➲ Investigation
94 MoDuLe 1: the chemical earth
• The ionic bonding model can be used to explain the properties of ionic crystals (see Table 5.8).
Table 5.8
Property Explanation
Hard and brittle ions are tightly bound by electrostatic forces in the crystal lattice non-conductivity of electricity when solid ions occupy fixed positions in the lattice. electrons are strongly held by nuclei of
individual ions Conductivity of electricity when molten or
in aqueous solution Mobile ions can transfer electric charge High melting and boiling points ions tightly bound by strong electrostatic forces
• A metal consists of a lattice of positively charged metallic ions surrounded by a ‘sea’ of mobile delocalised electrons.
• Metallic bonding is the electrostatic attraction between delocalised electrons and the positive metallic ions.
• The metallic bonding model can be used to explain the properties of metals as shown in Table 5.9.
Table 5.9
Property Explanation
relatively high density ions tightly packed in the lattice
Malleability and ductility Metallic bonding is non-directional, therefore metal lattices can be deformed without disrupting the metallic bonding Conductivity of electricity and heat Mobile delocalised electrons transfer charge and heat energy High melting and boiling points Strong metallic bonding exists throughout the lattice
1 a Describe a physical property that makes each of the following useful: iron,
copper, lead, alcohol, helium.
b Describe a chemical property that makes each of the following useful:
petrol, oxygen, carbon dioxide.
2 Explain, using copper(II) oxide as an example, why the properties of a simple
compound are quite different from those of the elements from which it was formed.
CHapter 5: Bonding and structure determine properties 95
3 The gems, rubies and sapphires, are slightly impure forms of aluminium oxide. Investigate the structures of these gems and why they display such different
colours.
4 a Briefly describe the three major types of bonding—metallic, ionic and
covalent—and explain how this can lead to four classes of solid substances.
b Describe why covalent molecular and covalent network substances have
such different melting points.
5 It has been found that most substances that have an odour are covalent
molecular substances. Propose a reason for this observation.
6 Consider the information below.
Compound Boiling point (ºC)
CH4 –162
H2o 100
nH3 –33
So2 –10
Order the substances in terms of increasing strength of forces between their molecules.
7 A certain solid is a non-conductor of electricity and has a high melting point. Identify the type of structure possessed by this solid. Describe further
tests that could be performed in order to make a definite decision about the type of solid.
8 Carbon and silicon both have four valence electrons and form compounds
with formulas CO2 and SiO2, respectively. These oxides have very different
properties: CO2 sublimes at 78ºC, whereas SiO2 melts at 1713ºC and boils at
2230ºC. Explain the differences in these properties in terms of the structures of these substances.
9 Silicon carbide has a structure similar to diamond.
a Construct a diagram representing the arrangement of atoms in a silicon
carbide crystal.
b Predict the properties of silicon carbide in terms of its structure.
c Investigate the properties of silicon carbide and compare them with your
predictions in b.
10 Neither solid sodium chloride nor water are electrical conductors. However,
when sodium chloride dissolves in water it forms a conducting solution.
Explain this observation.
11 Explain why metals are malleable while ionic solids are not.
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Investigation
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96 MoDuLe 1: the chemical earth
12 Classify the substances listed below into three groups: a those in which the bonds are metallic
b those in which the bonds are ionic c those in which the bonds are covalent.
SiH4, Cu, sulfur dioxide, MgBr2, P4O10, Na2S, sterling silver, K2O, F2O, strontium chloride, MgH2, SiC, Pb, AsH3, CsCl, B2O3, bronze, diamond
13 Identify the types of particles present (molecules, atoms, ions) in crystal
lattices of the following:
a sodium
b sodium chloride c diamond d carbon dioxide.
14 Identify which attractive forces must be overcome in the following processes: a I2(s) → I2(g)
b I2(g) → 2I(g)
c shattering a sodium chloride crystal with a hammer d bending an iron nail until it breaks.
15 Identify the charged species that conduct the current in the following: a molten NaI
b molten Cu c solid Al
d molten Mg(NO3)2
e a solution of Al(NO3)3.
16 Explain the following in terms of the structure of the substances involved. a In the solid state, copper is a good conductor of electricity and is malleable,
whereas diamond is a non-conductor and extremely hard.
b Naphthalene (C10H8) is a soft solid with a low melting point, whereas quartz (SiO2) is a hard solid with a high melting point.
17 Classify the following solid substances as ionic, covalent molecular, covalent
network or metallic:
a a lattice consisting of atoms covalently bonded to neighbouring atoms b a solid only at extremely low temperatures
c a good conductor of heat and electricity
d a good conductor of electricity only in the molten state
e a lattice consisting of positive ions sharing electrons with neighbouring
positive ions.
18 Classify, with explanations, the following solids as covalent molecular, metallic,
ionic or covalent network:
a a soft solid that decomposes at 250ºC; the solid does not conduct electricity b violet crystals that melt at 114ºC; neither the solid nor the liquid conducts
CHapter 5: Bonding and structure determine properties 97
c hard, colourless crystals that melt at 2045ºC; the solid is a non-conduct but
the liquid conducts electricity
d hard grey solid that melts at 3410ºC; both the solid and liquid conduct
electricity.
19 Consider the substances potassium bromide, gold, silicon carbide and sulfur. a Identify which of these solid substances will conduct electricity.
b Identify which of these substances will conduct electricity in the liquid state
but not the solid state.
c Explain how the ‘mode’ of electrical conduction in molten potassium
bromide differs from that in solid gold.
d Predict which of the four substances is the hardest. Justify your prediction. 20 Diamond is one form of carbon. Investigate the arrangement of the atoms and
the bonding in two other forms of carbon: graphite and ‘bucky balls’. On the basis of their properties, how would you classify these substances?
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*MoDULe 1 ReVieW
98 MoDuLe 1: the chemical earth