Question

In which of the following equations does sulfuric acid act as an oxidizing agent? In which does it act as an acid? Explain.

$$\begin{gathered} \mathbf{\left(}\mathbf{a}\mathbf{\right)}\mathbf{4}{\mathit{H}}^{\mathbf{+}}\mathbf{\left(}\mathbf{a}\mathbf{q}\mathbf{\right)}\mathbf{+}\mathit{S}{\mathit{O}}_{\mathbf{4}}^{\mathbf{2}\mathbf{-}}\mathbf{\left(}\mathbf{a}\mathbf{q}\mathbf{\right)}\mathbf{+}\mathbf{2}\mathit{N}\mathit{a}\mathit{I}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\mathbf{\to }\mathbf{2}\mathit{N}{\mathit{a}}^{\mathbf{+}}\mathbf{\left(}\mathbf{a}\mathbf{q}\mathbf{\right)}\mathbf{+}{\mathit{I}}_{\mathbf{2}}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\mathbf{+}\mathit{S}{\mathit{O}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}\mathbf{2}{\mathit{H}}_{\mathbf{2}}\mathit{O}\mathbf{\left(}\mathbf{l}\mathbf{\right)} \\ \mathbf{\left(}\mathbf{b}\mathbf{\right)}\mathit{B}\mathit{a}{\mathit{F}}_{\mathbf{2}}\mathbf{\left(}\mathbf{s}\mathbf{\right)}\mathbf{+}\mathbf{2}{\mathit{H}}^{\mathbf{+}}\mathbf{\left(}\mathbf{a}\mathbf{q}\mathbf{\right)}\mathbf{+}\mathit{S}{\mathit{O}}_{\mathbf{4}}^{\mathbf{2}\mathbf{-}}\mathbf{\left(}\mathbf{a}\mathbf{q}\mathbf{\right)}\mathbf{\to }\mathbf{2}\mathit{H}\mathit{F}\mathbf{\left(}\mathbf{a}\mathbf{q}\mathbf{\right)}\mathbf{+}\mathit{B}\mathit{a}\mathit{S}{\mathit{O}}_{\mathbf{4}}\mathbf{\left(}\mathbf{s}\mathbf{\right)} \end{gathered}$$

Short answer

Answer: You need to identify in which reaction sulfuric acid acts as an oxidizing agent and in which reaction it acts as an acid.

Step-by-step solution

Answer of subpart (a)

In this reaction sulfuric acid acts as an oxidizing agent.

Oxidation state of sulfur in H2SO4 

Oxidation state of Hydrogen attached to non-metal is +1.

Oxidation state of Oxygen attached to non-metal is -2.

Let, oxidation state of sulfur in H₂SO₄ is x.

Now you can write,

$$\begin{gathered} \mathbf{2}\left(+1\right)\mathbf{+}\mathbf{x}\mathbf{+}\mathbf{4}\left(-2\right)\mathbf{=}\mathbf{0} \\ \mathbf{x}\mathbf{=}\mathbf{+}\mathbf{6} \end{gathered}$$

Oxidation state of sulfur in SO2 

Oxidation state of Oxygen attached to non-metal is -2.

Let, oxidation state of sulfur in SO₂ is y.

No you can write,

$$\begin{gathered} \mathbf{y}\mathbf{+}\mathbf{2}\left(-2\right)\mathbf{=}\mathbf{0} \\ \mathbf{y}\mathbf{=}\mathbf{+}\mathbf{4} \end{gathered}$$

Conclusion

In the given reaction

$\mathbf{4}{\mathbf{H}}^{\mathbf{+}}\left(\mathrm{aq}\right)\mathbf{+}{\mathbf{SO}}_{\mathbf{4}}^{\mathbf{2}\mathbf{-}}\left(\mathrm{aq}\right)\mathbf{+}\mathbf{2}\mathbf{NaI}\left(s\right)\mathbf{\to }\mathbf{2}{\mathbf{Na}}^{\mathbf{+}}\left(\mathrm{aq}\right)\mathbf{+}{\mathbf{I}}_{\mathbf{2}}\left(s\right)\mathbf{+}{\mathbf{SO}}_{\mathbf{2}}\left(g\right)\mathbf{+}\mathbf{2}{\mathbf{H}}_{\mathbf{2}}\mathbf{O}\left(l\right)$

The oxidation state of sulfur in H₂SO₄ is +6 which reduces to +4 in SO₂. Therefore, H₂SO₄ undergoes reduction in this given reaction. Hence, it acts as an oxidizing agent.

Answer of subpart (b)

In this reaction sulfuric acid acts as an acid.

Oxidation state of sulfur in H2SO4 

Oxidation state of Hydrogen attached to non-metal is +1.

Oxidation state of Oxygen attached to non-metal is -2.

Let, oxidation state of sulfur in H₂SO₄ is x.

No you can write,

$$\begin{gathered} \mathbf{2}\left(+1\right)\mathbf{+}\mathbf{x}\mathbf{+}\mathbf{4}\left(-2\right)\mathbf{=}\mathbf{0} \\ \mathbf{x}\mathbf{=}\mathbf{+}\mathbf{6} \end{gathered}$$

Oxidation state of sulfur in BaSO4 

Oxidation state of Barium is +2.

Oxidation state of Oxygen attached to non-metal is -2.

Let, oxidation state of sulfur in BaSO₄ is y.

No you can write,

$$\begin{gathered} \mathbf{+}\mathbf{2}\mathbf{+}\mathit{y}\mathbf{+}\mathbf{4}\left(-2\right)\mathbf{=}\mathbf{0} \\ \mathit{y}\mathbf{=}\mathbf{+}\mathbf{6} \end{gathered}$$

Conclusion

In the given reaction

$\mathit{B}\mathit{a}{\mathit{F}}_{\mathbf{2}}\left(s\right)\mathbf{+}\mathbf{2}{\mathit{H}}^{\mathbf{+}}\left(aq\right)\mathbf{+}\mathit{S}{\mathit{O}}_{\mathbf{4}}^{\mathbf{2}\mathbf{-}}\left(aq\right)\mathbf{\to }\mathbf{2}\mathit{H}\mathit{F}\left(aq\right)\mathbf{+}\mathit{B}\mathit{a}\mathit{S}{\mathit{O}}_{\mathbf{4}}\left(s\right)$

The oxidation state of sulfur in H₂SO₄ and in BaSO₄ is +6. Therefore, it is not a redox reaction. Hence, it acts as an acid.