Quote-
Malachite:
2 Cu<sup>2+</sup>(aq) + 2 CO<sub>3</sub><sup>2-</sup>(aq) + H<sub>2</sub>O(l) === >
Cu<sub>2</sub>CO<sub>3</sub>(OH)<sub>2</sub>(s) + CO<sub>2</sub>(g)
So for every 2 mol of carbonate 1 mol is shed.
Azurite:
3 Cu<sup>2+</sup>(aq) + 3 CO<sub>3</sub><sup>2-</sup>(aq) + 2 H<sub>2</sub>O(l) === >
Cu<sub>3</sub>(CO<sub>3</sub><sub>2</sub>(OH)<sub>2</sub>(s) + CO<sub>2</sub>(g)
So for every 3 mol of carbonate 1 mol of CO2 is shed. (Of course Azurite cannot be prepared that way).
Unquote-
By weighing everything carefully and measuring the weight loss after reaction, the molar ratio of carbonate used/CO2 lost can be established.
Mass balances don't lie: if the molar ratio was 2 (or close), what you precipitated was Malachite, it simply cannot be otherwise stoichiometrically
speaking.
In any case, if that paper on M/A stability shows us one thing it's that in ordinary precipitation conditions pH will ALWAYS be (way) too high for
Azurite to form.
Reading that paper reinforces my belief that Debray had the right idea and that it's me who is doing it wrong.
[Edited on 24-12-2014 by blogfast25] |