Will acidified potassium dichromate(VI) oxidise chloride ions to chlorine?
The E° values are: Because the chlorine E° value is slightly greater than the dichromate(VI) one, there shouldn’t be any reaction. For a reaction to occur, the equilibria would have to move in the wrong directions. Unfortunately, in the test tube, potassium dichromate(VI) solution does oxidise concentrated hydrochloric acid to chlorine. The hydrochloric acid serves as the source of the hydrogen ions in the dichromate(VI) equilibrium and of the chloride ions. The problem here is that E° values only apply under standard conditions. If you change the conditions you will change the position of an equilibrium – and that will change its E value. (Notice that you can’t call it an E° value any more, because the conditions are no longer standard.) The standard condition for concentration is 1 mol dm-3. But concentrated hydrochloric acid is approximately 10 mol dm-3. The concentrations of the hydrogen ions and chloride ions are far in excess of standard. What effect does that have on the two pos
