# What are the main products (if any) of the reaction of "Fe"^(3+)(aq) with "SO"_2(g) in water in acidic conditions?

May 23, 2017

sf(Fe^(2+) and $\textsf{S {O}_{4}^{2 -}}$

#### Explanation:

Consider the thermodynamic data:

$\textsf{\text{ "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "" "E^@" } \left(V\right)}$

sf(Fe^(2+)" "" " +" "" "2e" "rightleftharpoons" "" "" "Fe" "" "" "" "-0.44)

$\textsf{S {O}_{4}^{2 -} + 4 {H}^{+} + 2 e \text{ "" "rightleftharpoons" "SO_2+2H_2O" } \textcolor{w h i t e}{. .} + 0.17}$

$\textsf{F {e}^{3 +} \text{ "+" "" "e" "" "color(white)(.)rightleftharpoons" "" "" "Fe^(2+)" "" "" } + 0.77}$

The most powerful oxidisers are to the bottom left. The most powerful reducers are at the top right when the $\textsf{{E}^{\circ}}$ values are listed like this.

We would expect sf(Fe^(3+) to oxidise $\textsf{S {O}_{2}}$ as it is the more powerful oxidiser so will take in electrons which are given out by $\textsf{S {O}_{2}}$.

The two half equations are:

$\textsf{S {O}_{2} + 2 {H}_{2} O \rightarrow S {O}_{4}^{2 -} + 4 {H}^{+} + 2 e \text{ "" "" "" } \textcolor{red}{\left(1\right)}}$

$\textsf{F {e}^{3 +} + e \rightarrow F {e}^{2 +} \text{ "" "" "" "" "" "" "" "" "" } \textcolor{w h i t e}{.} \textcolor{red}{\left(2\right)}}$

To get the electrons to balance you $\times$ $\textsf{\textcolor{red}{\left(2\right)}}$ by 2 then add:

$\textsf{2 F {e}^{3 +} + S {O}_{2} + 2 {H}_{2} O \rightarrow 2 F {e}^{2 +} + S {O}_{4}^{2 -} + 4 {H}^{+}}$

The data indicates that $\textsf{S {O}_{2}}$ is not a powerful enough reducer to reduce sf(Fe^(2+) down to $\textsf{F e}$.

If you were to bubble $\textsf{S {O}_{2}}$ gas through an acidified solution of iron(III) chloride you would observe the brown/yellow colour change to green showing the presence of iron(II).