Cation acids

The most famous example is the Aluminium ion dissolved in water, but there are lots more. Cations normally are hydrated polypositive metal ions: Al(H2O)63+,  Cu2+,  Fe2+ of Fe3+, any many more, and always hydrated in water.


The poly positive ions attract closely, in watery environment, the negative sides of the water molecules.
Then they cause a repulsive feeling between the central positive ion and the δ+)charges of the H atoms of the water molecules. There is a tendency to repulse H+.
A solution of, for example Iron(III)chloride can obtain in this way a rather acid pH value.

The drawing shows the attraction between Al3+ and the δ- part of Oxygen and the δ+ part of Hydrogen.
The water molecules (6 in total) surround the 3+ ion of Aluminium, because this positive ion attracts the δ- of water.
thus the distance between 3+ and δ+ becomes shorter, and with that, the repulsion between 3+ and δ+ becomes stronger.
The concequence is that the H can be (more of less) repelled. There could be even donation of H+ ionen, and all this we call: an acid character. (much more about this in module 9)