3.2.15 electrolysis of aqueous solutions (HL)

chem whatarethemechanismsofchemicalchange

Reactivity 3.2.15 - during electrolysis of aqueous solutions, competing reactions can occur at the anode and cathode, including the oxidation and reduction of water

• the electrolysis of molten salts is different to the electrolysis of aqueous solutions as it is possible for multiple reactions to occur at the anode and cathode

• with , sodium metal cannot be produced at the cathode as it would immediately react with water

the reaction at the cathode can be understood in terms of a sequence of two hypothetical steps

thus, water is preferentially reduced to hydrogen

generally, for a solute , it is possible for there to be more than one redox reaction possible at each electrode:

• anode: either or can be oxidised
• cathode: either or can be reduced

selective discharge is the discharge of an ion at the electrode

competing reactions during electrolysis of aqueous solutions

flowchart

A[1. identify all the ions present in the electrolyte and determine<br> which will migrate to which electrode] --> B(2. where there is more than one possible reaction at each<br> electrode, write the half-equation for the reaction at each<br> electrode as in the data booklet) --> C[3. the half reaction with the higher electrode potential is the<br>oxidation reaction at the cathode.] --> D[4. the half-reaction with the lower electrode potential is the<br>reduction reaction at the anode.] --> E[5. to deduce the overall equation, balance the electrons lost and<br>gained at the anode and the cathode, then add the two half-<br>equations to write the equation for the net reaction] --> F[6. consider what changes would be observed in the cell as a result<br>of the redox reactions. COBALT? pH?]

electrolysis of water

since pure water is not a good conductor of electricity, some sulfuric acid or sodium hydroxide is added.

electrolysis of

overall balanced equation for brine:

the preferential discharge of occurs at high concentration. values depend on the concentration.

electrolysis of with carbon (graphite) or other inert electrodes

graphite electrodes:

used to precipitate

copper electrodes:

no change in intensity of the blue colour of the solution as ions are both formed and removed from the solution simultaneously, so the concentration remains constant

this process can be used to purify copper

• impure copper anode
• small pure copper cathode
• the anode erodes as copper ions are formed and transferred to the cathode
• only copper atoms are formed at the cathode
• the impurities leave the anode as the copper ions are formed and fall to the bottom of the cell