Electrolysis Definition

The Role of Direct Current

A reliable direct current supply, such as a battery or power pack, is mandatory for electrolysis. Direct current provides a continuous, one-way flow of electrons through the external circuit. This maintains a fixed polarity at the electrodes: the anode remains permanently positive, and the cathode remains permanently negative.

If an alternating current (a.c.) were used, the polarity of the electrodes would rapidly swap back and forth many times a second. Because of this, the ions in the electrolyte would constantly change direction. They would never successfully reach an electrode to be discharged and separated, meaning decomposition would not occur.

The Step-by-Step Process

To fully describe how electrolysis breaks down a substance, you must follow the correct sequential steps:

• First, the ionic compound is melted or dissolved in water. This ensures the ions are free to move, unlike in a solid lattice.
• Then, a d.c. voltage is applied across two unreactive inert electrodes (like graphite or platinum), and electrons flow from the anode to the cathode in the external wires.
• Next, within the electrolyte, positively charged ions (cations) migrate toward the negative cathode, while negatively charged ions (anions) migrate toward the positive anode.
• Finally, as the ions reach their respective electrodes, they undergo a discharge process. Cations gain electrons (reduction) and anions lose electrons (oxidation), turning back into neutral elements.

Worked Example

A student uses a 6 V direct current battery and inert graphite electrodes to perform electrolysis on a sample of molten lead(II) bromide ($PbBr_2$). Describe the step-by-step decomposition of this electrolyte and provide the half-equations for each electrode.

Step 1: Identify the mobile ions present in the electrolyte.

• The molten compound contains lead cations ($Pb^{2+}$) and bromide anions ($Br^-$). Because it is molten, the ions are free to move.

Step 2: Describe the migration of the ions.

• The $Pb^{2+}$ cations migrate toward the negative cathode.
• The $Br^-$ anions migrate toward the positive anode.

Step 3: Describe the discharge at the cathode (reduction).

• At the cathode, the lead ions each gain two electrons to form neutral liquid lead metal.

$$Pb^{2+} + 2e^- \rightarrow Pb(l)$$

Step 4: Describe the discharge at the anode (oxidation).

• At the anode, pairs of bromide ions each lose an electron to form neutral bromine gas.

$$2Br^- \rightarrow Br_2(g) + 2e^-$$

Step 5: State the overall decomposition equation.

• Electrical energy has successfully decomposed the electrolyte into its elements.

$$PbBr_2(l) \rightarrow Pb(l) + Br_2(g)$$