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Electrolysis
• To revise electrolysis of molten compounds: aluminium oxide
• To revise electrolysis of aqueous solutions: brine and copper sulfate
Electrolysis
• The decomposition of ionic compounds using electricity
• Only works with molten or dissolved ionic compounds because the ions are free to move
Electrolysis
• Negative ions are attracted to the positive electrode (anode) – here they lose electrons = OIL
• Positive ions are attracted to the negative electrode (cathode) – here they gain electrons = RIG
+ -
PANIC!!!
6.1 Electrolysis6.1 Electrolysis
Electrolysis: splitting up using electricity
Ionic substance- molten (l)- dissolved (aq)
Non-metal ionMetal ion
Electrolysis
With molten compounds only the elements that are present will be produced at the electrodes, e.g.
Zinc chloride zinc + chlorineat the cathodeZn2+ + 2e- Zn at the anode 2Cl- - 2e- Cl2
Electrolysis
• With aqueous solutions, the water solvent must be taken into account
• It is also possible to have hydrogen or oxygen gas produced
• If a halide is present then the halogen gas will be produced
6.2 Changes at the 6.2 Changes at the electrodeselectrodes
Oxidation is loss
Reduction is gain
OIL RIG
Molten (PbBr) 2Br- Br2 + 2e- Pb2+ + 2e- Pb
Solution (KBr) 2Br- Br2 + 2e- 2H+ + 2e- H2
SolutionsWater contains the ions:
H+ and O2-
The less reactive element will be given off at electrode
When Salt Dissolves
• Sodium Chloride splits to give you:
• Na+ ions• and• Cl- ions
• Some water molecules also split up into:
• H+ ions• and• OH- ions
NegativeCathode
PositiveAnode
D.C.current
+ -
Na+
Na+Cl-
Cl-H+
H+
OH-
OH-
Where will the ions go?
H2
Cl22e-
Products
• The three main products are:
• Hydrogen gas
• Chlorine gas
• Sodium hydroxide solution
Cl2
H2
Na+ OH-
Electrolysis of sodium chloride solution
• At the negative electrode: hydrogen gas is produced (this comes from the water solvent, hydrogen is less reactive than sodium)
2H+ + 2e- H2
• At the positive electrode: chlorine gas is produced 2Cl- - 2e- Cl2
6.3 Electrolysing brine6.3 Electrolysing brine
At anode 2Cl- (aq) Cl2 (g) + 2e-
At cathode 2H+ (aq) + 2e- H2 (g)
In solution Na+ and OH-
Electrolysis of copper sulfate solution using carbon electrodes
• The products are different because water solvent is present:
• At the negative electrode: copper metal is produced (it is less reactive than hydrogen)
Cu2+ + 2e- Cu• At the positive electrode: oxygen gas is
produced (from the water solvent) 4OH- O2 + 2H2O + 4e-
Electrolysis
• This technique can be used to electroplate cheaper objects with a layer of a more expensive metal, e.g. Gold-plated jewellery
Purifying copper metal
• Electrolysis of copper sulfate solution using copper electrodes
• This shows how very pure copper is produced from crude copper metal.
• It needs to be very pure for things like ELECTRICAL WIRES so that they conduct very well.
6.4 Purifying copper6.4 Purifying copper
At anode 2H2O (l) 4H+ (aq) + O2 (g) + 2e-
At cathode Cu2+ (aq) + 2e- Cu (s)
Electrolysis of AluminiumElectrolysis of Aluminium Aluminium oxide has a very high melting point. It melts at 2050oC. So, chemists mix another ionic compound called CryoliteCryolite with the aluminium oxide. The moltenmolten mixture can now be electrolysed at 850-950oC.
• To revise the difference between acids, bases and alkalis.
• To revise the different methods used to make salts.
Acids, bases & salts
Metal oxides and metal hydroxides are bases e.g. sodium hydroxide, copper oxide.
If the hydroxides are soluble they are called alkalis
Hydrochloric + Sodium acid hydroxide
Sodium +Chloride
Water
The name of the salt produced when an acid and alkali react depends on the acid used and the metal in the base
Ammonia (NH3) dissolves in water to form an alkali solution, NH3 (aq).
It is used to produce ammonium salts
These salts are used as fertilisers
The presence of H+ ions makes solutions acidic.
The presence of OH- ions make solutions alkali.
1 2 14131211109876543
Strong acid Neutral Strong alkali
Weak acid Weak alkali
More alkaliMore acid
More OH- ionsMore H+ ions
H+(aq) + OH-(aq) H2O (l)
Salt + WaterAcid + Alkali
In neutralisation reactions, hydrogen ions react with hydroxide ions to produce water.
nitric + potassium acid hydroxide
potassium + nitrate
Water
7.1 Acids and alkalis7.1 Acids and alkalis
Acids = H+ ions
Alkalis = OH- ions
Alkalis = soluble bases
H+(aq) + OH-(aq) H2O (l)
State symbols are shown after substances in reactions
(s)= solid
(l)= liquid
(aq) = aqueous (dissolved in water)
(g) = gas
Soluble salts can be made by reacting acids with:1. metals – not all metals are suitable; some are too reactive and others are not reactive enough 2. insoluble bases – the base is added to the acid until no more will react and the excess solid is filtered off (copper oxide to make copper sulphate)3. alkalis – an indicator can be used to show when the acid and alkali have completely reacted to produce a salt solution. (neutralisation)
Soluble salts can be made by reacting acids with:1. metals – not all metals are suitable; some are too reactive and others are not reactive enough (MASH)
Metal + Acid Salt + Hydrogen
• Hydrochloric acid produces metal chlorides e.g. Sodium chloride
• Sulphuric acid produces metal sulphates e.g. Sodium Sulphate
• Nitric acid produces metal nitrates e.g. sodium nitrate
7.2 + 7.3 Salts7.2 + 7.3 Salts
AcidFormul
aSalt Example
Hydrochloric HCl Chloride Sodium chloride
Sulphuric H2SO4
Sulphat
eCopper sulphate
Nitric HNO3 NitratePotassium
nitrate
Soluble salts can be made by reacting acids with:2. insoluble bases – the base (copper oxide) is added to the acid until no more will react and the excess solid is filtered off (copper sulphate)
Soluble salts can be made by reacting acids with:3. alkalis – an indicator can be used to show when the acid and alkali have completely reacted to produce a salt solution. (neutralisation)
H+(aq) + OH-(aq) H2O (l)
Salt + WaterHydrochloric + Sodium acid hydroxide
Sodium +Chloride
Water
Acid + Alkali
7.2 + 7.3 Salts – metals, bases 7.2 + 7.3 Salts – metals, bases and alkalisand alkalis
Metals: Metal(s) + acid(aq) salt(aq) + hydrogen(g)
Bases: Acid(aq) + base(aq) salt(aq) + water(l)
Alkalis: Acid(aq) + alkali(aq) salt(aq) + water(l)
Ionic equation (neutralisation): H+ + OH- H2O
Insoluble salts can be made by reacting two soluble salts together. This is called a precipitation reaction.
Soluble except when with silver, mercury or lead.
Soluble except when with silver, lead, mercury, barium, strontium and calcium.
All soluble
Insoluble salts can be made by reacting two soluble salts together. This is called a precipitation reaction.
All insoluble except for NH4+,
barium and those of group 1 elements
All insoluble except when with NH4
+ and group 1 elements
All insoluble except for calcium, barium and group 1
An example precipitation reaction
Silver + Sodium nitrate chloride
Sodium + nitrate
Silverchloride
AgNO3(aq) + NaCl(aq) → NaNO3(aq) +AgCl(s)
Precipitation reactions are used to remove unwanted ions from solutions, for example to treat drinking water
or treating effluent