Detailed Notes on Electrolysis and Related Processes

ELECTROLYSIS

Definition of Electrolysis

  • Electrolysis is the process of decomposing a substance using electrical energy.
  • For electrolysis to occur, the substance must contain ions, which typically means it must be in a solution or molten state.
  • Substances containing ions that can conduct electricity are referred to as electrolytes.
  • The electrical conductivity is caused by the mobility of ions within the solution or liquid phase (molten ionic compounds).

Electrolytic Cell

  • Electrolysis occurs in an electrolytic cell which comprises two electrodes:
    1. Cathode:
    • Definition: A negatively charged (-) electrode, which has an excess of electrons.
    • Function: Attracts cations (positively charged ions) leading to reduction (Rig stands for Reduction is Gain) taking place at this electrode.
    1. Anode:
    • Definition: A positively charged (+) electrode, which is electron deficient.
    • Function: Attracts anions (negatively charged ions), leading to oxidation (Oil stands for Oxidation is Loss) taking place here.
  • Electrodes can be composed of metals or inert materials like carbon.
  • The electrodes are connected to a power supply that enforces a directed flow of electrons.
  • Electrons move from the anode to the cathode through the power supply.

Processes at the Electrodes

  • At the Cathode:

    • Cations from the electrolyte gain electrons and are converted into atoms.
    • This process is defined as reduction.

  • At the Anode:

    • Anions from the electrolyte lose electrons and become atoms.
    • This process is identified as oxidation.

Factors Influencing Preferential Discharge of Ions at the Electrodes

  1. Position of the Ion in the Electrochemical Series:

    • Different ions have varying ease of discharge. The following hierarchy is important:
      • Na+ > Mg2+ > SO4^2- > Al3+ > NO3- > Zn2+ > OH- > Fe2+ > Cl- > Pb2+ > Br- > H+ > I- > Cu2+ > Hg+ > Ag+ > Au+
    • Example for Cathode:
      • Hydrogen ions (H+) are discharged preferentially over more reactive metals.
      • E.g. In Sodium Chloride solution (Na+, Cl-, H+, OH- from water):
        2H^+ + 2e^-
        ightarrow H_2(g)
      • E.g. In Copper (II) sulphate solution (Cu2+, SO4^2-, H+, OH- ions):
        Cu^{2+} + 2e^-
        ightarrow Cu(s)

  2. Concentration of the Ion:

    • The increase in concentration of ions generally promotes their discharge.
    • Example: In concentrated hydrochloric acid, Cl- ions will be preferentially discharged, whereas dilute acid conditions would favor OH- ion discharge.

  3. Nature of the Electrode:

    • The choice of electrode impacts which ions are discharged:
      • Carbon Electrodes:
      • Carbon is inert, not participating in chemical changes, thus allowing Cu2+ to discharge at the cathode and OH- at the anode.
      • Copper Electrodes:
      • Copper participates; copper at the anode dissolves as it loses electrons replacement for the discharged Cu2+ at the cathode. This results in unchanged concentration of Copper (II) sulphate and a decrease in anode mass while increasing the cathode mass.
      • Reactions:
        • At Anode: Cu(s) - 2e^-
          ightarrow Cu^{2+}
        • At Cathode: Cu^{2+} + 2e^-
          ightarrow Cu(s)

  4. Purification of Copper:

    • The reaction is applicable in copper purification where the anode consists of impure copper and pure copper is deposited on the cathode.

Electroplating of Metals Using Copper

  • Electroplating Definition:
    • The electroplating process involves coating metals with other metals utilizing electrical current.
  • Purpose of Electroplating:
    • To prevent corrosion and enhance the appearance of metals.
  • Metals Used for Coating:
    • Common metals used for electroplating include Copper, Silver, Nickel, Tin, Chromium, and Gold.
Electroplating Process:
  1. The cathode is the metal to be coated.
  2. The anode is made of the metal that will coat another metal.
  3. The electrolyte must contain ions of the coating metal.
  • Example: Electroplating of Brass with Copper:
    • During electroplating, copper ions in the electrolyte are discharged at the cathode, resulting in copper metal formation that coats the brass surface:
    • Reaction: Cu^{2+} + 2e^-
      ightarrow Cu(s)
    • Copper atoms at the anode lose electrons to form Cu2+, replacing those discharged at the cathode:
    • Reaction: Cu(s) - 2e^-
      ightarrow Cu^{2+}
    • The anode thus dissolves while the electrolyte concentration remains unchanged.

Calculations in Electrolysis

  • Current (I):

    • Defined as the flow of charge or electrons. Measured in amperes (A).
    • A current of 1 ampere flowing for 1 second carries a charge of 1 Coulomb.
    • Charge in Coulombs formula:
      Q=IimestQ = I imes t
    • Where:
    • QQ is charge in Coulombs (C)
    • II is current in amperes (A)
    • tt is time in seconds (s)
    • Rearranging gives: I=I = \frac{Q}{t}

  • Faraday's Constant:

    • The charge of one mole of electrons is referred to as a Faraday.
    • It is defined as 96500 Coulombs per mole:
    • Thus:
      1 mole ext{ of electrons} = 96500 ext{ coulombs} = 1 ext{ Faraday (F)}$$