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CHE1501
EXAM PACK
DISTINCTION QUALITY
UNISA EXAM
,ANSWERS AND MEMO 2025
(1) 1a) Chemical
1b) Physical
1c) Physical
(2) Bromine is in Period 3, Group 7. It has the symbol Br. Its atomic number is 35. It is a non-metal.
(3) This is an example of ionic bonding. The magnesium atom loses one electron to each bromine atom to
form a magnesium cation and two bromide anions, which then form an ionic bond, MBr2:
(Section 8.2 in Brown – Ionic Bonding)
(4) 4a) A hydrocarbon is an organic compound consisting entirely of carbon and hydrogen.
4b) Isomers are two or more molecules with the same molecular formulae, but different structures.
4c) A functional group is a group of atoms or bonds within a molecule that is responsible for the
molecule’s characteristic chemical reactivity.
(5) Rhodium (Rh) has the ground state electron configuration [Kr]5s24d7. To form a 3+ ion, 3 electrons
must be removed. When electrons are removed from an atom to form a cation, they are always
removed first from the occupied orbitals having the highest principal quantum number, n – in this case,
from the 5s2 orbital. Thus, we remove two 2s electrons and one 1d electron, so the electron
configuration for the 3+ rhodium cation is [Kr]4d6.
(Section 7.4 in Brown – Electron Configurations of Ions)
(6) Sulphur has an electronegativity of 2.5 and oxygen has 3.5. The electronegativity difference is therefore
1.0, which indicates polar covalent bonding. Sulphur is positively charged and oxygen is negatively
charged.
3
,(7) The unbalanced chemical equation for the decomposition of nitroglycerin is:
C3H5N3O9 (l) N2 (g) + CO2 (g) + H2O (l) + O2 (g)
In the past (until the late 1800s), nitroglycerin was used as the primary explosive in construction.
However, it is highly sensitive to heat and shock, and even dropping or shaking a bottle of the substance
would make it explode. Historically, it resulted in many deaths among its users each year. Alfred Nobel
discovered that mixing nitroglycerin with silica (diatomaceous earth) would turn the dangerous liquid
into a safe, malleable paste, called dynamite. This paste does not explode due to shock, but needs
detonation. This revolutionized the use of explosives in construction, and he became extremely rich as
a result (his legacy has been funding the Nobel prizes since 1895, with the current prize fund of around
10 million US Dollars a year.)
Just on an interesting side note - there is not, and has never been a Nobel Prize for mathematics. Legend
has it that Alfred Nobel’s fiancée had an affair with a mathematician and he declared that not a cent of
his money would ever go to a mathematician. However, no-one really knows if this story is true or not.
We just know that there will never be a Nobel Prize for Mathematics.
(8) 8a) i)
Fluorine is in group 7 and therefore has 7 valence electrons in its ground state.
ii)
Bromine is in group 7 and therefore gains one electron to form a negatively charged
bromine ion with 8 valence electrons.
iii)
Two iodine atoms combine to form an iodine molecule, and a pair of electrons is shared.
8b)
It is vital that you know how to draw Lewis diagrams. You should be able to show all lone pairs in these
diagrams, assign formal charges to each atom, and indicate bond polarity on any polar bonds. The module
CHE1502 assumes that you are 100% proficient in drawing Lewis diagrams, and you will not be able to
pass that module without it. Now is an excellent time to make sure you know how, as these always feature
in CHE1501 exams as well.
4
, (9) 9a)
Total valence electrons:
S: 6
O: 6
Cl: 2x7=14
Total: 6+6+14=26
O
Cl S Cl
Check: Make sure the atom has the correct number of valence electrons:
2 single bonds = 4 electrons
1 double bond = 4 electrons
3 lone pairs on each chlorine atom: 12 electrons
2 lone pairs on the oxygen atom: 4 electrons
1 lone pair on the sulphur atom: 2 electrons
Total = 4+4+12+4+2 = 26
9b)
Total valence electrons:
H: 1
Cl: 7
O: 2x6=12
Total: 1+7+12=20
O
Cl O H
Check: Make sure the atom has the correct number of valence electrons:
2 single bonds = 4 electrons
1 double bond = 4 electrons
2 lone pairs on each oxygen atom: 8 electrons
2 lone pairs on the chlorine atom: 4 electrons
Total = 4+4+8+4+2 = 20
9c)
You should be able to do this one on your own now.
5
CHE1501
EXAM PACK
DISTINCTION QUALITY
UNISA EXAM
,ANSWERS AND MEMO 2025
(1) 1a) Chemical
1b) Physical
1c) Physical
(2) Bromine is in Period 3, Group 7. It has the symbol Br. Its atomic number is 35. It is a non-metal.
(3) This is an example of ionic bonding. The magnesium atom loses one electron to each bromine atom to
form a magnesium cation and two bromide anions, which then form an ionic bond, MBr2:
(Section 8.2 in Brown – Ionic Bonding)
(4) 4a) A hydrocarbon is an organic compound consisting entirely of carbon and hydrogen.
4b) Isomers are two or more molecules with the same molecular formulae, but different structures.
4c) A functional group is a group of atoms or bonds within a molecule that is responsible for the
molecule’s characteristic chemical reactivity.
(5) Rhodium (Rh) has the ground state electron configuration [Kr]5s24d7. To form a 3+ ion, 3 electrons
must be removed. When electrons are removed from an atom to form a cation, they are always
removed first from the occupied orbitals having the highest principal quantum number, n – in this case,
from the 5s2 orbital. Thus, we remove two 2s electrons and one 1d electron, so the electron
configuration for the 3+ rhodium cation is [Kr]4d6.
(Section 7.4 in Brown – Electron Configurations of Ions)
(6) Sulphur has an electronegativity of 2.5 and oxygen has 3.5. The electronegativity difference is therefore
1.0, which indicates polar covalent bonding. Sulphur is positively charged and oxygen is negatively
charged.
3
,(7) The unbalanced chemical equation for the decomposition of nitroglycerin is:
C3H5N3O9 (l) N2 (g) + CO2 (g) + H2O (l) + O2 (g)
In the past (until the late 1800s), nitroglycerin was used as the primary explosive in construction.
However, it is highly sensitive to heat and shock, and even dropping or shaking a bottle of the substance
would make it explode. Historically, it resulted in many deaths among its users each year. Alfred Nobel
discovered that mixing nitroglycerin with silica (diatomaceous earth) would turn the dangerous liquid
into a safe, malleable paste, called dynamite. This paste does not explode due to shock, but needs
detonation. This revolutionized the use of explosives in construction, and he became extremely rich as
a result (his legacy has been funding the Nobel prizes since 1895, with the current prize fund of around
10 million US Dollars a year.)
Just on an interesting side note - there is not, and has never been a Nobel Prize for mathematics. Legend
has it that Alfred Nobel’s fiancée had an affair with a mathematician and he declared that not a cent of
his money would ever go to a mathematician. However, no-one really knows if this story is true or not.
We just know that there will never be a Nobel Prize for Mathematics.
(8) 8a) i)
Fluorine is in group 7 and therefore has 7 valence electrons in its ground state.
ii)
Bromine is in group 7 and therefore gains one electron to form a negatively charged
bromine ion with 8 valence electrons.
iii)
Two iodine atoms combine to form an iodine molecule, and a pair of electrons is shared.
8b)
It is vital that you know how to draw Lewis diagrams. You should be able to show all lone pairs in these
diagrams, assign formal charges to each atom, and indicate bond polarity on any polar bonds. The module
CHE1502 assumes that you are 100% proficient in drawing Lewis diagrams, and you will not be able to
pass that module without it. Now is an excellent time to make sure you know how, as these always feature
in CHE1501 exams as well.
4
, (9) 9a)
Total valence electrons:
S: 6
O: 6
Cl: 2x7=14
Total: 6+6+14=26
O
Cl S Cl
Check: Make sure the atom has the correct number of valence electrons:
2 single bonds = 4 electrons
1 double bond = 4 electrons
3 lone pairs on each chlorine atom: 12 electrons
2 lone pairs on the oxygen atom: 4 electrons
1 lone pair on the sulphur atom: 2 electrons
Total = 4+4+12+4+2 = 26
9b)
Total valence electrons:
H: 1
Cl: 7
O: 2x6=12
Total: 1+7+12=20
O
Cl O H
Check: Make sure the atom has the correct number of valence electrons:
2 single bonds = 4 electrons
1 double bond = 4 electrons
2 lone pairs on each oxygen atom: 8 electrons
2 lone pairs on the chlorine atom: 4 electrons
Total = 4+4+8+4+2 = 20
9c)
You should be able to do this one on your own now.
5
