[Catalist] Acidic/ Basic Salts

Thu Jun 8 16:35:59 AEST 2017

I don’t tend to like wrote learning rules on the acidity or basicity or ions, I would rather understand it and not have to remember.

So in a case like this where you have two competing hydrolysis reactions, the answer is indeed which of these two hydrolysis reactions has a higher K.

Where one of the reactions produces a strong acid or strong base, then the K is extremely low because the strong acid or strong base by definition will not exist in water, if it forms it re-ionises/dissociates. So it will not hydrolyse water and wont produce any extra hydronium/hydroxide and the ion is neutral.
However, if a weak acid or base is produced, then an equilibrium would occur to a certain extent so you would get some reaction with water and a little extra hydronium or hydroxide in the process so the ion would have acidic or basic properties.

In this case if hydrogen sulphate acts as a Bronsted_Lowry base, it would form sulphuric acid and hydroxide ion, but do do so would produce sulphuric acid, which is a strong acid. So this equilibrium strongly favours the reactants and hydrogen sulphate ion wont act as a B-L base.
However, if it acts acts as a B-L acid, it produced sulphate ion and hydronium ion. Sulphate is not a strong base, so the equilibrium will not strongly favour the reverse reaction, which means the forward reaction will occur to a certain extent and you will get the hydrogen sulphate ion acting as a B_L acid. Put these two together and you get hydrogen sulphate is an acidic ion.

This method makes sense to me and works most of the time. You only get a bit of a problem when you get things like bicarbonate ion or dihyrogen phosphate ion that can produce carbonic acid or carbonate ion or phosphoric acid/ hydrogen phosphate. In both these cases both hydrolysis reactions will occur to some extent because the products are weak acid and weak bases.  In these cases you would need to rely on the rules/ knowledge to know which would occur to a greater extent. Bicarbonate is basic because the reaction to produce hydroxide and carbonic acid occurs to a greater extent than the reaction with water to form carbonate and hydronium. But dihydrogen phosphate is the opposite, the reaction to form hydrogen phosphate and hydronium ion occurs to a greater extent than the formation of phosphoric acid and hydroxide.

Regards
Brian

From: Catalist [mailto:catalist-bounces at lists.stawa.net] On Behalf Of Andrew Bland
Sent: Thursday, 8 June 2017 12:43 PM
To: Catalist at lists.stawa.net
Subject: [Catalist] Acidic/ Basic Salts

Question for the chemists:

Q: Is NaHSO4 and acidic or basic salt and why? Or in other words when HSO4- ion reacts with water, does it produce H2SO4 and H+ or SO42- and OH-? Why is one reaction favoured over the other?

Thanks

Andrew.

Andrew Bland

Head of Year 10 and Head of Geographe House

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