Let’s read!
Pages 82 to 89
Objectives
•
•
•
•
•
To know how to carry out electrolysis experiments.
To work out what happens to ions at each electrode.
To be able to write successful half-equations.
To predict the products of electrolysis.
To do electrolysis calculations using the idea of a Faraday
Stick it in!
Electrolysis
Lead bromide
Electrode - an
electrical conductor
which carries charge
to or from a liquid
undergoing
electrolysis.
Lead bromide
Copy
please
Electrolysis
Electrolysis is the
break-down of a
substance by
electricity
Electrolyte - a molten
or aqueous solution
through which an
electrical current can
flow.
Let’s try it!
Electrolysis experiments
• Electrolysis only happens in:
- molten ionic liquids or
- aqueous solutions containing
ions.
• There must be a complete
circuit.
• A lamp or ammeter shows that
electricity is flowing around the
circuit.
Moving charges
Can you stick the sheet in your book?
Electrolysis of zinc chloride
Draw the sentence
1. Electrolytes contain positive and negative ions.
2. During electrolysis, positive and negative
electrodes are put into the electrolyte.
3. The positive electrode is called the anode.
4. The negative electrode is called the cathode.
5. The negative ions (called anions) are attracted to
the anode.
6. At the anode, the negative ions lose electrons to
become atoms/molecules.
7. The positive ions (called cations) are attracted to
the cathode.
8. At the cathode, the positive ions gain electrons
to become atoms/molecules
Draw the sentence
1. Electrolytes contain positive and negative ions.
2. During electrolysis, positive and negative
electrodes are put into the electrolyte.
3. The positive electrode is called the anode.
4. The negative electrode is called the cathode.
5. The negative ions (called anions) are attracted to
the anode.
6. At the anode, the negative ions lose electrons to
become atoms/molecules.
7. The positive ions (called cations) are attracted to
the cathode.
8. At the cathode, the positive ions gain electrons
to become atoms/molecules
At the electrodes
Cathode (-)
(negative electrode)
• Positive ions go here
(cations).
• As metal ions are positive,
they go to the cathode.
• Ions gain electrons. They
are reduced and become
neutral atoms.
Anode (+)
(positive electrode)
• Negative ions go here
(anions).
• As non-metal ions are
negative, they go to the
anode.
• Ions lose electrons. They are
oxidised and become neutral
atoms
(which react together to form
molecules).
Common ions
•
•
•
•
•
•
•
Li+, Na+, K+,
Mg2+, Ca2+ , Zn2+
Cu+, Cu2+, Fe2+, Fe3+, Al3+
NH4+ (ammonium ion)
F-, Cl-, Br-, IO2-, S2OH- (hydroxide ion), CO32-, NO3- (nitrate
ion), SO42- (sulphate ion)
Questions!
• Let’s try some easy questions
Half equations
• Show what happens at each electrode.
• Are balanced equations.
• Consider the electrolysis of copper chloride:
Cu2+ + 2e-  Cu
2Cl- - 2e-  Cl2
Let’s read again!
Page 92 to 95 (Chemistry for You)
Electrolysis of solutions – Cathode
For solutions of highly
reactive metals:
Hydrogen gas, not the metal,
is produced at the cathode.
Electrolysis of solutions – Anode
The product at
the anode depends on:
The negative anions
present in the solution.
Electrolysis of aluminium oxide
CO2
Electrolysis
The Faraday
Objectives
• recall that one Faraday represents one mole
of electrons
• calculate the amounts of the products of the
electrolysis of molten salts and aqueous
solutions
The Faraday
• A Faraday is one mole of electrons, and is
equivalent to 96 500C (Coulombs)
• A current of 1A = 1C per second flowing
• For example,
Cu2+ + 2e  Cu
1 mole of Cu2+ ions reacts with 2 Faradays of
electrons, to produce 1 mole of Cu
• Quantity of electricity in coulombs =
current in amps x time in seconds
• Q (C) = I (A) x t (s)
Example
How much copper is deposited if a
current of 0.2 Amps is passed for 2
hours through a copper(II) sulphate
solution ?
current of 0.2 Amps is passed for
2 hours
At the cathode:
Cu2+(aq) + 2e-  Cu(s)
Q =I
x
t
= 0.2
x
(2 x 60 x 60)
=1440 Coulombs
1 mole electrons = 96500 Coulombs
So, 1440 / 96500 = 0.0149 moles of electrons
(Faradays)
Example
moles of electrons passed through circuit =
0.01492
Cu2+(aq) + 2e-  Cu(s)
• From equation, it takes two moles of
electrons to form one mole of copper
• moles copper = 0.01492 / 2 = 0.00746
Example
• moles Cu = 0.00746
• mass of Cu
= moles x Ar
= 0.00746 x 64
= 0.4775g of Cu deposited.
‘How To” Guide
1. Write out relevant half equation
2. Work out coulombs of electrons flowing (Q = It)
3. Convert C into moles of electrons (Faradays)
(Q/96500)
4. Work out moles of product using ratio from
equation
5. Convert into mass (mass = moles x Ar)
• In an electrolysis of sodium chloride
solution experiment a current of 2 A was
passed for 2 minutes.
– (a) Calculate the volume of chlorine gas
produced.
– (b) What volume of hydrogen would be
formed?
– (c) In practice the measured volume of chlorine
can be less than the theoretical value. Why?
– Electrode equations:
• (-) cathode 2H+ + 2e-  H2
• (+) anode 2Cl-  Cl2 + 2e
– (a) Calculate the volume of chlorine gas
produced.
• Q = I x t, so Q = 2 x 2 x 60 = 240 C
• 240 C = 240 / 96500 = 0.002487 mol electrons
• this will produce 0.002487 / 2 = 0.001244 mol Cl2
(two electrons/molecule)
• vol = mol x molar volume = 0.001244 x 24000 =
29.8 cm3 of Cl2
– (b) What volume of hydrogen would be
formed?
• 29.8 cm3 of H2 because two electrons transferred
per molecule, same as chlorine.
– (c) In practice the measured volume of chlorine
can be less than the theoretical value. Why?
• chlorine is moderately soluble in water and also
reacts with the sodium hydroxide formed.
• In the electrolysis of molten sodium
chloride 60 cm3 of chlorine was produced.
– Calculate ...
– (a) how many moles of were chlorine
produced?
– (b) what mass of sodium would be formed?
– (c) for how long would a current of 3 A in the
electrolysis circuit have to flow to produce the
60cm3 of chlorine?
(a) how many moles of chlorine produced?
• 60 / 24000 = 0.0025 mol Cl2
(b) what mass of sodium would be formed?
• from the electrode equations 2 mol sodium will be
made for every mole of chlorine
• so 0.0025 x 2 = 0.005 mol sodium will be formed.
Ar(Na) = 23
• mass = mol x atomic or formula mass = 0.005 x 23
= 0.115g Na
(c) for how long would a current of 3 A in the
electrolysis circuit have to flow to produce the
60cm3 of chlorine?
• To produce 0.0025 mol of Cl2 you need 0.005 mol
of electrons
• 0.005 mol electrons = 0.005 x 96500 coulombs =
482.5 C
• Q = I x t, so 482.5 = 2 x t, therefore t = 482.5 / 3 =
161 s (to nearest second)
Brine?
Brine is salty
water (sodium
chloride solution)
Electrolysis of brine
Electrolysis of brine
• Hydrogen is produced at the cathode
• Chlorine is produced at the anode
• The solution remaining is sodium hydroxide
Electrolysis of brine
• Hydrogen is produced at the cathode
• Chlorine is produced at the anode
• The solution remaining is sodium hydroxide
Cathy’s Ankles (CatHy’s AnCl)
Electrolysis of brine
• Hydrogen is produced at the cathode
• Chlorine is produced at the anode
• The solution remaining is sodium hydroxide
Copy
please!
Cathy’s Ankles (CatHy’s AnCl)
Electrolysis of brine
• Cathode (-)
2H+(aq) + 2eH2(g)
(SODIUM IS NOT FORMED (the sodium
ion is more stable than the hydrogen ion in
water H2O
H+ + OH-))
• Anode (+)
2Cl-(aq) – 2e-
Cl2(g)
Electrolysis of brine
• Cathode (-)
2H+(aq) + 2eH2(g)
(SODIUM IS NOT FORMED (the sodium
ion is more stable than the hydrogen ion in
water H2O
H+ + OH-))
• Anode (+)
2Cl-(aq) – 2e-
Copy
please!
Cl2(g)
Chemicals from salt
Chemicals from salt
Copy
please!
Let’s try some questions!