gelectrolpps

ELECTROLYSISELECTROLYSISA guide for GCSE studentsA guide for GCSE students

2010 2010 SPECIFICATIONSSPECIFICATIONSKNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING

ELECTROLYSISELECTROLYSIS

INTRODUCTION

This Powerpoint show is one of several produced to help students understand selected GCSE Chemistry topics. It is based on the requirements of the AQA specification but is suitable for other examination boards.

Individual students may use the material at home for revision purposes and it can also prove useful for classroom teaching with an interactive white board.

Accompanying notes on this, and the full range of AS and A2 Chemistry topics, are available from the KNOCKHARDY WEBSITE at...

www.knockhardy.org.uk

All diagrams, photographs and any animations in this Powerpoint are original and created by Jonathan Hopton. Permission must be obtained for their use in any work that is distributed for financial gain.

CONTENTSCONTENTS

• What is electrolysis?

• Electrolysis of ionic compounds

• Electrolysis of molten sodium chloride

• Electrolysis of aqueous sodium chloride (brine)

• Diaphragm cell

• Electrolysis of aluminium oxide

• Other electrolytic systems

• Electrolysis of water

Passing an electric current through ionic substances that are Passing an electric current through ionic substances that are molten or in or in aqueous solution, breaks them down into elements. solution, breaks them down into elements.

Passing an electric current through ionic substances that are molten or in aqueous solution, breaks them down into elements.

The process is known as ELECTROLYSIS

The substance broken down is called the ELECTROLYTE

During electrolysis…

Positively charged ions move to the negative electrode - CATHODE

Negatively charged ions move to the positive electrode - ANODE

During electrolysis…

Positively charged ions move to the negative electrode - CATHODE

Negatively charged ions move to the positive electrode - ANODE

CATHODE +ive ions gain electrons REDUCTION

ANODE - ive ions give up (lose) electrons OXIDATION

ELECTROLYSIS OF ELECTROLYSIS OF IONIC COMPOUNDSIONIC COMPOUNDS

CONDUCTING ELECTRICITYCONDUCTING ELECTRICITY

For a substance to conduct electricity it must have either…

electrons which are free to move about (metals / graphite)

or ions which are free to move about

When an ionic substance is melted or dissolved in water,the ions are free to move about within the liquid or solution.

ELECTROLYSIS OF IONIC COMPOUNDSELECTROLYSIS OF IONIC COMPOUNDS

Na+Cl- Na+Cl-

Na+Cl-Na+ Cl-

Na+Cl- Na+Cl-

WHEN SOLID, THE IONS ARE NOT FREE TO MOVE

NO CONDUCTIONOF ELECTRICITY

SOLID IONIC COMPOUNDS DON’T CONDUCT ELECTRICITYSOLID IONIC COMPOUNDS DON’T CONDUCT ELECTRICITY

Na+Cl- Na+Cl-

Na+Cl-Na+ Cl-

Na+Cl- Na+Cl-

Na+ Cl-

Na+Cl-

SOLID MOLTEN

WHEN MOLTEN, THE IONS ARE FREE TO MOVE

ELECTRICITY IS CONDUCTED

MOLTEN IONIC COMPOUNDS DO CONDUCT ELECTRICITYMOLTEN IONIC COMPOUNDS DO CONDUCT ELECTRICITY

Na+Cl- Na+Cl-

Na+Cl-Na+ Cl-

Na+Cl- Na+Cl-

SOLID IONIC COMPOUNDS DON’T CONDUCT ELECTRICITYSOLID IONIC COMPOUNDS DON’T CONDUCT ELECTRICITY

Na+Cl- Na+Cl-

Na+Cl-Na+ Cl-

Na+Cl- Na+Cl-

AQUEOUS SOLUTIONS OF IONIC COMPOUNDS DO CONDUCT ELECTRICITYAQUEOUS SOLUTIONS OF IONIC COMPOUNDS DO CONDUCT ELECTRICITY

Add to water

Na+Cl- Na+Cl-

Na+Cl-Na+ Cl-

Na+Cl- Na+Cl-

Na+ Cl-

Na+Cl-

SOLID IN AQUEOUS SOLUTION

WHEN DISSOLVED IN WATER, THE IONS ARE

FREE TO MOVE

ELECTRICITY IS CONDUCTED

AQUEOUS SOLUTIONS OF IONIC COMPOUNDS DO CONDUCT ELECTRICITYAQUEOUS SOLUTIONS OF IONIC COMPOUNDS DO CONDUCT ELECTRICITY

ELECTROLYSIS OF ELECTROLYSIS OF MOLTEN SODIUM MOLTEN SODIUM

CHLORIDECHLORIDE

ELECTROLYSIS OF SODIUM CHLORIDEELECTROLYSIS OF SODIUM CHLORIDE

DOWNS CELLDOWNS CELL

SOLID SODIUM CHLORIDE DOES NOT CONDUCT ELECTRICITY BECAUSE THE IONS ARE NOT FREE TO MOVE TO THE ELECTRODES

WHEN MOLTEN, THE IONS HAVE MORE FREEDOM AND CAN MOVE

SODIUM IONS GO TO THE CATHODE AND EACH PICK UP AN ELECTRON

Na+ + e- Na

CHLORIDE IONS GO TO THE ANODE AND EACH GIVE UP AN ELECTRON

Cl- Cl + e-

2Cl- Cl2 + 2e-

Na+ + e- Na

Cl- Cl + e-

2Cl- Cl2 + 2e-

Na+ + e- Na

Cl- Cl + e-

2Cl- Cl2 + 2e-

SUMMARY

ELECTROLYSIS OF ELECTROLYSIS OF AQUEOUS SODIUM AQUEOUS SODIUM CHLORIDE CHLORIDE (BRINE)(BRINE)

ELECTROLYSIS OF BRINEELECTROLYSIS OF BRINE

WHEN SODIUM CHLORIDE DISSOLVES IN WATER, THE IONS SEPARATE

WATER CONTAINS A FEW IONS

POSITIVE IONS ARE ATTRACTED TO THE NEGATIVE CATHODE...

POSITIVE IONS ARE ATTRACTED TO THE NEGATIVE CATHODE...OF THE TWO, HYDROGEN IONS ARE PREFERRED

NEGATIVE IONS ARE ATTRACTED TO THE POSITIVE ANODE...

NEGATIVE IONS ARE ATTRACTED TO THE POSITIVE ANODE...OF THE TWO, CHLORIDE IONS ARE PREFERRED

AT THE NEGATIVE CATHODEHYDROGEN IONS PICK UP AN ELECTRON AND BECOME ATOMS

H+ + e- H

2H+ + 2e- H2

The atoms form molecules and hydrogen gas H2 is produced

H+ + e- H

2H+ + 2e- H2

AT THE POSITIVE ANODECHLORIDE IONS GIVE UP AN ELECTRON AND BECOME ATOMS

Cl- Cl + e-

H+ + e- H

2H+ + 2e- H2

Cl- Cl + e-

2Cl- Cl2 + 2e-

The atoms form molecules and chlorine gas Cl2 is produced

SODIUM HYDROXIDE SOLUTION REMAINS

H+ + e- H

2H+ + 2e- H2

Cl- Cl + e-

2Cl- Cl2 + 2e-

SODIUM HYDROXIDE SOLUTION REMAINS

H+ + e- H

2H+ + 2e- H2

Cl- Cl + e-

2Cl- Cl2 + 2e-

H+ + e- H

2H+ + 2e- H2

Cl- Cl + e-

2Cl- Cl2 + 2e- SODIUM HYDROXIDE SOLUTION REMAINS

SUMMARY

INDUSTRIAL INDUSTRIAL ELECTROLYSISELECTROLYSIS

OF BRINEOF BRINE

SODIUM CHLORIDESODIUM CHLORIDE

CHLORINE GAS

THE DIAPHRAGM CELL

HYDROGEN GAS

SODIUM HYDROXIDESOLUTION(CAUSTIC SODA)

DIAPHRAGMSTOPS CHLORINE MIXING WITH SODIUM HYDROXIDE

THE DIAPHRAGM CELLTHE DIAPHRAGM CELL

IMPORTANT ASPECTS

WHAT IS AN ADVANTAGE OF ELECTROLYSING BRINE RATHER THAN MOLTEN

SODIUM CHLORIDE?

CHLORINE HYDROGEN

SODIUMHYDROXIDE(CAUSTIC SODA)

DIAPHRAGM

IMPORTANT ASPECTS

SODIUM CHLORIDE?

CHLORINE HYDROGEN

DIAPHRAGM

LESS ENERGY IS NEEDED BECAUSE YOU DON’T HAVE TO MELT THE SODIUM CHLORIDE; IT HAS A HIGH MELTING POINT

IMPORTANT ASPECTS

WHAT HAPPENS AT THE POSITIVE ANODE?

CHLORINE HYDROGEN

DIAPHRAGM

IMPORTANT ASPECTS

WHAT HAPPENS AT THE POSITIVE ANODE?

CHLORINE HYDROGEN

DIAPHRAGM

CHLORIDE IONS ARE OXIDISED

Cl¯ – e¯ Cl

2Cl¯ – 2e¯ Cl2

CHLORIDE IONS ARE OXIDISED

Cl¯ – e¯ Cl

2Cl¯ – 2e¯ Cl2

IMPORTANT ASPECTS

WHAT HAPPENS AT THENEGATIVE CATHODE?

CHLORINE HYDROGEN

DIAPHRAGM

IMPORTANT ASPECTS

WHAT HAPPENS AT THENEGATIVE CATHODE?

CHLORINE HYDROGEN

DIAPHRAGM

HYDROGEN IONS ARE REDUCED

H+ + e¯ H

2H+ + 2e¯ H2

HYDROGEN IONS ARE REDUCED

H+ + e¯ H

2H+ + 2e¯ H2

IMPORTANT ASPECTS

WHAT IS THEDIAPHRAGM FOR?

CHLORINE HYDROGEN

DIAPHRAGM

IMPORTANT ASPECTS

WHAT IS THEDIAPHRAGM FOR?

CHLORINE HYDROGEN

DIAPHRAGM

PREVENTS CHLORINE COMING INTO CONTACT WITH ANY SODIUM

HYDROXIDE

PREVENTS CHLORINE COMING INTO CONTACT WITH ANY SODIUM

HYDROXIDE

IMPORTANT ASPECTS

WHAT IS ALSOPRODUCED?

CHLORINE HYDROGEN

DIAPHRAGM

IMPORTANT ASPECTS

WHAT IS ALSOPRODUCED?

CHLORINE HYDROGEN

DIAPHRAGM

THE SOLUTION CONTAINS SODIUM IONS AND HYDROXIDE IONS SO

SODIUM HYDROXIDE IS PRODUCED

THE SOLUTION CONTAINS SODIUM IONS AND HYDROXIDE IONS SO

SODIUM HYDROXIDE IS PRODUCED

INDUSTRIAL IMPORTANCE

The products of the electrolysis of brine are of great industrial importance…

CHLORINE making bleachmaking plastics such as pvc

HYDROGEN used as fuelused in margarine production

SODIUM production of soaps and detergents HYDROXIDE used in the paper industry

EXTRACTION OF EXTRACTION OF ALUMINIUM BY ALUMINIUM BY ELECTROLYSISELECTROLYSIS

EXTRACTION OF ALUMINIUMEXTRACTION OF ALUMINIUM

Unlike iron, aluminium cannot be extracted using carbon.(Aluminium is above carbon in the reactivity series)

Reactive metals are extracted using electrolysis

Electrolysis is expensive - it requires a lot of energy…

- ore must be molten (ores have high melting points)

- electricity is needed for the electrolysis process

RAW MATERIALSRAW MATERIALS

BAUXITE aluminium ore

Bauxite contains alumina (Al2O3 aluminium oxide) plus impurities such as iron oxide – it is purified before use.

RAW MATERIALSRAW MATERIALS

BAUXITE aluminium ore

Bauxite contains alumina (Al2O3 aluminium oxide) plus impurities such as iron oxide – it is purified before use.

CRYOLITE Aluminium oxide has a veryhigh melting point.Adding cryolite lowers the

melting point and saves energy.

THE CELL CONSISTS OF A

CARBON ANODE

THE CELL CONSISTS OF A CARBON ANODE

STEEL CATHODE

CARBON LINING

THE CELL CONSISTS OF A CARBON LINED STEEL CATHODE

MOLTEN ALUMINA and

CRYOLITE

ALUMINA IS DISSOLVED IN MOLTEN CRYOLITE Na3AlF6

SAVES ENERGY - the mixture melts at a lower temperature

MOLTEN ALUMINA and

CRYOLITE

ALUMINA IS DISSOLVED IN MOLTEN CRYOLITE Na3AlF6

aluminium and oxide ions are now free to move

POSITIVE ALUMINIUM IONS ARE ATTRACTED

TO THE NEGATIVE CATHODE

Al3+ + 3e- Al

EACH ION PICKS UP 3 ELECTRONS AND IS DISCHARGED

CARBON CATHODE

O2- O + 2e-

NEGATIVE OXIDE IONS ARE

ATTRACTED TO THE POSITIVE

EACH ION GIVES UP 2 ELECTRONS AND IS DISCHARGED

CARBON ANODE

ELECTRONS

CARBON ANODE

CARBON CATHODE

ELECTRONS

ANODE 3O2- 1½O2 + 6e- OXIDATION

OXIDATION (LOSS OF ELECTRONS) TAKES PLACE

AT THE ANODECARBON ANODE

ELECTRONS

CATHODE 2Al3+ + 6e- 2Al REDUCTION

OXIDATION (LOSS OF ELECTRONS) TAKES PLACE

AT THE ANODE

REDUCTION (GAIN OF ELECTRONS) TAKES PLACE AT

THE CATHODE CARBON CATHODE

ELECTRONSOXIDATION (LOSS OF

ELECTRONS) TAKES PLACE AT THE ANODE

REDUCTION (GAIN OF ELECTRONS) TAKES PLACE AT

THE CATHODE

CARBON ANODE

CARBON CATHODE

CATHODE 2Al3+ + 6e- 2Al REDUCTION

CARBON ANODE

PROBLEMTHE CARBON

ANODES REACT WITH THE

OXYGEN TO PRODUCE

CARBON DIOXIDE

CARBON ANODE

PROBLEMTHE CARBON

ANODES REACT WITH THE

OXYGEN TO PRODUCE

CARBON DIOXIDE

THE ANODES HAVE TO BE REPLACED AT REGULAR INTERVALS, THUS ADDING TO THE

COST OF THE EXTRACTION PROCESS

CARBON DIOXIDE

USES OF ALUMINIUMUSES OF ALUMINIUM

THE IMPORTANCE OF ALUMINIUM

LOW DENSITY AND OVERHEAD CABLESELECTRICAL CONDUCTIVITY

LOW DENSITY AIRCRAFT BODIESneeds to be an alloyed withother metals for extra strength

GOOD HEAT CONDUCTIVITY SAUCEPANS

Aluminium is not as reactive as one would predict from its position in the reactivity series.

This is because a thin layer of oxide quickly forms on the surface and prevents any further reaction taking place.

THIN LAYER OF OXIDE

Aluminium is not as reactive as one would predict from its position in the reactivity series.

This is because a thin layer of oxide quickly forms on the surface and prevents any further reaction taking place.

ANODISING puts on a controlled layer of oxide so the metal can be used for household items such as pans and electrical goods.

THIN LAYER OF OXIDE

ANODISING

This puts on a controlled layer of oxide so that the metal can be used for household items such as pans and electrical goods.

The aluminium object is made the anode in the electrolysis of dilute sulphuric acid.

Oxygen given of at the anode reacts with the aluminium surface, to build up a very thin film of oxide.

aluminium object to be

anodised

dilute sulphuric

cathode

ELECTROLYSIS OF…ELECTROLYSIS OF…

MOLTEN LEAD BROMIDE

Br-Br-

CATHODE

MOLTEN LEAD BROMIDE

Br-Br-

CATHODE Pb2+ + 2e- Pb REDUCTION

ANODE 2Br- Br2 + 2e- OXIDATION

DILUTE SULPHURIC ACID

CATHODE

CATHODE 4H+ + 4e- 2H2 REDUCTION

ANODE 4OH- 2H2O + O2 + 4e- OXIDATION

CATHODE 4H+ + 4e- 2H2 REDUCTION

This electrolysis can be used to

deduce the formula of water

AQUEOUS COPPER SULPHATE (with carbon electrodes)

CATHODE

CATHODE 2Cu2+ + 4e- 2Cu REDUCTION

AQUEOUS COPPER SULPHATE (with copper anode)

A different reaction takes place if the anode

is made of copper

CATHODE

CATHODE Cu2+ + 2e- Cu REDUCTION

ANODE Cu Cu2+ + 2e- OXIDATION

CATHODE Cu2+ + 2e- Cu REDUCTION

ANODE Cu Cu2+ + 2e- OXIDATION

purify copper or copper plate

objects

purify copper or copper plate

objects

DILUTE HYDROCHLORIC ACID

CATHODE

CATHODE 2H+ + 2e- H2 REDUCTION

ANODE 2Cl- Cl2 + 2e- OXIDATION

COMPOSITION OF WATERCOMPOSITION OF WATER

WATER IS AN EXTREMELY POOR CONDUCTOR OFELECTRICITY. Dilute sulphuric acid must be added toimprove Its conductivity.

Fill both limbs of a Hoffman Voltameter with water, (acidified with a small amount of dilute sulphuric acid).Pass a direct current through the apparatus.

ANODE (+) - a colourless gas collected - the gas re-lit a glowing splint

CATHODE (-) - a colourless gas collected - its volume was twice that at anode- gas exploded with a squeaky pop when a lighted splint was applied

Conclusion Water can be split into its elementsby passing electricity through it.

water ——> hydrogen (2 vols) + oxygen (1 vol).

HOFFMANVOLTAMETER

THE ENDTHE END

gelectrolpps

sodium ions

electrolysis electrolyte

molten ionic compounds

ionic substances

electrons ive ions

electron na

electric current

aqueous solutionna

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