Question

All the reactions are correct , so what could be the appropriate answer then?

In which reaction is dilute sulfuric acid not behaving as an acid?
a) H2SO4 + 2NaOH = Na2SO4 +2H2O
b) H2SO4 + BaCl2 = BaSO4 + 2HCl
c) H2SO4 + CuO = CuSO4 + H2O
d) H2SO4 + Mg = MgSO4 + H2
* Pardon any tying error in the equations!

Answer 1

Is it not `B`?

Explanation:

The net ionic equation is simply..

`Ba^(2+) + SO_4^(2-) rarr BaSO_4(s)darr`...

The hydronium ion REMAINS in solution, i.e. `"2 equiv"` `H_3O^+`...

Just in response to a request for clarification...I will try to make what I have written less confusing... We often speak of acidity in terms of the characteristic cation of the solvent...for water we often write...

`H_2O(l)rightleftharpoonsH^+ + HO^-`...and `K_w=[HO^-][H^+]=10^-14`.

Here, `H^+` is CONCEIVED to be the acid principle...and `HO^-` is the base principle, the characteristic anion... An acid dissolved in water is INCREASES the concentration of the anion, i.e. for gaseous `HCl` we write...

`HCl(g) stackrel(H_2O)rarrH_3O^+ + Cl^-`

A saturated solution solution of `HCl(aq)` is approx `10.6*mol*L^-1` with respect to `H_3O^+`...

And of course the `H^+` is a label of convenience..in aqueous solution what we term `H^+` is possibly `H_7O_3^+` or `H_9O_4^+`...i.e. a CLUSTER of `2-3` or more water molecules WITH AN EXTRA `H^+` associated with the cluster. Certainly the `H^+` can move rapidly from cluster to cluster, tunnel if you like, given the vast aqueous mobility of `H^+` and `HO^-`...

And so we represent the acidium ion as `H^+` or even `H_3O^+`...this is a CONCEPTION rather than the reality...but it is useful to write chemical equations invoking them as actual species... I have written much the same thing here and here.

Anyway, I know that `"Inorganic Chemistry, Principles of"` `"Structure and Reactivity, J. E. Huheey"` is available in most university libraries (it may even be available for download but you might have to pledge your first-born to get it), and this has a good chapter on the concept of acidity...good luck...

Answer 2

The correct answer is option b).

Explanation:

A substance behaves as an acid if it provides `"H"^"+"` ions that get used up during a reaction.

It is easiest to see what is happening if we use net ionic equations.

a)

Molecular equation:
`"H"_2"SO"_4"(aq)" + "2NaOH(aq)" → "Na"_2"SO"_4"(aq)" + 2"H"_2"O(l)"`

Ionic equation:
`2"H"^"+""(aq)" + color(red)(cancel(color(black)("SO"_4^"2-""(aq)"))) + color(red)(cancel(color(black)("2Na"^"+""(aq)"))) +2"OH"^"-""(aq)" → color(red)(cancel(color(black)("2Na"^"+""(aq)"))) + color(red)(cancel(color(black)("SO"_4^"2-""(aq)"))) + 2"H"_2"O(l)"`

Net ionic equation:
`color(red)(cancel(color(black)(2)))"H"^"+""(aq)" +color(red)(cancel(color(black)(2)))"OH"^"-""(aq)" → color(red)(cancel(color(black)(2)))"H"_2"O(l)"`

`"H"^"+""(aq)" +"OH"^"-""(aq)" → "H"_2"O(l)"`

`"H"^"+"` is getting used up in this reaction, so the sulfuric acid is behaving as an acid.

b)

Molecular equation:
`"H"_2"SO"_4"(aq)" + "BaCl"_2"(aq)" → "BaSO"_4"(s)" + 2"HCl""(aq)"`

Ionic equation:
`color(red)(cancel(color(black)(2"H"^"+""(aq)"))) + "SO"_4^"2-""(aq)" + "Ba"^"2+""(aq)" + color(red)(cancel(color(black)("2Cl"^"-""(aq)"))) → "BaSO"_4"(s)" + color(red)(cancel(color(black)(2"H"^"+""(aq)"))) + color(red)(cancel(color(black)("2Cl"^"-""(aq)")))`

Net ionic equation:
`"SO"_4^"2-""(aq)" + "Ba"^"2+""(aq)" → "BaSO"_4"(s)"`

`"H"^"+"` is a spectator ion.

It is not getting used up in this reaction, so the sulfuric acid is not behaving as an acid.

c)

Molecular equation:
`"H"_2"SO"_4"(aq)" + "CuO(s)" → "CuSO"_4"(aq)" + "H"_2"O(l)"`

Ionic equation:
`2"H"^"+""(aq)" + color(red)(cancel(color(black)("SO"_4^"2-""(aq)"))) + "CuO(s)" → "Cu"^"2+""(aq)" + color(red)(cancel(color(black)("SO"_4^"2-""(aq)"))) + "H"_2"O(l)"`

Net ionic equation:
`2"H"^"+""(aq)" + "CuO(s)" → "Cu"^"2+""(aq)" + "H"_2"O(l)"`

`"H"^"+"` is getting used up in this reaction, so the sulfuric acid is behaving as an acid.

d)

Molecular equation:
`"H"_2"SO"_4"(aq)" + "Mg(s)" → "MgSO"_4"(aq)" + "H"_2"(g)"`

Ionic equation:
`2"H"^"+""(aq)" + color(red)(cancel(color(black)("SO"_4^"2-""(aq)"))) + "Mg(s)" → "Mg"^"2+""(aq)" + color(red)(cancel(color(black)("SO"_4^"2-""(aq)"))) + "H"_2"(g)"`

Net ionic equation:
`2"H"^"+""(aq)" + "Mg(s)" → "Mg"^"2+""(aq)" + "H"_2"(g)"`

`"H"^"+"` is getting used up in this reaction, so the sulfuric acid is behaving as an acid.