IChO 1968 · Theory problem 2

Equations of three redox reactions

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Write down equations for the following reactions:

2.1 Oxidation of chromium(III) chloride with bromine in alkaline solution (\(\ce{KOH}\)).

2.2 Oxidation of potassium nitrite with potassium permanganate in acid solution (\(\ce{H2SO4}\)).

2.3 Action of chlorine on lime water (\(\ce{Ca(OH)2}\)) in a cold reaction mixture.

Official English text of the problem.

Answers

  • 2.1 \[ \boxed{\ce{2CrCl3 + 3Br2 + 16KOH -> 2K2CrO4 + 6KBr + 6KCl + 8H2O}} \]

  • 2.2 \[ \boxed{\ce{5KNO2 + 2KMnO4 + 3H2SO4 -> 5KNO3 + 2MnSO4 + K2SO4 + 3H2O}} \]

  • 2.3 \[ \boxed{\ce{2Cl2 + 2Ca(OH)2 -> CaCl2 + Ca(ClO)2 + 2H2O}} \]

Solution

2.1

Bromine oxidizes chromium from oxidation state \(+3\) to \(+6\). In alkaline solution, the resulting chromium(VI) species is chromate, \(\ce{CrO4^2-}\). Bromine is reduced from oxidation state \(0\) to \(-1\), forming bromide, \(\ce{Br-}\).

The half-reactions in alkaline solution are \[ \ce{Cr^3+ + 8OH- -> CrO4^2- + 4H2O + 3e-} \] and \[ \ce{Br2 + 2e- -> 2Br-}. \]

Multiplying the first by \(2\) and the second by \(3\) balances the transfer of six electrons: \[ \ce{2Cr^3+ + 3Br2 + 16OH- -> 2CrO4^2- + 6Br- + 8H2O}. \]

Restoring the potassium and chloride spectator ions gives the molecular equation in 2.1. The salt products are potassium chromate, potassium bromide, and potassium chloride.

2.2

Nitrogen is oxidized from \(+3\) in nitrite, \(\ce{NO2-}\), to \(+5\) in nitrate, \(\ce{NO3-}\), losing two electrons per nitrogen atom. Manganese is reduced from \(+7\) in permanganate, \(\ce{MnO4-}\), to \(+2\) in acidic solution, gaining five electrons per manganese atom.

The half-reactions are \[ \ce{NO2- + H2O -> NO3- + 2H+ + 2e-} \] and \[ \ce{MnO4- + 8H+ + 5e- -> Mn^2+ + 4H2O}. \]

Multiplying the first by \(5\) and the second by \(2\), then cancelling electrons, water, and hydrogen ions, gives \[ \ce{5NO2- + 2MnO4- + 6H+ -> 5NO3- + 2Mn^2+ + 3H2O}. \]

The six hydrogen ions require three formula units of sulfuric acid. Restoring potassium and sulfate gives the equation in 2.2, with potassium nitrate, manganese(II) sulfate, and potassium sulfate as the salt products.

2.3

In a cold alkaline mixture, chlorine undergoes disproportionation:

  • One chlorine atom is reduced from \(0\) to \(-1\), forming chloride.
  • The other is oxidized from \(0\) to \(+1\), forming hypochlorite, \(\ce{ClO-}\).

Thus chloride and hypochlorite form in equal amounts. Balancing oxygen and hydrogen with hydroxide and water gives \[ \ce{Cl2 + 2OH- -> Cl- + ClO- + H2O}. \]

Calcium remains in oxidation state \(+2\). Using calcium hydroxide as the source of hydroxide gives calcium chloride and calcium hypochlorite: \[ \ce{2Cl2 + 2Ca(OH)2 -> CaCl2 + Ca(ClO)2 + 2H2O}. \]

The same stoichiometry can be written using the formal mixed-salt shorthand \[ \ce{Cl2 + Ca(OH)2 -> CaCl(OCl) + H2O}. \] Here calcium chloride hypochlorite, \(\ce{CaCl(OCl)}\), [Ca+2].[Cl-].[O-]Cl, records the calcium, chloride, and hypochlorite composition of the aqueous products.

Names and SMILES

The salts in these equations consist of ions; their constituent-ion SMILES are given below, with their proportions specified by the chemical formulas.

Molecule or ion SMILES
Bromine, \(\ce{Br2}\) BrBr
Chlorine, \(\ce{Cl2}\) ClCl
Water, \(\ce{H2O}\) O
Sulfuric acid, \(\ce{H2SO4}\) OS(=O)(=O)O
Hydrogen ion, \(\ce{H+}\) [H+]
Hydroxide ion, \(\ce{OH-}\) [OH-]
Potassium ion, \(\ce{K+}\) [K+]
Calcium ion, \(\ce{Ca^2+}\) [Ca+2]
Chromium(III) ion, \(\ce{Cr^3+}\) [Cr+3]
Manganese(II) ion, \(\ce{Mn^2+}\) [Mn+2]
Chloride ion, \(\ce{Cl-}\) [Cl-]
Bromide ion, \(\ce{Br-}\) [Br-]
Chromate ion, \(\ce{CrO4^2-}\) [O-][Cr](=O)(=O)[O-]
Nitrite ion, \(\ce{NO2-}\) [O-]N=O
Nitrate ion, \(\ce{NO3-}\) [O-][N+](=O)[O-]
Permanganate ion, \(\ce{MnO4-}\) [O-][Mn](=O)(=O)=O
Sulfate ion, \(\ce{SO4^2-}\) [O-]S(=O)(=O)[O-]
Hypochlorite ion, \(\ce{ClO-}\) [O-]Cl

These SMILES do not specify solvation or ionic crystal structures. Chromate, permanganate, and sulfate have tetrahedral oxygen arrangements around their central atoms; nitrate is trigonal planar, and nitrite is bent. The oxoanion SMILES show representative resonance forms rather than a unique localization of bonds and charge.

The solution was checked against the official solution.