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!!!

Electrolysis: splitting up using electricity Ionic substance - molten (l) - dissolved (aq) Non-metal ion Metal ion

Electrolysis With molten compounds only the elements that are present will be produced at the electrodes, e.g. Zinc chloride zinc + chlorine at the cathodeZn e - Zn at the anode2Cl - - 2e - Cl 2

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

Oxidation is lossReduction is gain OILRIG Molten (PbBr)2Br -  Br 2 + 2e - Pb e -  Pb Solution (KBr)2Br -  Br 2 + 2e - 2H + + 2e -  H 2 Solutions Water contains the ions: H + and O 2- The less reactive element will be given off at electrode

Electrolysis of sodium chloride solution: important industrial process

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

Negative Cathode Positive Anode D.C. current + - Na+ Cl- H+ OH- Where will the ions go?

Negative Cathode Positive Anode D.C. current + - Na+ Cl- H+ OH- At the anode Cl 2 2e -

Negative Cathode Positive Anode D.C. current + - Na+ Cl- H+ OH- At the cathode 2e - H2H2

Negative Cathode Positive Anode D.C. current + - Na+ Cl- H+ OH- Where will the ions go? H2H2 Cl 2 2e -

Products The three main products are: Hydrogen gas Chlorine gas Sodium hydroxide solution Cl 2 H2H2 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 - H 2 At the positive electrode: chlorine gas is produced 2Cl - - 2e - Cl 2

At anode2Cl - (aq)  Cl 2 (g) + 2e - At cathode2H + (aq) + 2e -  H 2 (g) In solutionNa + 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) Cu e - Cu At the positive electrode: oxygen gas is produced (from the water solvent) 4OH - O 2 + 2H 2 O + 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.

At anode2H 2 O (l)  4H + (aq) + O 2 (g) + 2e - At cathodeCu 2+ (aq) + 2e -  Cu (s)

Electrolysis of Aluminium Cryolite molten Aluminium oxide has a very high melting point. It melts at 2050 o C. So, chemists mix another ionic compound called Cryolite with the aluminium oxide. The molten mixture can now be electrolysed at o C.

Negative Electrode (Cathode) Positive Electrode (Anode) Al 3+ Al O 2- OO

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 (NH 3 ) dissolves in water to form an alkali solution, NH 3 (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 Strong acid NeutralStrong alkali Weak acidWeak alkali More alkaliMore acidMore OH- ionsMore H+ ions

H+ (aq) + OH- (aq)  H 2 O (l) Salt + Water Acid + Alkali  In neutralisation reactions, hydrogen ions react with hydroxide ions to produce water. nitric + potassium acid hydroxide potassium + nitrate Water

Acids = H + ions Alkalis = OH - ions Alkalis = soluble bases

H+ (aq) + OH- (aq)  H 2 O (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

AcidFormulaSaltExample HydrochloricHClChlorideSodium chloride SulphuricH 2 SO 4 SulphateCopper sulphate NitricHNO 3 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)  H 2 O (l) Salt + Water Hydrochloric + Sodium acid hydroxide Sodium + Chloride Water Acid + Alkali 

Salts solutions (aq) can be crystallised to produce solid salts (s)

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 -  H 2 O

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 NH 4 +, barium and those of group 1 elements All insoluble except when with NH 4 + and group 1 elements All insoluble except for calcium, barium and group 1

An example precipitation reaction Silver + Sodium nitrate chloride Sodium + nitrate Silver chloride AgNO 3 (aq) + NaCl(aq) → NaNO 3 (aq) +AgCl(s)

Precipitation reactions are used to remove unwanted ions from solutions, for example to treat drinking water or treating effluent

Solutions: solution(aq) + solution(aq)  precipitate(s) + solution(aq) Solid precipitate is filtered off and dried