In electrochemistry, we use REDOX and oxidation states (revise from inorganic booklet) when we deal with
the half cells.
Electrode Potentials:
In order to measure the standard electrode potential of a half cell, we have to use a Standard Hydrogen
Electrode. This has a potential difference of 0.00v under standard conditions (298k, 100kPa and
1.00mol/dm3
solutions). We also use a platinum electrode as platinum is unreactive and has a high
electrical conductivity (conducts electricity well);
The KCl salt bridge is used to allow ions to flow freely
between the 2 half cells, carrying the charge and therefore
completing the circuit. KCl is used because it is unreactive
(inert) and so will not react with the chemical in either half
cell
The high resistance voltmeter stops any current from
flowing, so will just measure the potential difference
between the 2 half cells
Looking at the half equations of this cell;
The half equations are always written as reductions, so the electrons are always
on the LHS. However, the more negative electrode (in this case Zn) is always an
oxidation so the half equation should be reversed in the overall equation. We can
then balance the equations to cancel out the electrons, getting the overall
equation of the reaction
Overall Electrode Potentials:
To find this, you simply reverse the sign of the oxidation half equation (the one
that has been reverse) and add the 2 number together. This is a simple way of
checking the standard cell notation method of working out the Electrode
Potentials.
Standard Cell Notation:
The more negative electrode goes on the LHS
LHS undergoes oxidation, RHS undergoes reduction
Species that are not in the same state are separated by a
phase boundary (|)
The 2 half equations are separated by the salt bridge (||)
Species on outside of electrode must be solid or a Platinum
(Pt) electrode must be used (good conductor & inert)
You don’t have to balance the 2 half equations in the
standard cell notation, only when forming overall equation
The ELHS & ERHS are the standard electrode potentials from
table (written as reductions). You don’t need to reverse the
sign or multiply by anything in this case.
When the Ecell value is positive, the reaction can be spontaneous
Electrochemical Series:
The standard electrode potentials can be written in a table, usually from most positive to most negative,
and they are all written as reductions (electrons on LHS);
The more negative the electrode potential, the better the
reducing agent (oxidise more easily) & therefore, the more
positive the electrode potential, the better the oxidising
agent (reduced more easily)
Once you have identified the reaction from which the
strongest [O] or [H] comes from, use the oxidation states of
the species in this equation to find which species hasn’t yet
been oxidised/ reduced. This is the [O] or [H].
Remember [O] are reduced and [H] are oxidised
Electrochemical Cells:
Most cells are non-rechargeable. This is because the redox reactions that occur in their half cells are
irreversible. Some cells however are rechargeable as the redox reactions are reversible, so the half cells
can be returned to their original state. The example you have to know is Lithium Ion Cells;
Negative Electrode:
Lithium is oxidised, forming Li+
ions and electrons
Positive Electrode:
Cobalt is reduced, forming an ionic compound that is aqueous, so a Pt electrode has to be used
Fuel Cells:
In normal electrochemical cells, the potential difference will decrease over time. This is because the
concentrations of the chemicals in the half cells changes, so the potential difference decreases. However,
in fuel cells, the fuel can be constantly added, so the potential difference is constant. The example of fuel
cells you need to know is the Hydrogen-Oxygen Fuel cell:
At the negative electrode, Hydrogen is oxidised, releasing electrons which flow around an external circuit,
doing work, before reducing Oxygen at the positive electrode. This can occur in either an acidic solution or
an alkaline solution;
The overall equation is the same in both instances!
Hydrogen-Oxygen fuel cells are much more energy efficient, there are no high temperatures or pressures,
and they only produce water as a by-product.
However, Hydrogen is very explosive, so any leaks are very dangerous, it is difficult to store, and
Hydrogen is often extracted using energy from power plants that burn fossil fuels.