Question

What will Cr2(SO4)3(aq)+(NH4)2CO3(aq) produce?

Answer 1

You get a precipitate of chromium(III) hydroxide and `sf(CO_2)` gas.

Explanation:

You might assume that you would get a precipitate of chromium(III) carbonate, but this is not the case.

This is due to the acidic nature of `sf(Cr_((aq))^(3+))`. This is a complex with 6 water ligands surrounding the central metal ion.

Because the `sf(Cr^(3+))` ion is small and highly charged this has the effect of removing electron density from the 0-H bonds, making them weaker. If a base is present then `sf(H^+)` ions will be lost:

`sf([Cr(H_2O)_6]^(3+)+H_2Orightleftharpoons[Cr(H_2O)_5OH]^(2+)+H_3O^(+))`

Many transitional salts are acidic because of this. In the presence of a stronger base such as `sf(CO_3^(2-))` then further protons can be lost:

`sf([Cr(H_2O)_5OH]^(2+)+H_2Orightleftharpoons[Cr(H_2O)_4(OH)_2]^(+)+H_3O^+)`

`sf([Cr(H_2O)_4(OH)_2]^(+)+H_2Orightleftharpoons[Cr(H_2O)_3(OH)_3]+H_3O^+)`

The last species is neutral so is not very water soluble and the substance drops out of solution as a green, gelatinous precipitate.

The hydrogen ions react with the carbonate ions in an acid/base reaction:

`sf(2H_3O^(+)+CO_(3)^(2-)rarr3H_2O+CO_2)`

Effervescence is observed due to the formation of `sf(CO_2)`.