Account for the following observations on the basis of electrochemical principles. The Standard Redu — Electrochemistry Chemistry Question
Problem Context
Account for the following observations on the basis of electrochemical principles. The Standard Reduction Potentials are provided.
2 HOCl(aq) + 2 H+(aq) + 2 e– → Cl2(g) + 2 H2O(l) E˚ = 1.61 V
Cl2(g) + 2 e– → 2 Cl–(aq) E˚ = 1.36 V
O2(g) + 4 H+(aq) + 2 e– → 2 H2O(l) E˚ = 1.23 V
Cu2+(aq) + 2 e– → Cu(s) E˚ = 0.34 V
Sn2+(aq) + 2 e– → Sn(s) E˚ = –0.14 V
Fe2+(aq) + 2 e– → Fe(s) E˚ = –0.44 V
Zn2+(aq) + 2 e– → Zn(s) E˚ = –0.76 V
In a voltaic cell made with Cu metal in a 1.0 M CuSO4 and Zn metal in 1.0 M ZnSO4 the Zn is the anode and the cell potential is more than 1.0 V. When aqueous sodium sulfide is added to the CuSO4 solution the cell potential decreases substantially.
Model Answer
Zn + Cu2+ → Zn2+ + Cu Ecell = 0.76 + 0.34 = 1.10 V
When S2- is added to the Cu2+/Cu half cell, CuS forms reducing [Cu2+], shifting the reaction to the left and decreasing Ecell.
Iron metal corrodes readily in moist air but this corrosion can be prevented when iron is coated with tin or zinc. Corrosion is prevented when the zinc coating is intact or broken. In contrast, corrosion is prevented by coating iron with tin only as long as the tin coating remains intact but actually occurs faster when there is a break in the tin coating.
Model Answer
Fe → Fe2+ + 2e– is 0.44 V so oxidation (corrosion) is spontaneous. Covering the surface with Sn or Zn prevents reaction with O2. If the Zn coating is broken, Zn will still oxidize preferentially. If the Sn coating is broken, Fe will oxidize more readily.
In acid solution chloride and hypochlorite ions react to form chlorine gas whereas in basic solution chlorine gas reacts to form chloride and hypochlorite ions.
Model Answer
2 HOCl + 2 H+ + 2 Cl– → 2 Cl2 + 2 H2O is spontaneous in acid. In basic solution [H+] is very low so the reaction shifts to the left and Cl2 forms Cl– and OCl–.