1. C2.5 Salts and Electrolysis
Mr D Powell

2. Animated Science
2015
C2 5.5 Changes at the electrodes
e) At the negative electrode, positively
charged ions gain electrons (reduction)
and at the positive electrode, negatively
charged ions lose electrons (oxidation).
f) If there is a mixture of ions, the
products formed depend on the reactivity
of the elements involved.

3. Animated Science
2015
Movement of Ions...
During electrolysis ions move towards the
electrodes.
The direction they move in depends on their
charge.
1. Positive ions move towards the negative
electrode (the cathode).
2. Negative ions move towards the positive
electrode (the anode).
When ions reach an electrode, they either lose
or gain electrons depending on
their charge.
Negatively charged ions lose electrons to
become neutral atoms. Positively charged ions
form neutral atoms by gaining electrons.
TASK: Can you draw a labelled
diagram (BASIC) and
explain how ions move
during electrolysis with a
paragraph of text
(Medium). Can you add a
real example you know of
(Harder)

4. Animated Science
2015
Reduction: In the electrolysis of molten lead bromide, positively
charged lead ions (Pb2+) move towards the cathode (+). When
they get there, each ion gains two electrons to become a
neutral lead atom. Gaining electrons is called reduction – we
say that the lead ions are reduced.
‘Reduction’ is simply another way of saying ‘gaining electrons’.
Oxidation: When molten lead bromide is
electrolysed, negatively charged bromide ions
(Br–) move towards the anode (+). When they get
there, each ion loses one electron to become a
neutral bromine atom. Two bromine atoms then form
a covalent bond to make a bromine molecule, Br2.
Losing electrons is called oxidation – we say that the
bromide ions are oxidised.
‘Oxidation’ is another way of saying ‘losing electrons’.
TASK: Can you draw a
labelled diagram
(BASIC) then explain
the idea behind
reduction and
oxidation.(Medium).
Oxidation
Reduction

5. Animated Science
2015
HT – “Half Equations” & “Redox”

6. Animated Science
2015
Effect of Water – (Harder)
When we carry out electrolysis in water the situation is
made more complicated by the fact that water contains
ions.
The rule for working out what will happen is to remember
that if two elements can be produced at an electrode, the
less reactive element will usually be formed.
Hydrogen is less reactive than potassium, so it is produced
at the negative electrode rather than potassium when we
electrolyse a solution of a potassium compound
If we think about the positive electrode
(anode) we find that we get OH- ions
from the water produced (giving O2(g))
unless the solution has a high
concentration of halide ion (group VII).
Then we have the halide discharged
instead.

7. Animated Science
2015
HT - Half Equations – Extra Help...
Step Result
1. Write the formulae of the reactant and product. Cl- → Cl2
2. Adjust the number of ions, if needed. 2Cl- → Cl2
3. Count the number of charges. Add enough electrons so
that both sides have the same total number of charges. 2Cl- → Cl2 + 2e-
Unbalanced Balanced
Al3+ + e– → Al Al3+ + 3e– → Al
Cu2+ + e– → Cu Cu2+ + 2e– → Cu
H+ + e– → H2 2H+ + 2e– → H2
Br- → Br2 + e- 2Br- → Br2 + 2e-
O2- → O2 + e- 2O2- → O2 + 4e-