Electrochemistry
Understand how chemical reactions produce electricity and how electricity drives chemical reactions -- the science behind batteries, fuel cells, and electroplating.
Galvanic (Voltaic) Cells
A galvanic cell converts chemical energy into electrical energy through spontaneous redox reactions. Two different metals are placed in solutions of their ions, connected by a salt bridge and an external circuit.
Galvanic Cell Structure
Anode (−)
Oxidation occurs
e.g. Zn(s)
Zn → Zn²+ + 2e−
Cathode (+)
Reduction occurs
e.g. Cu(s)
Cu²+ + 2e− → Cu
Cell notation (Daniell cell): Zn(s) | Zn²+(aq) || Cu²+(aq) | Cu(s). The anode is written on the left, the cathode on the right, and || represents the salt bridge.
Remember: AN OX, RED CAT -- Anode = Oxidation, Reduction = Cathode. In a galvanic cell, electrons flow from anode to cathode through the external circuit.
Electrolytic Cells
An electrolytic cell uses electrical energy to drive a non-spontaneous redox reaction. An external power source forces electrons to flow in the opposite direction to what would occur naturally.
Galvanic Cell
- • Spontaneous reaction
- • Produces electrical energy
- • Anode is negative (−)
- • Two separate solutions
- • Has a salt bridge
- • Example: batteries
Electrolytic Cell
- • Non-spontaneous reaction
- • Requires electrical energy
- • Anode is positive (+)
- • One shared solution
- • No salt bridge needed
- • Example: electroplating
Applications of electrolysis: Electroplating (coating objects with metal), extraction of aluminium from bauxite (Hall-Heroult process), purification of copper, and production of chlorine gas from brine.
Standard Electrode Potentials
The standard electrode potential (E°) measures the tendency of a half-cell to be reduced. All values are measured relative to the Standard Hydrogen Electrode (SHE), which is assigned E° = 0.00 V.
Selected Standard Reduction Potentials
Calculating cell EMF: E°cell = E°cathode - E°anode. A positive E°cell means the reaction is spontaneous. For the Daniell cell: E° = +0.34 - (-0.76) = +1.10 V.
Predicting reactions: In a galvanic cell, the half-cell with the more negative E° is oxidised (anode), and the half-cell with the more positive E° is reduced (cathode).
Key Vocabulary
Oxidation
The loss of electrons by a species in a chemical reaction. Occurs at the anode in both galvanic and electrolytic cells.
Reduction
The gain of electrons by a species in a chemical reaction. Occurs at the cathode in both galvanic and electrolytic cells.
Salt Bridge
A device connecting two half-cells that allows ion flow to maintain electrical neutrality while preventing direct mixing of solutions.
Standard Electrode Potential (E°)
The potential of a half-cell under standard conditions (25°C, 1 mol/L, 1 atm) measured against the Standard Hydrogen Electrode.
Worked Examples
Calculate the standard cell potential for a cell made from Zn/Zn²+ and Cu/Cu²+ half-cells.
Step 1: Identify E° values: Zn²+/Zn = -0.76 V, Cu²+/Cu = +0.34 V.
Step 2: The more negative value (Zn) is oxidised (anode). Cu is reduced (cathode).
Step 3: E°cell = E°cathode - E°anode = +0.34 - (-0.76) = +1.10 V.
Answer: The standard cell potential is +1.10 V. The positive value confirms the reaction is spontaneous.
Write the cell notation for a galvanic cell where magnesium is oxidised and silver ions are reduced.
Step 1: Anode (oxidation): Mg(s) → Mg²+(aq) + 2e−.
Step 2: Cathode (reduction): Ag+(aq) + e− → Ag(s).
Step 3: Cell notation: anode | anode ion || cathode ion | cathode.
Answer: Mg(s) | Mg²+(aq) || Ag+(aq) | Ag(s)
Explain what happens at each electrode during the electrolysis of molten sodium chloride.
At the cathode (−): Na+ ions are attracted, gain electrons, and are reduced: Na+ + e− → Na(l). Liquid sodium metal forms.
At the anode (+): Cl− ions are attracted, lose electrons, and are oxidised: 2Cl− → Cl2(g) + 2e−. Chlorine gas is produced.
Overall: 2NaCl(l) → 2Na(l) + Cl2(g). This is a non-spontaneous reaction driven by the external power supply.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
In a galvanic cell, oxidation occurs at the:
Question 2
The standard cell potential for a cell with E°cathode = +0.80 V and E°anode = -0.44 V is:
Question 3
The purpose of the salt bridge in a galvanic cell is to:
Question 4
Which of the following describes an electrolytic cell?
Question 5
A half-cell with a more negative standard electrode potential will tend to be:
Key Concepts Summary
- ●Galvanic cells convert chemical energy to electrical energy through spontaneous redox reactions.
- ●Electrolytic cells use electrical energy to drive non-spontaneous redox reactions.
- ●Oxidation occurs at the anode; reduction occurs at the cathode (AN OX, RED CAT).
- ●E°cell = E°cathode - E°anode. A positive value indicates a spontaneous reaction.
- ●The salt bridge maintains electrical neutrality by allowing ion migration between half-cells.