In the electrolysis of an aqueous solution of Na2 SO4, what reactions occur at the anode and the

In the electrolysis of an aqueous solution of Na2 SO4, what...

In the electrolysis of an aqueous solution of $\mathrm{Na}_{2} \mathrm{SO}_{4},$ what reactions occur at the anode and the cathode (assuming standard conditions)? $$\begin{array}{ll} {\text{}} & \quad{ \mathscr{E}^{\circ} } \\ \hline {\mathrm{S}_{2} \mathrm{O}_{8}^{2-}+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{SO}_{4}^{2-}} & {2.01 \mathrm{V}} \\ {\mathrm{O}_{2}+4 \mathrm{H}^{+}+4 \mathrm{e}^{-} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}} & {1.23 \mathrm{V}} \\ {2 \mathrm{H}_{2} \mathrm{O}+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2}+2 \mathrm{OH}^{-}} & {-0.83 \mathrm{V}} \\ {\mathrm{Na}^{+}+\mathrm{e}^{-} \longrightarrow \mathrm{Na}} & {-2.71 \mathrm{V}}\end{array}$$

In the electrolysis of an aqueous solution of $\mathrm{Na}_{2} \mathrm{SO}_{4},$ what reactions occur at the anode and the cathode (assuming standard conditions)?
$$\begin{array}{ll} {\text{}} & \quad{ \mathscr{E}^{\circ} } \\ \hline {\mathrm{S}_{2} \mathrm{O}_{8}^{2-}+2 \mathrm{e}^{-} \longrightarrow 2 \mathrm{SO}_{4}^{2-}} & {2.01 \mathrm{V}} \\ {\mathrm{O}_{2}+4 \mathrm{H}^{+}+4 \mathrm{e}^{-} \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}} & {1.23 \mathrm{V}} \\ {2 \mathrm{H}_{2} \mathrm{O}+2 \mathrm{e}^{-} \longrightarrow \mathrm{H}_{2}+2 \mathrm{OH}^{-}} & {-0.83 \mathrm{V}} \\ {\mathrm{Na}^{+}+\mathrm{e}^{-} \longrightarrow \mathrm{Na}} & {-2.71 \mathrm{V}}\end{array}$$

Key Concepts

Electrolysis

Electrolysis is the process of driving a non-spontaneous chemical reaction using an external electric current. In such processes, oxidation reactions occur at the anode and reduction reactions occur at the cathode. Understanding electrolysis involves analyzing which species in the solution are most easily oxidized or reduced based on their intrinsic properties and electrode potentials.

Standard Electrode Potentials

Standard electrode potentials provide a benchmark for comparing the relative tendencies of chemical species to gain or lose electrons under standard conditions. By comparing these potentials, one can predict which reactions will preferentially occur at the electrodes during electrolysis, as species with more favorable (less negative or more positive) potentials will be selected over less favorable ones.

Selective Electrode Reactions in Aqueous Solutions

In aqueous solutions, water molecules can participate in electrode reactions alongside the solute ions. The inherent properties of water mean that, under standard conditions, its oxidation or reduction often occurs preferentially over the direct oxidation or reduction of ions. Thus, even though other ionic species are present, the ease of water splitting can dictate the products at the electrodes.

Water Electrolysis

Water electrolysis involves the decomposition of water into oxygen and hydrogen gases. At the cathode, water is reduced to form hydrogen gas and hydroxide ions, while at the anode, water is oxidized to produce oxygen gas and protons. This process is central to understanding why, in many aqueous systems, electrolysis produces hydrogen and oxygen rather than altering the dissolved solute ions.

Competing Reactions and Reaction Selectivity

In electrolysis, competing reactions can occur based on the available species and their standard electrode potentials. In an aqueous sodium sulfate solution, the potentials indicate that reducing water to hydrogen is more favorable at the cathode than reducing sodium ions, and oxidizing water to oxygen is more favorable at the anode compared to the oxidation of sulfate to peroxodisulfate. This competition between possible electrode reactions is key to predicting and controlling the outcomes in electrolytic processes.

Transcript

00:01 We are given this list of standard reduction potentials for these four half reactions, and we are told that in the electrolysis of an na2 -s -o -4 solution, that we want to determine which of these half -reactions occurs in the anode and which occurs in the cathode.

00:15 We know that oxidation will occur in the anode, and that reduction will occur in the cathode.

00:21 And so since we have a solution of aqueous na -2 -s -o -4, that must mean that we have aqueous n a plus ions, aqueous s .o .4 .2 minus ions, and also water.

00:39 And now we need to use these standard reduction potentials for the half -reactions involving these species in order to determine which species will be reduced and which one will be oxidized, so we can determine which occurs in the anode and the cathode.

00:54 So starting with n -a -plus, we know that we can only reduce n -a -plus.

00:57 We cannot increase a charge of n .a.

01:00 Plus.

01:00 And so the only half reaction that we can have out of the four given half reactions for n .a.

01:08 Plus is a reduction by adding one electron.

01:11 And we see that out of the four it has the smallest reduction potential.

01:15 And we also know that for water, this can be either reduced or oxidized.

01:23 If we take this reaction and reverse it, then we can see that we can oxidize.

01:28 Water.

01:30 And for this reaction, this represents the reduction of water.

01:35 And for s .o4 -2 -minus, we know that we can only oxidize that.

01:39 We cannot increase the charge of that.