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

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

NegativeCathode

PositiveAnode

D.C.current

+ -

Na+

Na+Cl-

Cl-H+

H+

OH-

OH-

Where will the ions go?

NegativeCathode

PositiveAnode

D.C.current

+ -

Na+

Na+Cl-

Cl-H+

H+

OH-

OH-

At the anode

Cl22e-

NegativeCathode

PositiveAnode

D.C.current

+ -

Na+

Na+Cl-

Cl-H+

H+

OH-

OH-

At the cathode

2e-

H2

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)

Electrolysing aluminiumElectrolysing aluminium

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.

Negative Electrode (Cathode)

Positive Electrode (Anode)

Al3+

Al3+Al3+

Al Al Al

O2-O2-

O O

• 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

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

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

7.3 Salts – solutions7.3 Salts – solutions

Solutions: solution(aq) + solution(aq)

precipitate(s) + solution(aq)

Solid precipitate is filtered off and dried