IGCSE CHEMISTRY LESSON 4

Section 1

Principles of

Chemistry

a) States of matterb) Atomsc) Atomic structured) Relative formula masse) Chemical formulae and

chemical equationsf) Ionic compoundsg) Covalent substancesh) Metallic crystalsi) Electrolysis

Lesson 4

h) Metallic crystals

i) Electrolysis

1.37 describe a metal as a giant structure of positive ions surrounded by a sea of delocalised electrons.1.38 explain the malleability and electrical conductivity of a metal in terms of its structure and bonding.1.39 understand an electric current as a flow of electrons or ions.1.40 understand why covalent compounds do not conduct electricity.1.41 understand why ionic compounds conduct electricity only when molten or in solution.1.42 describe simple experiments to distinguish between electrolytes and non-electrolytes.1.43 recall that electrolysis involves the formation of new substances when ionic compounds conduct electricity.1.44 describe simple experiments for the electrolysis, using inert electrodes, of molten salts such as lead (II) bromide.1.45 write ionic half-equations representing the reactions at the electrodes during electrolysis.

Metallic crystals

++

+++ + + +

+ + ++ + ++

+ + + + +

Metal ions

Free electrons

Metals have a giant structure in which electrons in the highest energy level (orbit) are free to move through the whole structure.This effectively produces a regular arrangement (lattice) of metal ions in a ‘sea of electrons’.

Metallic crystals

++

+++ + + +

+ + ++ + ++

+ + + + +

Metal ions

Free electrons

These free electrons: Hold the atoms

together in a regular structure

Allow the atoms to slide over each other

Allow the metal to conduct heat and electricity.

Metal propertiesProperty Explanation

Metals are good conductors of heat and electricity

Delocalised electrons can move through the lattice, carrying negative charge

Metal propertiesProperty Explanation

Metals are good conductors of heat and electricity

Delocalised electrons can move through the lattice, carrying negative charge

Metals are malleable (can be bent or hammered into shape) and ductile (can be drawn into a wire.

Layers of positive metal ions can slide past each other without breaking metallic bonds. – there will always be delocalised electrons between the positive ions even when the position of ions is changed. Metallic bonds hold the metal ions together.

Lesson 4

h) Metallic crystals

i) Electrolysis

1.37 describe a metal as a giant structure of positive ions surrounded by a sea of delocalised electrons.1.38 explain the malleability and electrical conductivity of a metal in terms of its structure and bonding.1.39 understand an electric current as a flow of electrons or ions.1.40 understand why covalent compounds do not conduct electricity.1.41 understand why ionic compounds conduct electricity only when molten or in solution.1.42 describe simple experiments to distinguish between electrolytes and non-electrolytes.1.43 recall that electrolysis involves the formation of new substances when ionic compounds conduct electricity.1.44 describe simple experiments for the electrolysis, using inert electrodes, of molten salts such as lead (II) bromide.1.45 write ionic half-equations representing the reactions at the electrodes during electrolysis.

Back to a bit of basics here – what do we mean by the term “electric current”?

Back to a bit of basics here – what do we mean by the term “electric current”?

An electric current is a flow of electrons or ions. In effect, it is a flow of charge.

Back to a bit of basics here – what do we mean by the term “electric current”?

An electric current is a flow of electrons or ions. In effect, it is a flow of charge.So, why do covalent compounds not conduct electricity, but ionic compounds (when molten or in solution) do?

Remember our definition of the term “electric current”?

Remember our definition of the term “electric current”?

An electric current is a flow of electrons or ions. In effect, it is a flow of charge.

Remember our definition of the term “electric current”?

An electric current is a flow of electrons or ions. In effect, it is a flow of charge.

Ionic compounds conduct electricity (when in solution or molten) because they contain ions.

Ionic compounds conduct electricity (when in solution or molten) because they contain ions.

Covalent compounds do not conduct electricity because they do not contain ions

Ionic compounds conduct electricity (when in solution or molten) because they contain ions.

Covalent compounds do not conduct electricity because they do not contain ions

Simple, huh?

So, what is electrolysis

?

Electrolysis means “splitting

up with electricity”

It requires an electrolyte, a

liquid which will conduct

electriticty

Electrolytes conduct electricity, non-electrolytes do not.

Electrolyte or non-electrolyte?

Electrolyte or non-electrolyte?

Electrolytes are usually free ions

dissolved in water, eg.

NaCl solution

Cl- Cl-

Cl-

Na+

Na+

Na+

Electrolytes can also be molten ionic compounds

(but this involves higher

temperatures)

Cl- Cl-

Cl-

Na+ Na

+

Na+ Cl-

Cl-Cl-

Na+

Na+

Na+

Cl-

Na+

Cathode (-ve) Anode (+ve)

-----------

-----------

++++++++++++

++++++++++++

Cathode (-ve) Anode (+ve)

-----------

-----------

++++++++++++

++++++++++++

The electrical supply acts like an electron pump. It

takes electrons away from the +ve anode and onto

the –ve cathode.

Cathode (-ve) Anode (+ve)

-----------

-----------

++++++++++++

++++++++++++

The electrical supply acts like an electron pump. It

takes electrons away from the +ve anode and onto

the –ve cathode.

Ions gain or lose electrons at the electrodes and

neutral atoms and molecules are released.

Cathode (-ve) Anode (+ve)

-----------

-----------

++++++++++++

++++++++++++

In sodium chloride solution (brine) there are the

following ions:

Na+ H+ Cl- OH-

Cathode (-ve) Anode (+ve)

-----------

-----------

++++++++++++

++++++++++++

In sodium chloride solution (brine) there are the

following ions:

Na+ H+ Cl- OH- Look what happens when electricity flows through

the solution

Cathode (-ve) Anode (+ve)

-----------

-----------

++++++++++++

++++++++++++

H+

H+

H+

Cl-

Cl-

Na+

Na+

Na+

Na+

Cathode (-ve) Anode (+ve)

-----------

-----------

++++++++++++

++++++++++++

H

H+

H Cl Cl

Na+

Na+

Na+

Na+

Cathode (-ve) Anode (+ve)

-----------

-----------

++++++++++++

++++++++++++

H H Cl Cl

Na+

Na+

Na+

Na+

Each hydrogen ion gains

one electron to become a hydrogen

atom.

Each chlorine

ion loses an electron to become a chlorine

atom.

Cathode (-ve) Anode (+ve)

-----------

-----------

++++++++++++

++++++++++++

H H Cl Cl

Na+

Na+

Na+

Na+

+ve ions are called

cations because they are attracted

to the cathode

-ve ions are called

anions because they are attracted

to the anode

Half-equationsThese explain the reactions

happening at each electrode.

Half-equationsAt the cathode

(-ve)At the anode

(+ve)

2H+ (aq) + 2e-

H2 (g)

2Cl- (aq)

Cl2 (g) + 2e-

Lesson 4

h) Metallic crystals

i) Electrolysis

1.37 describe a metal as a giant structure of positive ions surrounded by a sea of delocalised electrons.1.38 explain the malleability and electrical conductivity of a metal in terms of its structure and bonding.1.39 understand an electric current as a flow of electrons or ions.1.40 understand why covalent compounds do not conduct electricity.1.41 understand why ionic compounds conduct electricity only when molten or in solution.1.42 describe simple experiments to distinguish between electrolytes and non-electrolytes.1.43 recall that electrolysis involves the formation of new substances when ionic compounds conduct electricity.1.44 describe simple experiments for the electrolysis, using inert electrodes, of molten salts such as lead (II) bromide.1.45 write ionic half-equations representing the reactions at the electrodes during electrolysis.

Half-equations

At the cathode (-ve)

At the anode (+ve)

Pb2+ (l) + 2e-

Pb (s)

2Br- (l)

Br2 (l) + 2e-

Electrolysis of molten lead bromide

Other examples of electrolysisEg. Electrolysis of copper sulphate solution

At the cathode (-ve) At the anode (+ve)

Electrode made of carbon Electrode made of carbon

Ions present: Cu 2+ H+ Ions present: SO4 2- OH-

Red deposit observed on electrode

Bubbles of colourless gas given off which relights a

glowing splint

2Cu 2+ (aq) + 4e- 2Cu 4OH- (aq) O2 (g) + 2H2O (l) + 4e-

Other examples of electrolysisEg. Electrolysis of dilute sulphuric acid

http://www.docbrown.info/page01/ExIndChem/ExtraElectrochem.htm

Other examples of electrolysisEg. Electrolysis of dilute sulphuric acid

At the cathode (-ve) At the anode (+ve)

Electrode made of carbon Electrode made of carbon

Ions present: H+ Ions present: SO4 2- OH-

Bubbles of gas (twice as much as at anode). Gas

burns with a squeaky ‘pop’

Bubbles of colourless gas given off which relights a

glowing splint

4H+ (aq) + 4e- 2H2 (g) 4OH- (aq) O2 (g) + 2H2O (l) + 4e-

End of Lesson 4

In this lesson we have covered:

Metallic crystals

Electrolysis