Why is Na2SO4 neutral but NaHSO4 acidic (1 Viewer)

[steven holwerda]

Was going over salts as acids and bases and got confused when I saw that Na₂SO₄ is neutral but NaHSO₄ is acidic.
According to the rule, a strong acid + strong base -> neutral salt and since the sodium ions come from the strong base sodium hydroxide and the sulphur ions come from the strong acid sulphuric acid, shouldn't both Na₂SO₄ and NaHSO₄​ neutral?

 

[someth1ng]

Was going over salts as acids and bases and got confused when I saw that Na₂SO₄ is neutral but NaHSO₄ is acidic.
According to the rule, a strong acid + strong base -> neutral salt and since the sodium ions come from the strong base sodium hydroxide and the sulphur ions come from the strong acid sulphuric acid, shouldn't both Na₂SO₄ and NaHSO₄​ neutral?

NaHSO4 is a very weak acid, in water it will dissociate into Na+ and HSO4- ions and HSO4- can give off some protons since it acts as a weak acid.
Na2SO4 is actually a very weak base since it can accept protons SO42-+H+ --> HSO4-.

If you consider H2SO4 to be a strong acid that donates both protons fully then Na2SO4 will be neutral and NaHSO4 will be acidic since the HSO4- ion will donate its proton.

 

[steven holwerda]

But according to: "Na2SO4 is actually a very weak base" shouldn't it be basic instead of neutral (because the SO4 accepts protons)?

 

[someth1ng]

But according to: "Na2SO4 is actually a very weak base" shouldn't it be basic instead of neutral (because the SO4 accepts protons)?

I said it was basic? If it's a base, it's basic.

Na2SO4 is a neutral salt as it is formed from HCl and H2SO4.

It's actually basic...

 

[steven holwerda]

I think you mean NaOH and H2SO4? but then what about NaHSO4, which is also formed from NaOH and H2SO4?

 

[steven holwerda]

but you said "then Na2SO4 will be neutral"?

 

[someth1ng]

but you said "then Na2SO4 will be neutral"?

I said this:

If you consider H2SO4 to be a strong acid that donates both protons fully then Na2SO4 will be neutral

When in solution, NaHSO4 makes HSO4- and Na2SO4 makes SO42-, they'll also make Na+ ions but we'll ignore them.

H2SO4 --> HSO4- + H+ <--> 2H+ + HSO4-
Note: first dissociation is strong but the second is weak but both are considered strong during calculations.

NaHSO4 produces HSO4- ions (assume all states as aqueous)
HSO4- <--> H+ + SO42- (HSO4- cannot accept any protons since H2SO4 will automatically dissociate to HSO4-)

As you can see, HSO4- is donating protons but is weak making NaHSO4 a weak acid.

Na2SO4 produces SO42- ions (assume all states as aqueous)
SO4 2- + H+ <--> HSO4-

In this one, SO4 2- is accepting protons but is a weak base making Na2SO4 a weak base.

 

[steven holwerda]

So my reactions would be:

NaHSO4 + H2O --> Na+ + HSO4- + H2O
Na+ + HSO4- + H2O --> Na+ + SO42- + H3O+

and

Na2SO4 + H2O --> 2Na+ + SO42- + H2O
2Na+ + SO42- + H2O --> 2Na+ + HSO4- + OH- ?

That would account for why you're saying that NaHSO4 is acidic and Na2SO4 is basic but why does it work in that way? Shouldn’t it look like this?

NaHSO4 + H2O --> NaOH +H2SO4

and

Na2SO4 + 2H2O --> 2NaOH + H2SO4

And since NaOH is a strong base and H2SO4 is a strong acid, (and there are no OH- or H+ ions in the products) shouldn’t they both be neutral?

 

[barbernator]

ok sorry yes I agree with something, Na2SO4 is a weak basic salt.

to ^ H2SO4 undergoes stepwise ionisation in which the first step goes to completion, yet the second step does not, resulting in SO42- being a weak base.

 

[someth1ng]

NaHSO4 + H2O --> Na+ + HSO4- + H2O
Na+ + HSO4- + H2O --> Na+ + SO42- + H3O+

Correct except in the second one, it's a double arrow (<-->) to show that there is an equilibrium.

Na2SO4 + H2O --> 2Na+ + SO42- + H2O
2Na+ + SO42- + H2O --> 2Na+ + HSO4- + OH- ?

These are correct but again, remember that you need a double arrow for the second equation.

That would account for why you're saying that NaHSO4 is acidic and Na2SO4 is basic but why does it work in that way? Shouldn’t it look like this?
NaHSO4 + H2O --> NaOH +H2SO4
and
Na2SO4 + 2H2O --> 2NaOH + H2SO4
And since NaOH is a strong base and H2SO4 is a strong acid, (and there are no OH- or H+ ions in the products) shouldn’t they both be neutral?

No, the reactions work in the opposite directions.
Remember that acids are proton donors and bases are proton acceptors.

An acid will donate a proton to water to produce hydronium ions
H+ + H2O --> H3O+

A base will accept a proton from water to produce hydroxide ions
H2O + A- --> OH- + HA

2NaOH +H2SO4 --> NaHSO4 + H2O
Break them up into ions
2Na+ + 2OH- + 2H+ + SO42- --> 2Na+ + 2H2O + SO42-
The OH- and the H+ react with each other (this is also the basis of Arrhenius' theory of acids)

Note: You CANNOT have an acid and a base in solution at the same time, the reaction of an acid and a base will result in the formation of a salt and water.

 

[steven holwerda]

I know that an acid + a base form water and a salt but don't you add H2O to a salt to illustrate whether it's acidic, basic or neutral?
(e.g. from the sample answer for 2008 q27b CH3COONa(aq) + H2O(l) --> H3COOH(aq) + Na+(aq) + OH–(aq) )

 

[someth1ng]

I know that an acid + a base form water and a salt but don't you add H2O to a salt to illustrate whether it's acidic, basic or neutral?
(e.g. from the sample answer for 2008 q27b CH3COONa(aq) + H2O(l) --> H3COOH(aq) + Na+(aq) + OH–(aq) )

Yes, you add it to see if it's acidic/basic but they DO NOT become two different acids, your equations were incorrect.

That sample answer just shows the CH3COO- ion accepting a proton making it a base and hence, making that solution basic.