First Exercise (7points)
Some Elements in the Earth’s Crust
The circle graph given at the right shows
the percentage by mass of some
elements in the Earth’s crust.
1- Calculate the percentage by mass of
the element silicon in the Earth’s crust.
2- Write the electron configuration of
each of the elements:
Silicon, magnesium and sulfur.
Given: O(z=8) , Mg (Z=12)
Si(Z=14) and S(Z=16)
3- Deduce the column (group) and the
row (period) of the elements sulfur and
magnesium on the periodic table.
4- Write the Lewis electron-dot symbol
for each of magnesium and sulfur
atoms.
5- The two elements magnesium and
sulfur react to form and a compound.
Explain the bond formation that takes place between magnesium and sulfur. Give the formula unit of
the compound formed.
Second Exercise (7points)
Redox Reactions
Modern photographic film is made of a plastic packing covered with a layer of gelatin, in which
millions of grains of silver bromide are embedded. When light strikes a grain, the silver and
bromide ions are converted into their elemental forms through an oxidation – reduction (redox)
reaction.
The equation of the redox reaction is:
2Ag+ + 2 Br-
2 Ag + Br2
1- Write the oxidation half- reaction and the reduction half - reaction involved in the above
given redox reaction.
2- One molecule of bromine (Br2), which is a halogen like chlorine (Cl2), is added to one
molecule of hydrocarbon (A) to produce one molecule of the compound 1, 2-dibromoethane
(E).
a- Write the condensed structural formula of compound (E).
b- Determine the molecular formula of hydrocarbon (A) and give is IUPAC name.
3- One molecule of ethane undergoes hydrogenation reaction (R) to produce a compound (C)
according to the following equation:
CH2 = CH2 + H2 → (C)
a- Give the molecular formula of compound (C).
b- Calculate the oxidation number of carbon before reacting and after reacting. State
whether the hydrogenation reaction (R) is an oxidation – reduction reaction Justify.
Third Exercise (6 Points)
Crude Oil: A mixture of Hydrocarbons
Crude oil is a mixture of many hydrocarbons. The
boiling point temperature of a hydrocarbon, whose
molecule consists of straight-chain of carbon atoms,
depends on the number of carbon atoms in the
molecule of the hydrocarbon.
Refer to Figure -1.
1- Copy and complete the following table:
Hydrocarbon Molecular
Boiling
Formula
Point (0C)
Butane
Hexane
Decane
2- Show that the boiling point temperature of
pentane (C5H12) is between 00C and 700C.
3- Long hydrocarbon molecules are made more
useful by splitting them. The process is
called cracking. The cracking of one
molecule of decane produces one molecule
of octane (C8H18) and one molecule of a
hydrocarbon (B).
a- Give the molecular formula of hydrocarbon
(B).
b- Indicate to what class (family) of
hydrocarbons does (B) belong.
4- An isomer of octane is used as car fuel.
a- Write the equation for the complete combustion reaction of the car fuel. Give the name of
the products obtained.
b- The products of this combustion contribute to the greenhouse effect, which leads to global
warming. State one consequence of global warming.