IGCSE Chemistry Physical Chemistry Energetics

1. IGCSE CHEMISTRY LESSON 4

2. Section 1
Principles
of
Chemistry
a) States of matter
b) Atoms
c) Atomic structure
d) Relative formula mass
e) Chemical formulae and
chemical equations
f) Ionic compounds
g) Covalent substances
h) Metallic crystals
i) Electrolysis

3. 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.

4. 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’.

5. 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.

6. Metal properties
Property Explanation
Metals are good conductors
of heat and electricity
Delocalised electrons can
move through the lattice,
carrying negative charge

7. Metal properties
Property 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.

8. 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.

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

10. 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.

11. 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?

13. Remember our
definition of the term
“electric current”?

14. 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.

15. 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.

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

17. 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

18. 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?

19. So, what is
electrolysis?

20. Electrolysis
means “splitting
up with
electricity”

21. It requires an
electrolyte, a
liquid which will
conduct
electriticty

22. Electrolytes
conduct electricity,
non-electrolytes do
not.

23. Electrolyte or non-electrolyte?

24. Electrolyte or non-electrolyte?

25. Electrolytes
are usually
free ions
dissolved in
water, eg.
NaCl solution
Cl- Cl-
Cl-
Na+
Na+
Na+

26. 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+

28. Cathode (-ve) Anode (+ve)
-
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-
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+

29. Cathode (-ve) Anode (+ve)
-
-
-
-
-
-
-
-
-
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-
-
-
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-
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-
-
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-
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
The electrical supply acts
like an electron pump. It
takes electrons away from
the +ve anode and onto the
–ve cathode.

30. 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.

31. Cathode (-ve) Anode (+ve)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
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-
-
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
In sodium chloride solution
(brine) there are the
following ions:
Na+ H+ Cl- OH-

32. 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

33. Cathode (-ve) Anode (+ve)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
H+
H+
H+
Cl-
Cl-
Na+
Na+
Na+
Na+

34. Cathode (-ve) Anode (+ve)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
H
H+
H Cl Cl
Na+
Na+
Na+
Na+

35. 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.

36. 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

37. Half-equations
These explain the reactions
happening at each electrode.

38. Half-equations
At the cathode
(-ve)
At the anode
(+ve)
2H+
(aq) + 2e-
 H2 (g)
2Cl-
(aq) 
Cl2 (g) + 2e-

39. 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.

40. 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

41. Other examples of electrolysis
Eg. 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-

42. Other examples of electrolysis
Eg. Electrolysis of dilute sulphuric acid
http://www.docbrown.info/page01/ExIndChem/ExtraElectrochem.htm

43. Other examples of electrolysis
Eg. 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-

44. End of Lesson 4
In this lesson we have covered:
Metallic crystals
Electrolysis