Weller, Overton, Rourke & Armstrong: Inorganic Chemistry 6e
Chapter 17: Videos of chemical reactions
The starch-iodine clock reaction
This particular version is, in fact, the Landolt Reaction. The first, very slow step, is the rate determining step in which iodate is reduced to iodide:
HSO3-(aq) + H2O (l) ? HSO4-(aq) + 2H+(aq) + 2e-
IO3-(aq) + 6H+ (aq) ? I-(aq) + 3H2O(l)
The iodide ion then undergoes a fast conproportionation reaction with excess iodate ion:
2I-(aq) ? I2 (aq) + 2e-
2IO3- (aq) + 12H+(aq) + 10e-? I2 (aq) + 6H2O(l)
The iodine is then reduced to iodide ion again in an ?immeasurably fast? reaction with hydrogen sulfite ion:
I2 (aq) + 2e-? 2I-(aq)
HSO3-(aq) + H2O(l) ? HSO4-(aq) + 2H+(aq) + 2e-
It is only when all the hydrogen sulfite ion is completely consumed that the iodine concentration suddenly starts to build up and the characteristic deep blue-black starch-iodine complex forms (this consists of the starch molecules wrapping themselves helically around a chain of iodine molecules).
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Comparative solubility of the silver halides
Addition of chloride ion to silver ion produces a precipitate of white silver chloride.
Ag+(aq) + Cl-(aq) ? AgCl (s)
Addition of iodide ion to silver ion produces a precipitate of yellow silver iodide.
Ag+(aq) + I-(aq) ? AgI (s)
The more soluble silver chloride reacts with ammonia to give the soluble colorless diamminosilver(I) complex ion
AgCl(s) + 2NH3(aq) ? [Ag(NH3)2]+(aq) + Cl-(aq)
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Hypochlorite ion as an oxidizing agent I
Hypochlorite ion reacts with the chloride ion in a conproportionation reaction to give chlorine gas:
2ClO-(aq) + 4H+(aq) + 2e-? Cl2(g) + 2H2O(l)
2Cl-(aq) ? Cl2(g) + 2e-
to give a combined reaction of:
ClO-(aq) + Cl(aq) + 2H+(aq) ? Cl2(g) + H2O(l)
Damp colorless starch-iodide paper turns purple-black (the reaction between iodine and starch) upon exposure to the chlorine gas:
2I-(aq) ? I2(s) + 2e-
2Cl2(g) + 2e-? 2Cl-(aq)
Blue litmus paper turns a pale pink as a result of the combination of acidic and oxidizing properties of chlorine gas.
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Hypochlorite ion as an oxidizing agent II
Initially, lead(II) ion reacts with hypochlorite ion to give a white precipitate of lead(II) hypochlorite:
Pb2+(aq) + 2ClO-(aq) ? Pb(ClO)2(s)
The lead(II) hypochlorite is then oxidized to lead(IV) oxide. Reaction is slow at room temperature but much faster when the tube is placed in boiling water.
Pb2+(aq) + 2H2O(l) ? PbO2(s) + 4H+(aq) + 2e-
ClO-(aq) + 2H+(aq) + 2e-? Cl-(aq) + H2O(l)
