CHEM1001 Answers to Problem Sheet 4
1.
(a)
A bonding pair is located between two atoms and acts
to glue the atoms together. In water, there are two
bonding pairs – one located in each O-H bond.
(b)
A non-bonding pair (sometimes called a lone pair) is
localized on one atom. It does not play a role in
bonding but often does play a part in determining the
molecular shape. In water, there are two non-bonding
pairs – both located on the oxygen atom.
(c)
2.
Valence level electrons are those in the outer shell of
an atom. Oxygen has 6 electrons in the outer (n = 2)
level. Hydrogen has 1 electron in its outer (n = 1)
level.
Valency:
non-bonding
pairs
O
H
H
bonding
pairs
C = 4, H = 1, N = 3, I = 1, Cl = 1
Compound
Structure
Bonds
Valency?
Comment
H
CH3
C
H
H makes 1 bond

C makes 3 bonds

H makes 1 bond

C makes 4 bonds

Unlikely to exist:
C has a valency of 4
H
H
C2H6
H
C
H
C
H
H
All valencies satisfied:
molecule exists
(ethane)
H
I
I makes 1 bond
N
I
NI3
N makes 3 bonds


All valencies satisfied:
molecule exists
(nitrogen triodide)
I
Cl makes 1 bond
NCl
C2H2
N
H
C
Cl
C
H

N makes 1 bond

H makes 1 bond

C makes 4 bonds

Unlikely to exist:
N has a valency of 3
All valencies satisfied:
molecule exists
(ethyne)
The molecular formula shows the actual number of atoms of each element in a
compound. The empirical formula is the simplest integer ratio of the elements in that
compound:
3.
Molecular formula
Empirical formula
C2H6
CH3
NI3
NI3
C2H2
CH
Unsaturated species contain multiple (double, triple, …) bonds.
Molecule
Structure
CS2
S
C
Comment
S
Unsaturated:
C=S double bonds
H
Saturated:
only C-H single bonds
H
CH4
C
H
H
H
H
H
CH2CH2
H
C
H
4.
Saturated:
only O-H single bonds
O
H2O
C
H
Unsaturated:
C=C double bond (and C-H single bonds)
Diamond consists of a vast, extended array of covalent bonds (four to each carbon
atom). Melting diamond requires many bonds to be broken so requires a very high
temperature.
Iodine molecules contain a covalent bond but there no covalent bonds between one
iodine molecule and its neighbour, consequently it is relatively easy to separate one
molecule from another.
5.
Ionic bonding involves attraction between each cation and each anion in a regular
crystal lattice made up of essentially an infinite number of ions. The ions are made by
transfer of one or more electrons from the cation to the anion.
Covalent bonding involves sharing of electrons between the directly connected atoms
making up the bonds in a covalent molecule. These molecules can involve only a few
atoms, like H2 and H2O, many millions of atoms, like a protein, or essentially an infinite
number of atoms, like diamond.
6.
Metallic bonding – in the electron ‘sea’ model, the valence electrons of the metal atoms
in a same are highly delocalised and attract the metal cations together.
7.
8.
(a)
Metals conduct electricity and heat because the electrons are mobile.
(b)
Metals can be deformed because the electron sea prevents repulsions among the
cations. The attraction is long range and acts to pull the atoms back into place if
they are moved as the solid is bent or stretched.
(a)
2K + Cl2  2KCl
(b)
Ba + 2H2O  Ba(OH)2 + H2
(c)
2H2S + 3O2  2H2O + 2SO2
(d)
2C2H6 + 7O2  4CO2 +6H2O
(e)
6Li + N2  2Li3N
(f)
Fe2O3 + 3CO  2Fe + 3CO2
(g)
4Al + 3O2  2Al2O3
(h)
2C6H14 + 19O2  12CO2 + 14H2O
(a)
Water solutions of sodium sulfate and barium chloride
Full equation:
BaCl2(aq) + Na2SO4(aq)  BaSO4(s) + 2NaCl(aq)
Omitting spectator ions:
Ba2+(aq) + SO42(aq)  BaSO4(s)
(b)
Water solutions of potassium hydroxide and copper(II) nitrate
Full equation:
Cu(NO3)2(aq) + 2KOH(aq)  Cu(OH)2(s) + 2KNO3(aq)
Omitting spectator ions:
Cu2+(aq) + 2OH(aq)  Cu(OH)2(s)
(c)
Water solutions of silver nitrate and sodium chloride
Full equation:
AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq)
Omitting spectator ions:
Ag+(aq) + Cl(aq)  AgCl(s)
(d)
Water solutions of cobalt(II) sulfate and ammonium carbonate
Full equation:
CoSO4(aq) + (NH4)2CO3(aq)  CoCO3(s) + (NH4)2SO4(aq)
Omitting spectator ions:
Co2+(aq) + CO32(aq)  CoCO3(s)