Electrolytic cells
Electrolytic cells use external electrical power to drive nonspontaneous redox reactions — the reverse of galvanic cells. See how electroplating and electrorefining work.
Quick answer
An electrolytic cell is an electrochemical cell in which external electrical energy drives a nonspontaneous redox reaction; oxidation still occurs at the anode and reduction at the cathode, but the anode is positive and the cathode is negative.
Example: In copper electrorefining, impure copper dissolves at the anode (Cu → Cu2+ + 2e−) and pure copper plates out at the cathode (Cu2+ + 2e− → Cu).
What you will learn
- Define an electrolytic cell and contrast it with a galvanic cell
- State the sign convention: anode positive, cathode negative in electrolysis
- Explain how an external power supply drives a nonspontaneous reaction
- Describe electroplating and electrorefining with half-reactions
- Compare galvanic and electrolytic cells across spontaneity, signs, and purpose
Forcing a reaction uphill
A galvanic cell harvests energy from a reaction that wants to happen. An electrolytic cell does the reverse: it spends electrical energy to force a reaction that would not happen on its own. Connect a battery or power supply across the electrodes, and the external voltage pushes electrons uphill against the spontaneous direction.
This is how we charge a rechargeable battery (forcing the discharge reaction backward), plate chrome onto a bumper, purify copper for wiring, and extract metals like aluminum and sodium that are too reactive to win their electrons back by ordinary chemical means.
Because the energy comes from outside, an electrolytic cell is an energy consumer — the electrochemical opposite of a galvanic cell's energy production.
How electrolytic cells differ from galvanic cells
The chemistry definitions never change: An Ox (oxidation at the anode) and Red Cat (reduction at the cathode) hold in every cell. What changes is who supplies the push:
- Galvanic cell: the reaction is spontaneous; the chemistry pushes electrons out of the anode, making the anode negative and the cathode positive.
- Electrolytic cell: the power supply pushes electrons; it rams electrons into the cathode (making it negative) and drags them out of the anode (making it positive).
So the signs flip, but the chemistry roles do not. The most common exam trap is labeling the anode negative in an electrolysis diagram — in electrolysis, the anode is the positive electrode.
A quick comparison:
- Spontaneity: galvanic = spontaneous (E > 0); electrolytic = nonspontaneous (E < 0), driven externally.
- Energy: galvanic converts chemical → electrical; electrolytic converts electrical → chemical.
- Anode sign: galvanic negative; electrolytic positive.
- Cathode sign: galvanic positive; electrolytic negative.
Electroplating and electrorefining
Electroplating coats an object with a thin layer of metal for appearance or corrosion protection — chrome on car parts, gold on jewelry, tin on food cans.
Setup: the object to be plated is the cathode (negative), and a bar of the plating metal is the anode (positive). Both sit in a solution of the metal's ions.
- At the anode: metal atoms are oxidized and dissolve (M → M^n+ + ne−), keeping the solution supplied with ions.
- At the cathode: metal ions are reduced and deposit as a uniform layer (M^n+ + ne− → M).
Because deposition happens ion by ion, the coating follows every contour and bonds tightly — far better than paint.
Electrorefining purifies metals the same way. In copper refining, slabs of impure copper serve as the anode and dissolve; pure copper plates onto thin starter sheets at the cathode. Impurities either stay in solution or fall to the bottom as 'anode mud' (which is collected — it often contains valuable silver and gold). The copper in electrical wiring is about 99.99% pure thanks to this process.
Products depend on what is in the cell
What forms at each electrode depends on the species present, and the easiest reduction and easiest oxidation win. Two classic cases:
- Molten NaCl: only Na+ and Cl− exist. At the cathode, Na+ is reduced to sodium metal; at the anode, Cl− is oxidized to chlorine gas. Clean and simple.
- Aqueous NaCl (brine): water is also present and competes. At the cathode, water is reduced to hydrogen gas and OH− instead of sodium metal (water is easier to reduce than Na+). At the anode, chloride is oxidized to chlorine gas. So brine electrolysis gives H2, Cl2, and NaOH — the industrial chlor-alkali process.
The lesson: always list every species that could react at each electrode and pick the one with the most favorable potential. Water's presence changes the products completely.
Common mistakes
They share the hardware idea, but the chemistry differs: galvanic cells run spontaneous reactions, while electrolytic cells force nonspontaneous ones using external power. The electrode signs flip.
The anode is defined by chemistry (oxidation), not by sign. In an electrolytic cell the external supply makes the anode positive — it pulls electrons out of it.
Electroplating deposits metal atom by atom as ions are reduced at the cathode. That is why the coating is uniform and bonds tightly to the surface.
Real-world applications
Electroplating jewelry, car parts, and electronics with metal coatings
Electrorefining impure copper to high purity for wiring
Extracting reactive metals like aluminum and sodium from their ores
Charging rechargeable batteries by reversing the discharge reaction
Splitting water into hydrogen and oxygen for fuel and industry
Frequently asked questions
If the anode is positive here, does oxidation still happen at the anode?
'An Ox' and 'Red Cat' always hold: oxidation at the anode, reduction at the cathode. Only the + and − signs flip, because an external supply — not the chemistry — is pushing the electrons.
Is charging a phone battery electrolysis?
Rechargeable batteries run as galvanic cells while discharging and as electrolytic cells while charging. The external charger forces the discharge reaction to run backward, restoring the reactants.
Why is an electrolyte added when electrolyzing water?
Pure water conducts poorly, so industrial cells use concentrated brine or add an electrolyte like sulfuric acid or sodium hydroxide. The added ions carry the current; in water splitting they are not consumed.
If electrolysis needs so much electricity, why is it used industrially?
Cost, safety, and product purity. Electrolysis is used where the metal is too reactive for chemical reduction (aluminum, sodium) or where extreme purity is required (copper for wiring), even though the electricity is expensive.