Electrolysis

Electrolysis of Molten substances

Ionic compound

Positive ion Negative ion

Sodium Chloride

NaCl

Lead (ll) Oxide

PbO

Magnesium Chloride

MgCl2

Zinc Bromide

ZnBr2

Ionic compound

Positive ion Negative ion

Sodium Chloride

NaCl

Na+

Cl-

Lead (ll) Oxide

PbO2

Pb2+

O2-

Magnesium Chloride

MgCl2

Mg2+

Cl-

Zinc Bromide

ZnBr2

Zn2+

Br-

Which is electrolyte and non-electrolyte?

• Sodium Chloride

• Lead (ll) Oxide

• Magnesium Chloride

• Zinc Bromide

These ionic compounds are electrolytes

The term electrolysis was introduced by Michael Faraday.

• ‘Lysis’ means loosening in Greek.

• Thus electrolysis means ‘loosening by electricity’.

Electrolysis

Electrolysis is a process occurs when electricity is pass through a solution (electrolyte) and cause it to decompose into its constituent elements.

Electrolytes are substances able to conduct electricity in [ ] state or [ ] state and undergo [ ] change.

Electrolysis is a process where the [ ] are broken down into its [ ] elements by passing [ ] through it.

electrolytes molten chemical

liquid electricity constituent

Electrolytes are substances able to conduct electricity in [molten] state or [liquid] state and undergo [chemical] change.

Electrolysis is a process where the [electrolytes] are broken down into its [constituent] elements by passing [electricity] through it.

Electrolytic Cell

The electrolytic cell is the set of apparatus needed to conduct electrolysis.

Electrolytic cell consists of

•a battery connected to a switch in series with

•2 metal plates called electrodes. Electrodes will carry electricity to the electrolyte.

Electrolytic Cell

Battery

Electrolyte

Metal Plates (Electrodes)

ammeter

a)Electrode connected to the positive terminal of an electric source is called anode.

b)Electrode connected to the negative terminal of an electric source is called cathode.

Anode (+) Cathode (-)

• When electric current is allowed to pass through, the electrolytes which are ionic compound will split up into positive (+) and negative (-)ions.

• Example:

NaCl Na+ + Cl-

Ionic compound +ve ion -ve ion

• Ions will migrate to the oppositely charged electrodes.

• Negatively charged (-) ions will move to the positively charged anode (+).

Therefore called anions.

• Positively charged (+) ions will move to the negatively charged cathode (-) and are called cations.

• At the anode, the negative ions will lose their electrons to the anode.

• Anode will readily accepts electrons because it is positively charged.

ANODE

CATHODE

• At the cathode, the positively charged cations will accept electrons from the cathode.

• This discharging of ions at the electrode results in chemical decomposition of the electrolyte.

• The movement of ions allows electricity to be conducted.

When describing electrolysis, state:

• The ions present in the electrolyte

• The movement of ions to the anode and cathode

• Half equation at the anode and cathode

Electrolysis of Molten Lead (ll) Bromide

PbBr2

• Lead (ll) bromide is an ionic compound electrolyte.

• PbBr2 consist of lead (ll) cations, Pb2+ and bromide anions, Br-.

• In solid lead (ll) bromide, these ions do not move freely but are held in fixed positions in lattice.

• It can only can conduct electricity when dissolved in water or in molten state.

Molten lead (ll) bromide

Crucible

Electrolysis of molten lead (ll) bromide

• A bulb is connected to the circuit to show when the electricity is flowing.

• Carbon electrodes are used because it will not react with the products formed.

• When a current is passed through the molten lead bromide, lead (ll) cations, Pb2+ and bromide anions, Br- are formed.

•Bromide anions which are negatively charged are attracted to the positive anode.

•Lead (ll) cations which are positively charged are attracted to the negative cathode.

ANODE

•Bromide ions will

move towards anode.

•At anode, each

bromide ion will

release an electron to

form a neutral bromine

atom.

•Two bromine atoms

combine to form

bromine gas.

•Thus, bromine gas

evolves at the anode.

• Lead (ll) cations

attracted to

cathode.

• At the cathode,

each lead (ll) ion

accept 2

electrons to form

lead atom.

• Hence, silvery

lead globule

deposited.

CATHODE

Pb2+, Br-

Br- Pb2+

release accept

1 2

bromine gas evolves

silvery grey globule

Half Equation

Half equation is equation used to show the reaction occur at the anode or cathode.

• Bromide ion, Br- releases an electron to form a bromine atom, Br.

Br- Br + e-

• Two bromine atoms combine to form a bromine gas, Br2 molecule.

Br + Br Br2

Half equation:

2 Br- (l) Br2 (g) + 2e-

ANODE

• lead (ll) ion, Pb2+ accept 2 electrons to form lead atom.

Half equation:

Pb2+ (l) + 2e- Pb (s)

CATHODE

Combining the two half equations, we get the overall equation that represent the electrolysis of molten lead (ll) bromide

Anode: 2 Br-(l) Br2 (g) + 2e-

Cathode: Pb2+ (l) + 2e- Pb (s)

Pb2+ (l) + 2 Br- (l) Pb (s) + Br2 (g)

Electrolysis of Molten Lead (ll) Oxide

PbO

Pb2+, O2-

O2- Pb2+

release accept

2 2

oxygen gas evolves

silvery grey globule

• The Pb2+ ions move towards the cathode

• The O2- ions move towards the anode

• At cathode,

a lead (ll) ion accepts two electrons to form a lead atom.

• At anode,

an oxide ion releases two electrons to form an oxygen atom.

Two oxygen atoms combine to form one molecule of oxygen gas.

Half Equation

ANODE

Anode: 2 O-(l) O2 (g) + 2e-

CATHODE

Cathode: Pb2+ (l) + 2e- Pb (s)

Pb2+ (l) + 2 O- (l) Pb (s) + O2 (g)

Overall Equation

Web Link

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