ELECTROLYSIS Level 4 Chemistry KNOCKHARDY PUBLISHING

ELECTROLYSISELECTROLYSISLevel 4 ChemistryLevel 4 Chemistry

KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING

ELECTROLYSISELECTROLYSIS

INTRODUCTION

This Powerpoint show is one of several produced to help students understand selected National 4 Chemistry topics. It is based on the requirements of the SQA specification.

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 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.

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 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




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

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-

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

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-



SOLID

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-

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





Na+ + e- Na





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



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

ELECTROLYSIS OF…ELECTROLYSIS OF…

MOLTEN LEAD BROMIDE


MOLTEN LEAD BROMIDE

Br-Br-

Pb2+

CATHODE

ANODE


MOLTEN LEAD BROMIDE

Br-Br-

Br Br

Pb2+

Pb

CATHODE Pb2+ + 2e- Pb REDUCTION

ANODE 2Br- Br2 + 2e- OXIDATION


DILUTE SULPHURIC ACID



SO42-

CATHODE

ANODE



SO42-

CATHODE 4H+ + 4e- 2H2 REDUCTION

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

OO

H H

H H



SO42-

CATHODE 4H+ + 4e- 2H2 REDUCTION


OO

H H

H H

This electrolysis can be used to

deduce the formula of water


deduce the formula of water


AQUEOUS COPPER SULPHATE (with carbon electrodes)



Cu2+

SO42-

CATHODE

ANODE



Cu2+

SO42-

CATHODE 2Cu2+ + 4e- 2Cu REDUCTION


OOCu


AQUEOUS COPPER SULPHATE (with copper anode)

A different reaction takes place if the anode

is made of copper



Cu2+

SO42-

CATHODE

ANODE

Cu



Cu2+

SO42-

CATHODE Cu2+ + 2e- Cu REDUCTION

ANODE Cu Cu2+ + 2e- OXIDATION

Cu

Cu



Cu2+

SO42-

CATHODE Cu2+ + 2e- Cu REDUCTION

ANODE Cu Cu2+ + 2e- OXIDATION

Cu

Cu


purify copper or copper plate

objects


purify copper or copper plate

objects


DILUTE HYDROCHLORIC ACID



CATHODE

ANODE

Cl-



CATHODE 2H+ + 2e- H2 REDUCTION

ANODE 2Cl- Cl2 + 2e- OXIDATION

H H

Cl-

Cl Cl

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

© 2011 © 2011 JONATHAN HOPTON & KNOCKHARDY PUBLISHINGJONATHAN HOPTON & KNOCKHARDY PUBLISHING


THE ENDTHE END

Documents

ELECTROLYSIS Level 4 Chemistry KNOCKHARDY PUBLISHING