Year 12 Chemistry: ~ Chapter 26 Electricity from Chemical Reactions
26.1 Galvanic Cells
Daniell cell
The external circuit is the write and light globe, where an electrical current can flow
through. The globe converts the electrical energy into light and heat (thermal).
The current flows because of a chemical reaction taking place in the cell. If you leave
the cell with a light globe connected for several hours a number of changes can be
observed:



These changes provide evidence of a chemical reaction occurring.
A galvanometer can be connected into the circuit (an instrument for detecting electric
current) and indicates that electrons are flowing from the zinc electrode, through the
wire and to the copper electrode. Current will only flow if there is a ______
__________ present.
After you have read
the appropriate
section label the
anode and cathode
on this diagram
 The reaction in the cell is a redox reaction, as electrons are being produced and
consumed.
 The zinc electrode is eaten away (corroded) forming zinc ions in solution:
Zn (s)  Zn2+(aq) + 2e The oxidation of the zinc metal releases electrons, which flow through the wire to
the copper electrode.
 Electrons are accepted by copper ions in the solution when the ions collide with
the copper electrode:
Cu2+(aq) + 2e-  Cu(s)
Copper atoms are insoluble and deposit on the electrode producing a brown-black
coating.
 The salt bridge forms an essential part of the cell.
 Overall equation: Zn (s) + Cu2+(aq)  Zn2+(aq) + Cu(s)
OILRIG
A chemical that oxidised another substance is called an oxidant and is itself reduced in
the process. A substance that reduces another substance is called a reductant.
Reductants are oxidised in redox reaction.
Why is electrical energy released?
A galvanic cell is designed so that half reactions occur in two separate compartments of
the cell. Because the oxidant and reductant do not come in contact with each other,
electrons can only be transferred by travelling through an external circuit connecting
the negative and positive electrodes. Chemical energy of the reactants is transformed
into electrical creating a current.
Half Cells
In the Daniell cell, one half cell contains Cu and Cu2+, the other contains Zn and Zn2+.
The species present in each half cell form a conjugate redox pair.
In general, if one member of the conjugate pair in a half cell is a metal, it is usually used
as the electrode. If no metal is present an inert electrode such as platinum or graphite
is used. In some half cells, one of the conjugate pairs may be a gas.
 The electrode at which oxidation occurs is called the anode. In a galvanic cell the
anode, where electrons are produced, is described as the ___________ terminal.
 The electrode at which reduction occurs is called the cathode. The cathode where
electrons are consumed, is the ___________ terminal.
 In a Daniell cell the zinc electrode is the __________ and the copper electrode
is the ____________.
ANOILRIGCA
T
The Purpose of the Salt Bridge
If no salt bridge were present, the solution in one half cell would accumulate ________
charge and the solution in other half cell would accumulate a __________ charge as the
reaction proceeded.
The salt bridge contains ions that are free to move so that they can balance charges
formed in the half cells. Cations (__________ ions) migrate to the cathode and anions
(__________ ions) migrate to the anode
Cations  cathode
Anions  anode
Writing half cell equations
In balanced half and overall equations:
 The number of atoms of each elements is equal on both sides.
 The total charge on each side is equal.
Knowing that the conjugate redox pair Zn2+ ions and Zn:
Zn2+(aq) + 2e- Zn(s)
Half cell equations involving polyatomic ions. The equation for a half cell containing the
dichromate ion (Cr2O72-) and Cr3+ ion redox pair.
K O H E S
Cr2O72(aq) + 14H+(aq) + 6e-  2Cr3+(aq) + 7H2O(l)
26.2 The Electrochemical series
Why does the zinc metal in the Daniell cell react with Cu2+ ions, rather than the copper
metal reacting with Zn2+ ions? Zinc loses its electrons much more readily than copper,
zinc is more reactive than copper. In a Zn2+/Zn and Cu2+/Cu combination the electrons
flow from the zinc metal in one half cell to the copper ions in the other half cell.
By looking at the electrochemical series (you have a laminated one), zinc is a stronger
reductant (lower in the series) than copper, therefore losing electrons more readily. Cu2+
is a stronger oxidant than Zn2+, and accepts electrons more readily.
Example:
 Highlight the charges on the anode and cathode
 Write an overall reaction, combining the two half equations.
The electrochemical series is used to predicted half equations.
The electrochemical series is strictly valid only for the conditions under which it was
determined:
 A temperature of ________
 Pressure of _________

Concentration of solutions.
Predicting half equations using the table:
The higher of the two half reactions in the electrochemical series goes forward and
the lower one is reversed. The higher half reaction occurs at the positive electrode;
the lower reaction occurs at the negative one.
Example: Use the electrochemical table to predict the behaviour of a cell composed of
Ag+/Ag and Fe2+/Fe.
 Find the two equations in the electrochemical series
 Ag+ is higher on the left side of the table than Fe2+, it is a stronger oxidant.
 Iron is lower on the right hand side and is a stronger reductant.
 The strongest oxidant reacts with the strongest reductant.
 Half equations:
(Ag+(aq) + eAg(s)) x2 – so electrons balance
2+
Fe(s)
Fe (aq) + 2e Overall equation: 2Ag+(aq) + Fe(s)
2Ag(s) + Fe2+(aq)
 ANOILRIGCAT: using this and the above information draw and label the below
diagram of the cell Ag+/Ag and Fe2+/Fe.
Label:
 Anode / cathode
 Positive / negative
 ½ equations and overall equations
 electrodes
 Salt bridge and draw in the direction the cations and anions would migrate
 Direction of electrons flowing in wire by using arrows.
Questions: 1, 2, 5, 8 & 12.
Year 12 Chemistry: Chapter 26.3 Predicting Redox
26.3 Potential difference
A current flows in a galvanic cell because one half cell has a greater tendency to push
electrons into the external circuit than the other half cell. A potential difference
exists between the two half cells. The potential difference of a cell is sometimes called
the emf.
A rough indication of cell voltage can be obtained from the electrochemical series. Large
potential differences are obtained when a half cell containing a strong oxidant is
connected to a half cell with a strong reductant.
Potential differences of cells at standard conditions can be calculated using the E values
found on the electrochemical series. Reduction reactions are most likely to occur in half
cells with high E values, eg F2(g) / F-(aq) (E= 2.87 V), whereas oxidation reactions are most
likely to occur in half cells with low E values, eg Li+(aq) / Li(s) (E= -3.05).
E values for half cells are measured by connecting the cells to a reference half cell, a
hydrogen half cell, H+(aq) / H2(g).
The potential difference of a cell at standard conditions is the difference between the
E values of its two half cells:
Cell potential difference = higher half cell E – lower half cell E
26.4 Predicting direct redox reactions
In a galvanic cell, the higher half reaction in the electrochemical series occurs in the
forward direction (as reduction) and the lower reaction occurs in reverse (as oxidation).
For an appreciable reaction to occur, a chemical on the left of the electrochemical
series must react with a chemical on the right that is lower in the series.
Oxidants
Reductants
Example:
Consider the following equations that appear in the order shown in the electrochemical
series.
Br2(aq) + 2e2Br-(aq)
Ni2+(aq) + 2eNi(s)
2+
Mg (aq) + 2e
Mg(s)
Predict the effect of mixing:
a) Br2(aq) and Mg2+(aq)
b) Mg2+(aq) and Ni(s)
c) Ni2+(aq) and Mg(s)
Limitations of predictions
The standard half cell potentials are measured under standard conditions.
 Temperature:
 Pressure:
 Concentration:
It is important to bear in mind that the electrochemical series gives no information
about the rate at which reactions occur.
Questions: 3, 4, 6 & 14.