TEC 111 QUESTIONS
1. The elements
A, B, X, Y and Z have atoms with outer electron shell configurations containing
1, 2, 4, 7 and 8 electrons respectively. State and describe the type of bonding
which is likely to occur in the following cases:
a) Between a large number of atoms of A.
Metallic bond; the
outer electron is readily
released from A and all the valence electrons can
move about freely in A ions. The positively charged ions are held together by
their attraction to the cloud of negative electrons in which they are embedded.
It is a good electrical conductor.
b) Between a large number of atoms of X.
Covalent bond; it
involves sharing of electrons. Each of X atom is surrounded by 8 electrons
which a stable arrangement is forming a rigid structure with high MP and does
not conduct electricity.
c) Between atoms of Y.
Covalent bond; this
is a single covalent bond formed when each of atoms of Y donates one electron.
The bond formed has high melting and boiling point and does not conduct
electricity.
d) Between atoms of Z
Van der Waal’s bond; Van der Waals forces are the attractive forces of one
transient dipole for another. A
transient dipole is a temporary imbalance of positive and negative charge. At
particular instants, even atoms that are spherical on average, such as those of
the noble gases, will have
greater electron density on one side of the atom than
another. At that instant, the atom will possess a temporary dipole with a
negative charge concentration on
the side of the atom with greater electron density. Does not conduct
electricity and has low MP.
e) Between equal numbers of A and Y atoms.
Ionic bond; it’s
formed when there is total electron transfer from A to Y. A and Y are held by
strong electrostatic attractions between the positive
and negative charges. It has high melting point conducts
electricity.
f) Between one atom of X and 4 atoms of Y?
Covalent bond; It
involves sharing of electrons to form a structure that is tetrahedral and has a
high melting point. The compound so formed conducts electricity since it
hydrolyses in water.
2. Two elements X
and Y which have atomic numbers 12 and 17 respectively combine to form a
compound. What type of bond exists in such a compound and what would be the
chemical formula of the compound?
Ionic bond; XY2
3. By use of dots
and cross diagrams show covalent bonding between nitrogen to yield N2
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4. What do you
understand by the term valency (valence of an element)?
It is the measure of
the number of bonds formed by an atom.
N/B: Valence
of an element depends on the number of valence electrons that may be involved
in the forming of the valence bond.
A univalent (monovalent) ion/ group have a valence of one
and can form one covalent bond.
A divalent molecular entity as a valence of two and can form
two sigma bonds to two different atoms or one sigma bond plus one pi bond to a
single atom.
5. Diagrammatically
show both hydrogen and covalent bonding in a water molecule.
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6. Why does the
water molecule look bent?
-There are lone pair
electrons on the oxygen that tend to cause repulsion.
-Hydrogen atoms are
tetrahedrally arranged around the oxygen.
7. What are lone
pairs?
These are electrons
that are not involved in covalent bonds.
8. When does the
hydrogen atom emit a photon?
When its electron
changes from n=7 to n=3.
9. What is the
energy of a photon of black light (ultraviolet) that has a frequency of 2.00 ×
1016
Hz?
E=hf
= 6.62 ×10-34 × 2.00 ×1016
=1.324 ×10-17 J
=1.324 ×10-20 KJ
10. What is the electronic configuration of
potassium, 19?
1s2 2s2
2p6 3s2 3p6
4s1
11. What do you
mean by hydrogen bond?
It is the attractive interaction
of a hydrogen atom with an electronegative atom such as oxygen, nitrogen/
fluorine that comes from another molecule/chemical group.
12. Why do
different polymers have different properties?
They have different
long chain and some have different monomer units.
13. Determine the
number of molecules in 1 kg of MgCl2. (Mg= 24.32 Cl=35.46)
Moles = mass in grams
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Molecular mass
= 1000
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95.24
=10.49979
1 mole =6.023×1023 molecules
10.49979
=?
=
(10.49979 × 6.023×1023) ×3
=6.324×1024moloecules.
14. By use of
illustration, differentiate clearly between steel-reignforced concrete and
pre-stressed concrete.
Steel reignforced concrete
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Concrete
Steel
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-In steel reinforced, the concrete is set
around steel wire in the form of a mesh or rods.
-In pre-stressed concrete, steel reignforcing wires are elastically
deformed while allowing concrete to set around them. When the concrete has set
the tensile stress on the wires is removed and the wires contract elastically
producing compressive stress on the concrete.
15. What do you understand by the term anisotropy as used in the
wooden materials?
It is the property of possessing properties varying with the
crystallographic orientation
N/B: When a material has its properties same in all directions, it is
said to be isotropic.
16. What is the effect of increase in molecular length of a polymer
on the melting temperature?
The temperature increases since more heat would be needed to break the
increasing chains.
17. Define the following terms:
Crystallinity- Refers to the degree of structural order in a solid.
Thermosets-Polymers which form crosslinks between their long chains of molecule;
when heated, they undergo chemical decomposition hence cannot be remuolded.
Cleaving- It is the tendency of crystalline material to split along a definite
crystallographic structural planes.
Elastomer- A group of polymer materials of very low elastic moduli giving them
great flexibility and the ability to suffer elastic deformation.
18. What conditions are
necessary to fulfill when preventing corrosion of a metal by anodizing/
plating.
The coated metal must be less reactive than the coating metal in order
to form an electrochemical cell; in which case the coating material will react
in preference to the coated metal.
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