posted by Stuck on .
I am predicting acid-base equilibria using the Brondsted-Lowry predictions. I am asked to do "solid aluminum sulfate added to water".
So I know there is an Al^2+ ion and a SO4^-2 ion, but does water dissociate into H30^+ and OH^-? If so, then H30^+ and OH^- would be the strongest acid and base...and I would end up with a neutral reaction (shouldn't it be acidic?). If not, then I would only have a strong base (SO4^-2), and no acid to work with. Help!
Also, if products are favoured, does that mean that the resulting solution is basic? And if reactants are favoured, the solution is acidic? Or is there no distinction?
Let's start over.
Al2(SO4)3 + H2O ==> Al^+3 (you had +2) + SO4^-2. Not balanced.
But it doesn't stop there.
Al^+3 actually is not that but Al(H2O)6 with +3 charge in solution. It is a hydrated ion with a +3 charge. It is acidic because it ionizes. It's tough to write on the board because I can't do subscripts and superscripts. Here is what happens. One of the H2O molecules that is in the hydrated ion splits into H^+ and OH^-. The H^+ reacts with H2O to form the H3O^+. What is left is the Al(H2O)5(OH)^+2. Now that you know what happens I'll do my best to write the equation.
Al(H2O)6(+3 charge) + H2O ==> Al(H2O)5(OH)^+2 + H3O^+.
The solution is acidic because of the H3O^+. The products are favored. But that means the solution is acidic. Where is this knowledge useful. Some plants don't grow well in basic soil; they need acidic soil. If a person has soil that is basic, s/he adds a little Al2(SO4)3 to the soil (solid S will do the same thing but it's a different reaction) and it becomes acidic (pH 5.5 -6--something like that) and those acidic plants thrive. (The S works because it's a bacteria found in the soil that eats on it and converts it to an acid.).