 Chemical periodicity
The role of Fe2+ in the I-/S2O82- reaction
The oxidation of iodide ions by peroxodisulfate ions is another example of homogeneous catalysis. In this case, all the species are in the aqueous phase. It is believed
that in the presence of Fe 3+ ions this oxidation occurs in two steps:
Overall reaction:
Hodder CIE revision guide
2010
Catalysed
reaction:
Chemistry fig 8.8
25 August 2010
Eleanor Jones
S2O82-(aq) + 2I-(aq) → 2SO 42-(aq) + I 2(aq)
S2O82-(aq) + 2Fe2+(aq) → 2SO 42-(aq) +2Fe 3+(aq)
2Fe 3+(aq) + 2I-(aq) → 2Fe2+(aq) + I 2(aq)
Although there are two steps in the reaction, the overall activation energy is lower
than in the single-step reaction, as can be seen in Figure 8.8.
Energy
Intermediate (no catalyst)
Ea uncatalysed
Intermediate step 1
Intermediate step 2
Ea
catalysed
S2O82− + I− (intermediate) + Fe2+
SO42− + Fe3+ + I− (reactants)
SO42− + I2 (products) + Fe2+
Progress of reaction
Figure 8.8 Effect of a catalyst on the reaction profile
9 Chemical periodicity
All of the material in this chapter is needed for the AS exam.
Physical properties of elements
You need to know and be able to explain the variation of four key physical properties
of elements across the third period (sodium to argon). These are:
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atomic and ionic radius
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melting point
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electrical conductivity
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ionisation energy