Metal Aqua Ions: Acidity, Ligand Exchange and Precipitation
A metal ion in water is already a complex, and a highly charged one pulls hard enough on its water to make the solution acidic. Adding alkali, ammonia or an excess of either swaps ligands and drops or redissolves a hydroxide.
What a learner can do afterwards
- Explains why a 3 plus aqua ion gives a more acidic solution than a 2 plus one
- Writes the equation for a hydroxide precipitating when sodium hydroxide is added
- Predicts which precipitates redissolve in excess alkali and which redissolve in excess ammonia
- Identifies an unknown transition metal ion from the colours in a sequence of tests
1 · Read
A metal ion in water is already a complex, and a 3+ ion packs more charge into its pull than a 2+ one. It drags harder on the water around it, weakens the O-H bonds, and frees more H+, so its solution is more acidic.
Add sodium hydroxide to copper(II) and blue copper hydroxide falls out: Cu2+ in water plus 2OH- gives Cu(OH)2 solid. Pour in concentrated HCl instead and the pale blue water complex swaps to the yellow-green chloro complex.
Some hydroxides redissolve in excess. Copper, nickel and zinc hydroxides clear in excess ammonia, with copper turning deep blue, while aluminium, chromium, zinc and lead hydroxides clear in excess alkali. Iron(II) green stays put in both.
A green precipitate that refuses to redissolve in excess alkali points to iron(II), since chromium(III) is green too but redissolves. Back it up with ammonia, which also gives green with no redissolve for iron(II).
Charge sets acidity, alkali drops hydroxides, excess sorts them, and green that stays put means iron(II).
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.