SOUTH EASTERN KENYA UNIVERSITY
UNIVERSITY EXAMINATIONS 2015/2016
SECOND YEAR
DEGREE:
BACHELOR OF SCIENCE (CHEMISTRY, GEOLOGY, EDUCATION)
SCH 201:
CHEMISTRY OF MAIN BLOCK ELEMENTS
TIME:
2h
DATE…24/5/15
______________________________________________________________
INSTRUCTIONS TO CANDIDATES
(a) Answer question One and any other Two questions
(b) Question 1 carries 30 marks while the other questions carry
20 marks each
(c) Illustrate your answers with well label diagrams where applicable
QUESTION ONE (30 MARKS)
a) Explain the following observations
i) The density of of ice is smaller than that of water
(1 marks)
ii) Anions are usually larger than the corresponding atom
(1 marks)
iii) Cations are usually smaller than the corresponding atom
(1 marks)
iv) The largest electron affinities are those of the halogens
(1 marks)
b) Define electronegativity and explain its important trends in the periodic table
(3 marks)
c) How can one distinguish NaF, NaBr and NaI from each other
(4 marks)
d) Explain why
1
i) Lithium can NOT form higher oxides
(2 marks)
ii) Lithium fluoride is not soluble
(2 marks)
iii) Lithium carbonate is insoluble in water
(3 marks)
iv) Lithium carbonate behaves like magnesium carbonate and yet they don’t belong to the
same group
(2 marks)
e) Explain the chemical test of hydrogen sulphide
(2 marks)
f) compare the first and second ionization energy of magnesium
( 2 marks)
g) Beryllium salts never have more than four molecules of water of crystallization. Explain
(3 marks)
QUESTION TWO (20 MARKS)
a) Explain the following observations
i Group two elements have higher first ionization energy than group one
(2 marks)
ii The solubility of most salts in group two decreases with increasing atomic weight (3 marks)
b) What is the importance of nitrogen to animals
c) Describe why hydration increases the effective size of beryllium ion
(1 mark)
(8 marks)
d) Explain why
i
Nitrogen can form negative oxidation state
(2 marks)
e) Explain how soap removes dirt during washing
(4 marks)
QUESTION THREE (20 MARKS)
a) Explain the following observations
i) Nitrogen has maximum coordination number four while phosphorous can have five or six
coordination numbers
ii) When the bond between sulphur and oygen is measured, it is found to be shorter
2
(3 marks)
than expected
(3 marks)
iii) Sulphur exist as eight membered ring while oxygen exist as diatomic
(2 marks)
iv) Oxygen mostly form divalent compounds
(2 marks)
b) How do the electron affinity of sulphur and oxygen compare?
(2 marks)
c) What changes in viscosity are observed when sulphur is heated
(8 marks)
QUESTION FOUR (20 MARKS)
a) What is water hardness
b) Explain five methods used to remove water hardness
(2 marks)
(10 marks)
c) Name two advantages of water hardness
(2 marks)
d) What is the use of calcium chloride during electrolysis of fused halides
(1 mark)
e) Explain the following observations
i PH5 does not exist but PH3 is known to exist
ii Between CsF and KBr, which one is more ionic?
(2 marks)
(3 marks)
QUESTION FIVE (20 MARKS)
a) What are chlorofluorocarbons?
(2 marks)
b) Explain how chlorofluorocarbons pollute the environment
(7 marks)
c) Explain the anomalous behavior of beryllium
(4 marks)
d) Hydrogen sulphide is a gas while water is a liquid though they form similar bond. Explain
(2 marks)
e) By use of chemical equations explain how some oxides of nitrogen pollute the atmosphere
(5marks)
3