Chemistry Grade 12

Chemistry Past Papers 12th Class Peshawar Board

Section A-(MCQs)

i) Which element is not a Ferromagnetic?                                                 (Mark 1)

A. Sb
B. Fe
C. Ni
D. Co
Answer:
A. Sb

Section B-(Short Questions)

Q.2 ii) Why aluminium is passive towards oxyacids?                        (Marks 4)

Answer:
Oxyacid:
Oxyacid is defined as an acid that contains an oxygen atom in it.
Aluminium is passive towards the oxyacids due to the formation of aluminium oxide. This aluminium oxide protects the further contact of aluminium with water and makes aluminium passive towards oxyacids.
When aluminium is added into conc. HNO3, it does not react with HNO3 at any concentration. This is probably due to the formation of the protective layer of aluminium oxide. The acid is said to render aluminium passive. HNO3 is therefore frequently transported in aluminium containers.

Section C-(Long Questions)

Q.3 a) Write the trend in chemical properties of compounds of alkali and alkaline earth metals.                                                                             (Marks 4.5)

Answer:
General trends in properties of compounds of alkali and alkaline-Earth metals
Oxides:
Alkali metal oxide:

Alkali metal oxides dissolve in water to give strongly alkaline solutions. For example:
Li2O(s) + H2O(I) →   2LiOH(aq)
2Na2O2(s) + 2H2O(I)   →     4NaOH(aq) + O2(g)
The reaction of an alkali metal oxide with water is an acid-base reaction and not an oxidation-reduction reaction since no element undergoes a charge in its oxidation number. The reaction simply involves the decomposition of the water molecule by an oxide ion.
O2- +H2O  →  2OH-
The basic character of alkali metal oxides increases down the group.
Alkaline metal oxide:
The solubility of alkaline earth metal oxide in water increases down the group. BeO and MgO are insoluble but CaO, SrO, and BaO are soluble and react with water to form the corresponding hydroxides.
The basic character of oxides of alkaline earth metals increases down the group. The tendency for group IIA oxides to form alkaline solution is relatively less than that of alkali metals.
BeO is amphoteric in nature since it reacts with both acids and bases.
Carbonates:
Alkali metal carbonates:
Carbonates of alkali metals are stable enough to be isolated as solids. However, they decompose on heating forming carbonates.
2NaHCO3 →Na2CO3 + CO2 +H2O
Thermal stability of hydrogen carbonates of group I increases down the group. The reason is increasing size and decreasing charge density of metal ions. The polarizing power of a cation increases with increasing charge on the ion and decreasing the radius on the ion.
Alkaline earth metal carbonates:
Carbonates of group II become more thermally stable as we go down the group. The ones at the lower position have to be heated more strongly than those at the top before they decompose.
XCO3 →XO + CO2
2X(NO3)2→ 2XO + 4NO2+ O2

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