Topic 19.1: Electrochemical cells – Redox processes
Energy conversions between electrical and chemical energy lie at the core of electrochemical cells.
• Understanding: A voltaic cell generates an electromotive force (EMF) resulting in the movement of electrons from the anode
(negative electrode) to the cathode (positive electrode) via the external circuit. The EMF is termed the cell potential (Eº).
▪ Electromotive force (EMF): potential difference between two electrodes when no current is flowing in the circuit.
• Cell potential (Eº): EMF is termed the cell potential when the cell is operating
• Due to the cell potential, electrons flow from anode to cathode via the external circuit
• Understanding: The standard hydrogen electrode (SHE) consists of an inert platinum electrode in contact with 1 mol dm-3
hydrogen ion and hydrogen gas at 100 kPa and 298 K. The standard electrode potential (Eº) is the potential (voltage) of the
reduction half-equation under standard conditions measured relative to the SHE. Solute concentration is 1 mol dm-3 or 100 kPa for
gases. Eº of the SHE is 0 V.
▪ Standard hydrogen electrode (SHE): universal reference gas electrode used to measure
electrode potential of half-cells
• Conditions of SHE
▪ Inert platinum electrode: unreactive electrode that does not corrode
or ionize, act as a heterogeneous catalyst
▪ Standard condition: 100 kPa
▪ Concentration: 1 mol dm-3 H+ (aq), 1 mol dm-3 H2 (g)
• Eº of SHE: always 0 V under all temperature
• Reaction in SHE: 2H+ (aq) + 2e- ⇌ H2 (g)
▪ Standard electrode potential (Eº): potential of the reduction half-equation under standard conditions measured relative to SHE
• Standard condition: 100 kPa, temperature relative to temperature of SHE
• Standard concentration: 1 mol dm-3 solute
• Understanding: Gº = -nFEºcell. When Eº is positive, Gº is negative indicative of a spontaneous process. When Eº is negative, Gº
is positive indicative of a non-spontaneous process. When Eº is 0, then Gº is 0.
▪ Gibbs free energy and standard electrode potential are associated to one other: Gº = -nFEº
• Gibbs free energy (Gº): extensive property; changes with quantity of sample (hence Eº has to be multiplied by n)
• Standard cell potential (Eº): intensive property; does not change with quantity of sample
• Faraday’s constant (F): 95600 C mol-1; charge that 1 mol of e- carries
Gº Eº Reaction under standard state condition
Negative Positive Spontaneous; product formation favoured
Positive Negative Non-spontaneous; reactant formation favoured
Zero zero Product and reactant formation favoured equally
• Understanding: When aqueous solutions are electrolysed, water can be oxidized to oxygen at the anode and reduced to hydrogen
at the cathode.
▪ Electrolysis of aqueous solution (dilute sulfuric acid)
• Reaction of water at cathode: H+ (aq) + e- → ½ H2 (g) Eº = 0.00 V
• Reaction of water at anode: H2O (l) → ½ O2 (g) + 2H+ (aq) + 2e- Eº = -1.23 V
• Understanding: Current, duration of electrolysis and charge on the ion affect the amount of product formed at the electrodes
during electrolysis.
▪ Equation for electrical charge: Q = It
• Q = coulomb; unit of electrical charge
• I = current
• t = time
▪ Factors affecting amount of product formed at the electrodes
• Current (I): the greater the current, the greater the charge, Q, which means that there is an increased quantity of
electricity passing through electrolysis and according to Faraday’s first law, the greater the amount of product formed
• Duration of electrolysis (t): the greater the given time, the greater the amount of product deposit (see equation)
• Charge on ions (z): the amount of electron needed to discharge 1 mol of an ion at an electrode is equal to the charge
on the ion according the Faraday’s second law
• Understanding: Electroplating involves the electrolytic coating of an object with a metallic thin layer.
• Applications and skills: Explanation of the process of electroplating.
▪ Electroplating: electrolytic coating of an object with a metallic thin layer through electrolysis
• Anode: consists of the metal that would be used as a coat (e.g. silver)
• Cathode: with the metal that would be coated (e.g. silver spoon)
• Solution: solution containing metal ions (e.g. Na[Ag(CN)2])
Energy conversions between electrical and chemical energy lie at the core of electrochemical cells.
• Understanding: A voltaic cell generates an electromotive force (EMF) resulting in the movement of electrons from the anode
(negative electrode) to the cathode (positive electrode) via the external circuit. The EMF is termed the cell potential (Eº).
▪ Electromotive force (EMF): potential difference between two electrodes when no current is flowing in the circuit.
• Cell potential (Eº): EMF is termed the cell potential when the cell is operating
• Due to the cell potential, electrons flow from anode to cathode via the external circuit
• Understanding: The standard hydrogen electrode (SHE) consists of an inert platinum electrode in contact with 1 mol dm-3
hydrogen ion and hydrogen gas at 100 kPa and 298 K. The standard electrode potential (Eº) is the potential (voltage) of the
reduction half-equation under standard conditions measured relative to the SHE. Solute concentration is 1 mol dm-3 or 100 kPa for
gases. Eº of the SHE is 0 V.
▪ Standard hydrogen electrode (SHE): universal reference gas electrode used to measure
electrode potential of half-cells
• Conditions of SHE
▪ Inert platinum electrode: unreactive electrode that does not corrode
or ionize, act as a heterogeneous catalyst
▪ Standard condition: 100 kPa
▪ Concentration: 1 mol dm-3 H+ (aq), 1 mol dm-3 H2 (g)
• Eº of SHE: always 0 V under all temperature
• Reaction in SHE: 2H+ (aq) + 2e- ⇌ H2 (g)
▪ Standard electrode potential (Eº): potential of the reduction half-equation under standard conditions measured relative to SHE
• Standard condition: 100 kPa, temperature relative to temperature of SHE
• Standard concentration: 1 mol dm-3 solute
• Understanding: Gº = -nFEºcell. When Eº is positive, Gº is negative indicative of a spontaneous process. When Eº is negative, Gº
is positive indicative of a non-spontaneous process. When Eº is 0, then Gº is 0.
▪ Gibbs free energy and standard electrode potential are associated to one other: Gº = -nFEº
• Gibbs free energy (Gº): extensive property; changes with quantity of sample (hence Eº has to be multiplied by n)
• Standard cell potential (Eº): intensive property; does not change with quantity of sample
• Faraday’s constant (F): 95600 C mol-1; charge that 1 mol of e- carries
Gº Eº Reaction under standard state condition
Negative Positive Spontaneous; product formation favoured
Positive Negative Non-spontaneous; reactant formation favoured
Zero zero Product and reactant formation favoured equally
• Understanding: When aqueous solutions are electrolysed, water can be oxidized to oxygen at the anode and reduced to hydrogen
at the cathode.
▪ Electrolysis of aqueous solution (dilute sulfuric acid)
• Reaction of water at cathode: H+ (aq) + e- → ½ H2 (g) Eº = 0.00 V
• Reaction of water at anode: H2O (l) → ½ O2 (g) + 2H+ (aq) + 2e- Eº = -1.23 V
• Understanding: Current, duration of electrolysis and charge on the ion affect the amount of product formed at the electrodes
during electrolysis.
▪ Equation for electrical charge: Q = It
• Q = coulomb; unit of electrical charge
• I = current
• t = time
▪ Factors affecting amount of product formed at the electrodes
• Current (I): the greater the current, the greater the charge, Q, which means that there is an increased quantity of
electricity passing through electrolysis and according to Faraday’s first law, the greater the amount of product formed
• Duration of electrolysis (t): the greater the given time, the greater the amount of product deposit (see equation)
• Charge on ions (z): the amount of electron needed to discharge 1 mol of an ion at an electrode is equal to the charge
on the ion according the Faraday’s second law
• Understanding: Electroplating involves the electrolytic coating of an object with a metallic thin layer.
• Applications and skills: Explanation of the process of electroplating.
▪ Electroplating: electrolytic coating of an object with a metallic thin layer through electrolysis
• Anode: consists of the metal that would be used as a coat (e.g. silver)
• Cathode: with the metal that would be coated (e.g. silver spoon)
• Solution: solution containing metal ions (e.g. Na[Ag(CN)2])
