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December 21, 2011

TEC 111

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













































 








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.


Hydrogen bond
 


Covalent bond
 
 


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 2p3s2 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                                                                                                        


 
                        Molecular mass

= 1000


 
   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


 






                                                                                                                                    Concrete
Steel











 
Text Box: Pre-stressed concrete

                                                                                                        -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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