8.1
INVESTIGATING ELECTRICAL
CONDUCTIVITY OF COMPOUNDS
All pure metals conduct an electric current.
With the exception of carbon (in the form of graphite), all pure
non-metals do not conduct electricity.
6 V battery
_
+
+
light bulb
_
carbon (graphite)
rods
crocodile
clip
solid piece
under test
(a)
crucible
(b)
Figure 8.1 Investigating the electrical conductivity of compounds.
liquid(molten)/
aqueous
under test
(Non-conductors when solid,
but conductors when molten
or in aqueous solution)
Constituent elements
(Non-conductors
whether solid, liquid or
in aqueous solution)
Constituent elements
Sodium chloride
sodium, chlorine
Wax
carbon, hydrogen
Lead(II) bromide
lead, bromine
Sugar
carbon, hydrogen,
oxygen
Potassium iodide
potassium, iodine
Polythene (a plastic)
carbon, hydrogen
Calcium nitrate
calcium, nitrogen,
oxygen
Distilled water
hydrogen, oxygen
Magnesium sulphate
magnesium, sulphur,
oxygen
Ethanol (an alcohol)
carbon, hydrogen,
oxygen
Copper(II) sulphate
copper, sulphur,
oxygen
Hexane
carbon, hydrogen
Potassium permanganate
potassium,
manganese, oxygen
Tetrachloromethane
chlorine, carbon
Sodium hydroxide
sodium, hydrogen,
oxygen
1,1,1-trichloroethane
chlorine, carbon,
hydrogen
Table 8.1 Electrical conductivity and constituent elements of some compounds.
Ionic compounds
ELECTROLYTES are compounds which allow electricity to pass
through them only when molten or in aqueous solution.
1
Compounds
(do not conduct
electricity)
(conduct electricity)
Electrolyte
2
Compounds
solid, liquid or in
aqueous solution
(do not conduct
electricity)
Non-Electrolyte
• Why ionic compound is an electrolyte ?
• How do ionic compound conduct electricity
in molten or aqueous state?
Solid lead(II) bromide
does not conduct electricity
Molten lead(II) bromide
conducts electricity
6 V battery
_
+
Glass tubing (connected to
a rubber tubing which leads
to the fume cupboard)
light bulb
clamp
crocodile
clip
boiling tube
negative electrode
(cathode)
positive
electrode
(anode)
molten lead(II)
bromide
heat
Thus electric current is carried through molten lead(II) bromide by
mobile (freely moving) ions.
At positive electrode
(anode)
Reddish brown
bromine vapour is
liberated
At negative electrode
(cathode)
A silvery grey
substance (lead)
is deposited
ELECTROLYTES AND IONS
Electrolytes
ELECTROLYTES are compounds which allow electricity to pass
through them only when molten or in aqueous solution.
ELECTROLYTES are compounds which allow electricity to pass
through them only when molten or in aqueous solution. There is
chemical decomposition during the conduction.
Molten lead(II) bromide is thus decomposed to the two elements
lead and bromine.
The whole process is called
'decomposition by electricity').
electrolysis
(meaning
Lead(II) bromide does not conduct electricity in solid state. This is
because ions in solids are not mobile (i.e. unable to move
freely).
Pb2+
Br -
Pb2+
Br -
Pb2+
Br -
Br -
Pb2+
Br -
Pb2+
Br -
Pb2+
Pb2+
Br -
Pb2+
Br -
Pb2+
Br -
Conclusions
• Ionic compounds in molten or aqueous states can
conduct electricity. It is because there are mobile ions.
• Ionic compounds in solid state cannot conduct electricity.
It is because ions are not mobile.
• Electrolytes are compounds which allow electricity to
pass through them only when molten or in aqueous
solution. There is chemical decomposition during the
conduction.
• Electrolysis means 'decomposition by electricity'.
•
You do not need to remember the observations in electrolysis.
ELECTROLYSIS
OF
CHLORIDE SOLUTION
AQUEOUS
COPPER(II)
Aqueous copper(II) chloride conducts electricity.
During electrolysis, copper(II) chloride is decomposed to form
copper (reddish brown solid) at cathode and chlorine gas (with
smell of swimming pool) at anode.
Electrolysis of aqueous copper(II) chloride solution.
Q8.2
(a)
What charged particles does copper(II) chloride
consist of?
(b)
Explain why solid copper(II) chloride does not
conduct electricity, but its aqueous solution does.
A8.2
(a)
Positively charged copper(II) ions and negatively charged
chloride ions.
(b)
Solid copper(II) chloride consists of ions which are not
mobile, therefore it does not conduct electricity. In aqueous
solution, the ions become mobile and thus the solution
conducts electricity.
Q8.3
Which of the following are electrolytes?
Wax, sodium chloride, sodium, mercury,
potassium iodide, sugar, carbon monoxide
A8.3
Sodium chloride and potassium iodide only.
ethanol,
An ION is an atom or a group of atoms having an overall electric
charge.
A simple (monoatomic) ion is derived from a single atom. A
polyatomic ion is derived from a group of atoms.
Cations and anions
There are two kinds of ions: positively charged ions (cations) and
negatively charged ions (anions).
are attracted towards
cations
anions
cathode
are attracted towards
anode
MIGRATION OF IONS
We can observe the migration (movement) of coloured ions
during electrolysis.
20V d.c. supply
+
_
carbon anode
+
_
carbon cathode
dilute hydrochloric acid
this region slowly
becomes
orange
due to the migration
of
negative
dichromate
ions
towards the positive
anode
dilute hydrochloric acid
this
region
slowly
becomes blue due to the
migration of positive
copper(II) ions towards
the negative cathode
a gel containing copper(II)
ions and dichromate ions
Figure 8.7 To show the migration of coloured ions during electrolysis (using a Utube).
To show the migration of purple permanganate ions during under the influence of an
electric field(using a strip of filter paper on a microscope slide).
Q8.6
Refer to Figure 8.8 again.
(a)
Towards which electrode are potassium ions
migrating? Why?
(b) Can we see the movement of potassium ions? Why?
(c)
If a chromium(III) sulphate crystal was used instead
of a potassium permanganate crystal, what would
be observed? Why?
A8.6
(a)
The cathode. Potassium ions are positively charged. They
are thus attracted towards the negative electrode
(cathode).
(b)
No. Potassium ions are colourless.
(c)
A green patch would move towards the negative electrode
(cathode). Chromium(III) ions are green in colour and
positively charged. They are attracted towards the negative
electrode.
Objectives
• To define what is chemical bonding
• To define what is ionic bonding
-
Electrostatic attractions of opposite ions
Transfer of electron(s)
Non-directional force
Obey the rule of stability
• To draw electron diagrams
• To name and give chemical formula of the ionic
compound ( both from mono or polyatomic ions)
• To plan and describe experiments to realize what are
electrolytes and non-electrolytes.
• To realize the nature of ions of an ionic compound.
• To figure out the colour of an ionic compound in aqueous
state.
Chemical bonding (attraction)
Metallic bonding
Covalent bonding
Forms between
nonmetals and nonmetals
Ionic bonding
Forms between
metals and nonmetals
Ionic bonding
• The reaction of sodium and chlorine .
Sodium Chloride
sodium atom (Na) 2, 8, 1
Na + ion





Na


unstable and
reactive



loses 1 e-
Na


stable
CATION
+

chlorine atom (Cl) 2, 8, 7
Cl
xx
xx
x
x
x x
Cl
x
xx
xx
unstable and
reactive
-
ion
_
xx
xx
x
 x
x x
x x
x x
gains 1 e-
Cl
x
xx
xx
stable
ANION
x x
x x
IONIC BONDING AND IONIC SUBSTANCES
FORMATION OF IONIC BOND
When a sodium atom and a chlorine atom react, the sodium atom
loses one electron to the chlorine atom. As a result of this
transfer of electron, two ions are formed.
Sodium Chloride (Salt)
Nature of ionic bond
•The attraction force is non-directional
In sodium chloride
+
Na
_
Cl
_
attracted by ionic bond
Cl
Figure 8.15 Crystal of an ionic compound,
sodium chloride.
Sodium chloride has a giant ionic structure. It consists of Na+ and Cl– ions held
together by ionic bonds.
Sodium chloride has a giant ionic structure. It consists of Na+ and Cl– ions held
together by ionic bonds.
IONIC BOND is the strong non-directional electrostatic force of
attraction between oppositely charged ions.
An ionic bond is formed by the transfer of one or more electrons
from one atom (or group of atoms) to another.
Monoatomic ion
(consists of 1 atom
only)
Na+
Polyatomic ion
(consists of 2 or more
atoms )
OH-
K+
NO3-
Ag+
SO42-
Hg+
IClBr-
1) All simple (single) metal ions are cations.
2a) All simple (single) non-metal ions are anions (except
H+).
2b) All polyatomic ions are anions (except NH4+)
3) There is only one common polyatomic cation - NH4+.
4) Polyatomic ions usually consist of non-metals only, but
some consist of a metal and a non-metal.
5) When a metal forms only one cation, the ion has the
same name as the metal.
6) Transition metals are rather unusual- the same metal
can form more than one simple cation with different
charges. To name each ion, a Roman numeral
indicating the charge is written in brackets after the
name of the metal.
7) Simple (single) anion names ending in –ide
8) Polyatomic anions containing oxygen ofter have
names ending in –ate or –ite. The polyatomic
anion with more oxygen is named as –ate, and
that with less oxygen as –ite.
9)Ions with 4+ or 4- are uncommon. (unstable)
Questions
•
•
•
•
•
•
•
•
Lead(II) dichromate
Iron(III) oxide
Potassium permanganate
Zinc hydroxide
KNO3
CaCO3
Ca(HCO3)2
Fe2(SO4)3
Objectives
• To define what is chemical bonding
• To define what is ionic bonding
-
Electrostatic attractions of opposite ions
Transfer of electron(s)
Non-directional force
Obey the rule of stability
• To draw electron diagrams
• To name and give chemical formula of the ionic
compound ( both from mono or polyatomic ions)
END