ELECTROCHEMISTRY SERIES PART 2

Electrolysis

Electrolysis is the passage of an electric current through an electrolyte causing it to decompose.

  • Electrode – The point at which current enters and leaves the electrolyte
  • Anode- positive electrode.
  • Cathode – negative electrode

electrolysis

  • Cations (+) move towards the cathode.  Please note that cations are positively-charged ions and are attached to the cathode (negative electrode). 
  • Anions (-) move towards the anode.  Please note that anions are negatively-charged ions and are attached to the anode (positive electrode). 
  •  Electrodes may be active or inert.

oducts-20of-20electrolysis

During electrolysis, ions lose their charge (discharge).

At the anode:    Xn- -ne   →  X       (Oxidation (loss of electrons))

e.g. 2Cl-    –  2e   →   Cl2

At the cathode: Mn+    +   ne   →   M   (Reduction (gain of electrons))

e.g. Cu2+   +  2e  →   Cu

When ions are discharged they become neutral atoms.

In the electrolysis of MgO, show what happens at the anode and the cathode.

During electrolysis;

  1. Metal deposit at the cathode
  2. Hydrogen evolve at the cathode
  3. Metals break away from the anode
  4. Non – metals discharged at the anode

How do we know which ion is discharged?

We use the electrochemical/ reactivity series

ELECTROCHEMICAL SERIES

Example: Dilute sulphuric acid(H2SO4)

Ions

From water        H+               OH-

From acid           H+                So42-

At cathode: 2H+   +     2e    →   H2

At anode:  4OH-   –  4e  →  2H2O  +  O2

Electrolysis of dilute sodium hydroxide (NaOH)

Ions present – from water:  H+           OH-

–  from NaOH:  Na+         OH-

Cathode:  2H+  +   →  H2

Anode:    4OH-  –  4e    →   2H2O  +  O2

If the electrodes are active they will react.

e.g. electrolysis of  copper sulphate using copper electrodes

H+                       OH-

Cu2+                    SO42-

At cathode:  Cu2+   +   2e  →   Cu  [Cu2+ leaves the solution and is deposited at the cathode.]

At anode:  Cu   –   2e   →  Cu2+  [ copper anode breaks down to Cu2+ and enters the solution.

This is used in copper refining and electroplating

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