Electrolysis

• Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.

Simple electrolytic cell

• A simple electrolyic cell consist of:
  • an anode as a positive (+) electrode
  • a cathode as a negative (-) electrode
  • the electrolyte as the molten or aqueous solution that undergoes electrolysis.

Molten lead(II) bromide

• When the power is turned on and electricity is passed through the molten substance, the positive metal lead ions will move to the cathode.
  • Here, the lead ions will gain electrons to become lead atoms, and lead will form.
  • The lead will be deposited on the surface of the electrode.
• The negative bromide ions will move to the anode
  • Here, each bromide ion will lose one electron, forming bromine ions
  • Two of the bromine atoms formed will combine to form bromine molecules, and you will see bubbles of bromine gas given off
• Equation: $PbBr_{2}$ -> $Pb_{(l)}$ + $Br_{2 (g)}$

Concentrated aqueous sodium chloride

• The products formed in the electrolysis of $NaCl_{(aq)}$ are:
  • Hydrogen gas at the cathode
  • Chlorine gas at the anode
  • Leaves behind sodium hydroxide solution
• The observations at:
  • the cathode: efferversence/bubbles of a colorless gas
  • the anode: bubbles of a pale yellow-green gas formed by chlorine
• NOTE: Metals or hydrogen are formed at the cathode. Non-metals (except hydrogen) are formed at the anode.

Molten binary compounds

• A binary compound consists of only two elemets. When molten, the ions are free to move and are discharged at the electrodes.
• RULE In a molten binary compound, there are only two ions present, so predicting the products is straightforward.
• In order to predict the products, you do these steps:
  1. Identify compound and its ions A binary compound splits into a cation and an anion.
  2. Apply electrode rules

Electroplating

• Metal objects are electroplated to improve their appearance and resistance to corrosion.
• Electroplating is using electricity to coat one metal with another.
• To electroplate an object with metal x, the set-up is:
  • the cathode: object that is undergoing electroplating
  • the anode: metal x
  • electrolyte: a solution of a soluble compound of x.

Transfer of charge during electrolysis

Movement of electrons in the external circuit

• The power supply drives electrons through the external circuit
• Electrons flow from the negative terminal of the battery to the cathode
• At the other end, electrons flow from the anode to the positive terminal of the battery
• So in the external circuit, electrons travel from anode to cathode through the wire.

LOSS or GAIN of electrons at the electrodes

• NOTE: Remember this mnemonic: OIL RIG, Oxidation is Loss, Reduction is Gain.

Movement of ions in the electrolyte

• Ions in the molten/dissolved electrolyte carry the charge through the liquid
• Cations move toward the cathode
• Anions move toward the anode
• This ion movement completes the circuit through the electrolyte. Notice how opposite charges attract?

When electrodes are NOT inert

• Inert means chemically inactive. This example uses a solution of copper(II) sulfate, which has blue copper ions, sulfate ions, and hydrogen and hydroxide ions.
• Using carbon/graphite electrodes (inert)
  • At the cathode: Copper ions are discharged. The copper coats the electrode. $$2Cu^{2+}(aq) + 4e^{-} \rightarrow 2Cu (s)$$
  • At the anode: Oxygen bubbles off. $$4OH^{-}(aq) \rightarrow 2H_{2}O(l) + O_{2}(g) + 4e^{-}$$
  • So copper and oxygen are produced. The blue color of the solution fades because the concentration of copper ions in it decreases as the ions are discharged.
• Using copper electrodes (not inert)
  • At the cathode: Copper is formed and coats the electrode. $$Cu^{2+}(aq) + 2e^{-} \rightarrow Cu(s)$$
  • At the anode: The anode dissolves, giving copper ions in solution. $$Cu(s) \rightarrow Cu^{2+}(aq) + 2e^{-}$$
  • As the anode dissolves, the copper ions move to the cathode to form copper. So, copper moves from the anode to the cathode. The solution stays blue because the concentration of copper stays the same.

Rules for electrolysis of a solution

• At the cathode, either a METAL or HYDROGEN forms.
  • The more reactive an element is, the longer it stays as an ion. Therefore, if a metal is more reactive than hydrogen, its ions stays in solution and hydrogen bubbles off.
  • If a metal is less reactive than hydrogen, the metal forms.
• At the anode, a NON-METAL OTHER THAN HYDROGEN forms.
  • If it is a concentrated solution of a halide (a compound containing Cl-, Br-, or I- ions), then chlorine, bromine, or iodine form.
  • If the halide solution is dilute, or there is no halide, oxygen forms.