ELECTROLYSIS

Module C2

Splitting up ionic compounds (F)

Molten compounds

Ionic solutions & discharge rules

Q = It and OILRIG

Brine and purifying copper

+

-

+-+

- +

-+-

+-

+ +

-

Cl- ION

Na+ ION

SPLITTING UP IONIC COMPOUNDS 1

Ionic compounds (eg sodium chloride) are made from:

POSTIVE IONS (atoms which LOST negative electrons)

NEGATIVE IONS (atoms which GAINED negative electrons)

As these ions have OPPOSITE CHARGES they attract each other strongly to form IONIC BONDS

SPLITTING UP IONIC COMPOUNDS 2

2 ways to split up the ions:

-+ +- + -

-+ +

- + -

+

-

+-+

- +

-+-

+-

+ +

-

MELT

+ --

++ --+

- +

DISSOLVE

H2O800°C 20°C

SEPARATING THE IONS 1

+ _

Battery pulls electrons off one

electrode and pushes them onto the other

Metal ELECTRODEELECTRON

This IS SHORT OF electrons so becomes

POSITIVELY CHARGED “ANODE”

This HAS EXTRA electrons so becomes

NEGATIVELY CHARGED “CATHODE”

+

+ -+

- +

-

-

+

--

+ +

-

+

MOLTEN IONIC COMPOUND

+ ANODE - CATHODE

When the battery is switched on,

the + IONS move to the – CATHODE

the – IONS move to the + ANODE

SEPARATING THE IONS 2

This gives a way to SPLIT UP IONIC COMPOUNDS: “ELECTROLYSIS”

Example 1: Splitting up MOLTEN SODIUM CHLORIDE (salt)

+

Cl-

Cl-

Cl-

Cl-

ClCl

Cl Cl

- = chloride ION, extra 1 electronCl- Cl

chlorine ATOM, NEUTRAL

Cl Cl Cl2

molecule

- chloride IONS lose their extra electrons and turn into neutral chlorine ATOMS

At ANODE: Cl- e- + Cl

then: Cl + Cl Cl2 (gas)

Both together:

2Cl- → 2e- + Cl2

Na+

Example 1: Splitting up MOLTEN SODIUM CHLORIDE (salt)

+

At CATHODE: Na+ + e- Na

Na+

Na+ Na+

Na

NaNa

Na

= sodium ION, missing1 electron

sodium ATOM, NEUTRALNa++ Na

+ sodium IONS gain an extra electron and turn into neutral sodium ATOMS

molten sodium metal sinks to

bottom

Example 1: Splitting up MOLTEN SODIUM CHLORIDE (salt)

MOLTEN SODIUM

CHLORIDE

+ ANODE - CATHODE

CHLORINE gas Cl2

SODIUM metal Na

At ANODE: Cl- e- + Cl

At CATHODE: Na+ + e- Na

Cl + Cl Cl2 (gas)

ELECTRONS

Cl- Na+

Example 2: Splitting up MOLTEN LEAD BROMIDE PbBr2

MOLTEN LEAD

BROMIDE

+ ANODE - CATHODE

BROMINE gas Br2

LEAD Metal Pb

At ANODE: Br- e- + Br

At CATHODE: Pb2+ + 2e- Pb

Br + Br Br2 (gas)

ELECTRONS

Br- Pb2+

Both together:

2Br- → 2e- + Br2

What happens when the ionic compounds are dissolved in water?

Here, water molecules break up into HYDROGEN IONS, H+ and HYDROXIDE IONS OH-

H2O H+ + OH-

So, in an ionic solution (eg sodium chloride solution), there will be FOUR types of ion present:

TWO from the ionic compound and TWO from the water (H+ + OH-)

SODIUM CHLORIDE SOLUTION NaCl (aq)

Cl-

Na+

OH-H+Na+

Cl-OH-H+

H+

OH-Cl-

Na+

IONIC SOLUTION

Cl-

Na+

OH- H+Na+

Cl-

OH-

H+

H+

OH-Cl- Na+

Which ions gain or lose electrons (“get discharged”) and which stay in solution?

IONIC SOLUTIONS: At the CATHODE

+

At CATHODE: 2H+ + 2e- H2

sodium ION, missing 1 electron

Hydrogen ATOM, NEUTRALNa+ H

Na+

Na+

H+

H+

H

H

H+

hydrogen ION, missing 1 electron

As HYDROGEN is LESS REACTIVE than SODIUM, it is discharged. The sodium ions stay in solution.

which ions?

+

Cl-

Cl-

Cl-

Cl-

ClCl

Cl Cl

chloride ION, extra 1 electronCl- Cl

chlorine ATOM, NEUTRAL

At ANODE: 2Cl- 2e- + Cl2

IONIC SOLUTIONS: At the CATHODE – halogen compounds

If the – ion is a HALOGEN (Cl, Br, I) it is discharged and chlorine (or Br or I) is given off and the

OH - ions stay in solution

OH

O HOH

O H

O Hhydroxide ION, from water extra electron

which ions?

+

nitrate ION, extra 1 electronNO3

-Oxygen atom

At CATHODE: 4OH- 2H2O + O2 + 4e-

OH

H

IONIC SOLUTIONS: CATHODE – non halogen compounds

NO3-

NO3-

O Hhydroxide ION, OH- from water, extra electron

O

NO3-

NO3-

OH

O H

O

If the – ion is NOT a halogen (eg nitrate, sulphate etc) then the HYDROXIDE ions from the water are discharged to make WATER and OXYGEN gas. The other ions stay in solution.

which ions?

+ ANODE

Attracts – ions (‘Anions’)

If – ions are HALOGENS ie

chloride Cl-

bromide Br-

iodide I-

the HALOGEN is produced.If – ions are NOT HALOGENS

Eg sulphate SO42-,

nitrate NO3-

carbonate CO32-

OXYGEN is produced.

- CATHODE

Attracts + ions (‘Cations’)

If + ions (metals) are MORE REACTIVE than hydrogen

K, Na, Ca, Mg, Zn, Fe

Then HYDROGEN is produced

If + ions (metals) are LESS REACTIVE than hydrogen

Cu, Ag, Au

Then the METAL is produced

RULES FOR IONIC SOLUTIONS

potassium chloride

Compound State Anode (+)Cathode (-)Ions

molten K+ Cl- potassium chlorine

aluminium oxide molten Al3+ O2- aluminium oxygen

copper chloride solution Cu2+ Cl- H+ OH-copper chlorine

sodium bromide solution Na+ Br- H+ OH- hydrogen bromine

silver nitrate solution Ag+ NO3- H+ OH- silver oxygen

potassium chloride solution K+ Cl- H+ OH- hydrogen chlorine

zinc sulphate solution Zn+ SO42- H+ OH- hydrogen oxygen

(REACTIVITY: K+ Na+ Ca2+ Mg2+ Al3+ Zn2+ Fe3+ H+ Cu2+ Ag+ Au3+ )

(REACTIVITY: K+ Na+ Ca2+ Mg2+ Al3+ Zn2+ Fe3+ H+ Cu2+ Ag+ Au3+ )

ELECTROLYSIS makes a CIRCUIT

Complete electric circuit:

Current carried by:

ELECTRONS in electrodes/wires

IONS in the electrolyte

To DOUBLE the MASS of substance discharged at electrodes:

2 x CURRENT (2x batt. voltage)

2 x TIME current flows for

(Q = I t)

-

+

+-

Cl-

Cl-

Cl-

Cl- Na+ Na+

Na+ Na+

OILRIG

- ions LOSING electrons to become atoms is called ‘OXIDATION’ (even though oxygen may not be involved)

+ ions GAINING electrons to become atoms is called ‘REDUCTION’

Oxidation is loss, reduction is gain

‘OILRIG’

INDUSTRIAL USES OF ELECTROLYSIS

1. To extract reactive metals such as ALUMINIUM, sodium, magnesium etc from their compounds. This is EXPENSIVE due to the large amounts of electrical energy needed. Aluminium is extracted from bauxite (Al2O3).

2. Electrolysis of BRINE (salt solution) to produce CHLORINE (for disinfectants and plastics)

HYDROGEN (for ammonia fertilisers, margarine) SODIUM HYDROXIDE (for soap and cleaning agents)

3. Purifying copper. The copper for wiring etc needs to be more pure than that produced in a blast furnace. Electrolysis is used to convert impure copper to pure copper

see below

see below

INDUSTRIAL ELECTROLYSIS OF BRINE

ANODEOH- and Cl-

2Cl- 2e- + Cl2

OH- left in solution so concentration grows

CATHODEH+ and Na+

2H+ + 2e- H2

Na+ left in solution so concentration grows

Chlorine gas Hydrogen gas

Sodium chloride solution (neutral) slowly changed to

sodium hydroxide solution (alkaline)

BRINE (NaCl solution)

Industrial chlorine production from electrolysis of brine

IMPURE COPPER ANODE

PURE COPPER CATHODECopper sulphate

CuSO4 solution

PURIFYING COPPER

Cu2+

Cu

CuCu2+

Cu2+Copper atoms from impure copper are OXIDISED to copper ions

Copper ions transported from anode to cathode

Copper ions from impure copper are REDUCED to copper atoms

As the atoms of the impurities are not transported, the copper that builds up on the anode is extremely pure.

IMPURE COPPER ANODE

PURE COPPER CATHODE

IMPURE COPPER ANODE

PURE COPPER CATHODE

Over time, the impure anode dissolves away and the impurities sink to the bottom. The pure cathode grows as more pure copper is deposited on it.

Why will the concentration of the solution stay the same?