C3.4 – Electrolysis and cells

Learning Objectives:

• To describe the properties of metals

• To explain the processes involved in electrolysis

• To describe the principle behind a dry cell

Name all the properties of metals that you can

Iron and copper are both __________ metals. They have __________ melting points, they are good conductors of __________ and electricity and they have a high __________. Transition metals have uses linked to their __________. For example, copper is __________ and so is used to make wires. Some transition metals are used as __________ , speeding up chemical reactions.

catalysts density ductileheat

high properties transition

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

To make ions move:

-+ +- + -

-+ +

- + -

+

-

+-+

- +

-+-

+-

+ +

-

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”

Define the terms:

•Electrolyte•Electrode•Anode •Cathode•Inert

Electrolysis Practical1. Select one of the test substances

and connect it into the circuit.2. Switch on the power supply. Does

the bulb light?3. Look carefully for any signs of

reaction. These might include:

• gases being given off• changes taking place on the

surface of the electrodes • changes taking place in the liquid.

+

+ -+

- +

-

-

+

--

+ +

-

+

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

1. Zinc atoms turn into ions, and electrons travel around the circuit.

2. When they reach the metals cap they travel down the positive pole.

3. The manganese dioxide absorbs the electrons.

4. There is an electrolyte (ammonium chloride in this case) which is kept damp and which the ions can travel through.

Recipe Time

Write a recipe

of the lesson (or of your learning)