Electrolytic Reactions and Applications
Competing Reactions in Electrolysis
Definition
Electrolysis
Electrolysis uses electrical energy to drive non-spontaneous redox reactions.
- In aqueous solutions, both water and dissolved ions can participate in these reactions, creating competition at the electrodes.
- The products depend on the standard electrode potentials ($ E^\Theta $) of the reacting species.
Reduction at the Cathode
- At the cathode, reduction occurs as electrons are supplied by the external circuit.
- The potential reductions in an aqueous solution include:
- Reduction of water:$$2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-(aq) \quad E^\Theta = -0.83 \, \text{V}$$
- Reduction of the cation from the dissolved salt, such as $ \text{Na}^+ $ or $ \text{Cu}^{2+} $.
The species with the less negative (or more positive) reduction potential is preferentially reducedExample
- In $ \text{NaCl(aq)} $, the reduction of $ \text{Na}^+ $ ($ E^\Theta = -2.71 \, \text{V} $) is less favorable than the reduction of water.
- Thus, hydrogen gas is formed at the cathode.
- In $ \text{CuSO}_4(aq) $, the reduction of $ \text{Cu}^{2+} $ ($ E^\Theta = +0.34 \, \text{V} $) is more favorable than the reduction of water.
- As a result, copper metal is deposited at the cathode.
Compare the standard electrode potentials to predict which species will be reduced at the cathode.
Oxidation at the Anode
- At the anode, oxidation occurs as electrons are removed.
- The possible oxidations in an aqueous solution are:
- Oxidation of water:$$2H_2O(l) \rightarrow O_2(g) + 4H^+(aq) + 4e^- \quad E^\Theta = -1.23 \, \text{V}$$
- Oxidation of the anion from the dissolved salt, such as $ \text{Cl}^- $ or $ \text{SO}_4^{2-} $.
The species with the less negative (or more positive) oxidation potential is preferentially oxidized.Example
- In $ \text{NaCl(aq)} $, the oxidation of $ \text{Cl}^- $ ($ E^\Theta = -1.36 \, \text{V} $) is slightly less favorable than the oxidation of water.
- However, at high concentrations of $ \text{Cl}^- $, chlorine gas ($ Cl_2 $) is preferentially produced at the anode.
- In $ \text{CuSO}_4(aq) $, sulfate ions ($ \text{SO}_4^{2-} $) cannot be oxidized further, so water is oxidized to oxygen gas ($ O_2 $) at the anode.
- Many students assume that the oxidation of water always occurs at the anode.
- However, high concentrations of certain ions, like $ \text{Cl}^- $, can shift the reaction to produce other products.
Electrolysis of $ \text{NaCl(aq)} $
- At the cathode:$$2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-(aq)$$Hydrogen gas is formed.
- At the anode:$$2Cl^-(aq) \rightarrow Cl_2(g) + 2e^-$$
Chlorine gas is formed. - Overall reaction:$$2NaCl(aq) + 2H_2O(l) \rightarrow H_2(g) + Cl_2(g) + 2NaOH(aq)$$
- Sodium hydroxide remains in the solution.
