Half Cell Reactions
Skills to develop
- Explain chemical reactions for each electrode of a battery or galvanic cell.
- Use notations to depict a electrode.
- Describe oxidation and reduction reactions.
- Construct a hydrogen electrode.
Half Cell Reactions
A half cell is one of the two electrodes in a galvanic cell or simple battery.
For example, in the Zn-Cu battery, the two half cells make an
Placing a piece of reactant in an electrolyte solution makes a half cell.
Unless it is connected to another half cell via an electric conductor and
salt bridge, no reaction will take place in a half cell.
On the cathode, reduction takes place.
Oxidant + n e- ® Reductant
Example: Cu2+ + 2 e ® Cu
Cu2+ is the oxidizing agent and Cu the reducing agent.
On the anode, oxidation takes place.
Reductant ® n e- + Oxidant
Example: Zn ® Zn2+ + 2 e-.
Zn is the reducing agent, and Zn2+ the oxidizing agent.
A battery requires at least two electrodes, the anode at which oxidation
occurs, and the cathode at which reduction occurs. Reduction and oxidation
are always required in any battery setup.
A battery operation requires
an anode, a cathode, a load, and a salt bridge
(if the salt bridge is not there already). These are the key elements
of a battery.
Some example problems are given below to illustrate the kind of problems
you are expected to solve.
Write the anode and cathode reactions for a galvanic cell that
utilizes the reaction
Ni(s) + 2 Fe3+ ® Ni2+ + 2 Fe2+
Oxidation takes place at the anode, and the electrode must be Ni | Ni2+,
Ni(s) ® Ni2+(aq) + 2 e
and the reduction occurs at the cathode: Fe3+, Fe2+:
2 Fe3+ + 2 e ® 2 Fe2+
For every Ni atom oxidized, two Fe3+ ions are reduced.
The electrons from the Ni metal will flow from the anode, pass the load,
and then carry out the reduction at the surface of the cathode to reduce
the ferric (Fe3+) ions to ferrous ions. In the mean time the
ions in the solution move accordingly to keep the charges balanced.
Ni(s) | Ni2+(aq) || Fe3+(aq), Fe2+(aq) | Pt(s)
where "Fe3+(aq), Fe2+(aq)" represent a solution
containing two types of ions. An innert Pt electrode is placed in the
solution to provide electrons for the reduction.
The galvanic cell is:
The charge on an electron is 1.602x10-19 C (coulomb).
What is the charge on 1 mole of electrons?
F = (6.022045x1023 / mol) × (1.602x10-19 C)
The charge on one mole (Avogadro's number of) electrons is called
a Faraday (F).
= 96485 C/mol
The chemical history involving the determination of Avogadro's number,
and the charge on an electron, and how the two values agree with each
other is very interesting.
Who determined the charge on a single electron?
Robert Millikan was awarded with the Nobel Prize for his
determination of electron charge at University of Chicago.
If 96485 C of charge is required to deposit 107.9 g of silver,
What is the charge of an electron?
A galvanic cell with a voltage of 1.1 V utilizes the reaction
Zn + Cu2+ ® Cu + Zn2+
as a source of energy. If 6.3 g of Cu and 11 g Zn are used,
what is the maximum usable energy in this battery?
Max. Energy = (1.1 V)(96485 C)(2)(0.10)
The 6.3 g Cu and 11 g Zn correspond to 0.10 and 0.17 mol of Cu and Zn
respectively. Thus, Cu is the limiting reagent, and 0.10 mol corresponds
to a charge of 2×96485×0.10 C (2 significant figures). The maximum available
energy is then
= 22000 J (1 J = 1 VC)
This energy corresponds to 2500 cal, which is enough to bring 25 g water
from 273 K to its boiling point (373 K). Another way of looking at it:
22000 J is enough energy to send a 20-gram rocket to a height of 56 m.
If the galvanic cell of Example 3 is used to power a calculator,
which consumes 1 mW, how long theoretically will the battery
last in continuous operation?
Power consumption of 1 mW is equivalent to 0.001 J/sec.
------------ = 2.2E7 sec
= 6200 hrs
This is a realistic example. Most recent calculators use very little
power. I noted that a SHARP programmable calculators uses 15 mW, a
Casio calculator uses 0.5 mW, and an HP 25 uses 500 mW.
= 254 days
The Hydrogen Half Cell
A half cell consists of an electrode and the species to be oxidized or
reduced. If the material conducts electricity, it may be used as an electrode.
The hydrogen electrode consists of a Pt electrode, H2 gas and H+. This
half cell, is represented by:
Pt(s) | H2(g) | H+(aq)
where the vertical bars represent the phase boundaries.
Conventionally, the cell potential for the hydrogen electrode is defined to
be exactly zero if it has the condition as given below:
Pt | H2 (g, 1 atm) | H+(aq), 1 M
The notations for half cells are not rigid, but a simplified way to
represent a rather complicated setup.
Standard Reduction Potential
The tendency for a reduction reaction is measured by its
Oxidant + n e- ® Reductant . . . Eo
For example: Cu2+ + 2 e ® Cu . . . Eo = 0.339 V
The reduction potential is a quantity measured by comparison.
As mentioned earlier, the reduction potential of the standard hydrogen
electrode (SHE) is arbitrary defined to be zero as a reference point
When a half cell Cu2+ || Cu for the reaction
Cu2+ + 2 e ® Cu
is coupled with the Standard Hydrogen Electrode (SHE), the copper electrode
is a cathode, where reduction takes place. The potential accross the cell
Pt | H2 (g, 1 atm) | H+(aq), 1 M || Cu2+
has been measured to be 0.339 V. This indicates that Cu2+ ions
is easier to reduce than the hydrogen ions, and we usually
represent it by
Cu2+ + 2 e ® Cu . . . Eo = 0.339 V
A positive cell potential indicates a spontaneous reaction.
When the cell Zn | Zn2+ is coupled with the SHE,
Zn | Zn2+> (aq) 1 M || H+(aq), 1 M | H2 (g, 1 atm) | Pt
The potential has been measured to be 0.76 V. However, in this
cell, Zn is oxidized, and its electrode is the anode. Therefore, the reduction
potentail has a negative value for the reduction reaction
Zn2+ + 2 e ® Zn . . . Eo = - 0.76 V
This means that Zn2+ ions are less readily to accept electrons
than hydrogen ions.
Ideally, for every redox couple, there is a reduction potential.
Reduction potentials of standard cells have been measure against the
SHE or other standards, their potentials are measured.
This values are usually tabulated in handbooks. A short
Standard Reduction Potentials
table is available from the HandbookMenu, but
you may also click the live link to see one.
Confidence Building Questions
Choose the single correct or the single incorrect statement
- At the anode, oxidation takes place only when used as a battery
- At the anode, oxidation takes place in a battery and in
a electrolysis operation
- At the cathode, oxidation takes place only when used as a battery
- At the cathode, oxidation takes place in a battery and in
a electrolysis operation
It is a convention to call the reduction electrode a cathode
in a battery or in a electrolysis operation.
The oxidation reaction occurs at the anode.
All chemical reactions that supply the power to a battery are oxidation
reduction reactions. True or false?
Only RedOx reactions involve electron transfer. Even concentration cells
involve oxidation and reduction of the same material.
Note that Ag+ + Cl- ® AgCl(s) is an ionic reaction,
not a redox reaction.
The half-cell using the reaction:
2 H+(aq, 1.00 F) + 2 e ® H2(g, 1atm)
has a half cell potential of zero because
- it is so defined,
- hydrogen is not very reactive,
- its potential is absolutely zero,
- it is not a useful electrode.
No cell potential is ABSOLUTELY zero.
H2 is reactive. This is not the reason at all.
The notation to indicate a boundary between two phases in a electro-
chemical cell is
The vertical bar | is used to indicate boundary between two phases.
Pt | H2 | H+ (1.0 M) represents the hydrogen half cell.
The notation to indicate a salt bridge between two half electro-
chemical cells is
Only | and || are used among the four notations.
Two vertical bars, ||, represent a salt bridge.