This questions was popular to the candidates and their performance was good. In (a), majority of the candidates were able to identify the elements that belongs to the same group and the same period. In (b), majority of the candidates did not give a complete definition of electronegativity while others defined it using element in place of atom in the definition. Also candidates lost marks because they could not correctly state the trend of electronegativity in the periodic table and could not adequately explain why atomic number increase down the group but decrease across the period.
In (c), Majority of the candidates were able to list the properties of transition elements, though some candidate listed the general properties of metals.
In part (d), majority of the candidates were able to write and balance the equation for the thermal decomposition of kClO3 and were also able to state the role of MnO2 in the reaction.
The expected answers include:
1.
(a) (i) Q and S
(ii) R, S, U. Q, T
(iii) I. Q/R/S
II. U
(iv) T
(v) R, U; S, U; Q, U;
(b) (i) Electronegativity is defined as the power of an atom to attract (shared pair or
bonded) electrons in a molecule.
(ii) Electronegativity of elements increases across the period and decreases down
the group.
(iii) There is an increase in the number of shells down the group and less nuclear
attraction hence increase in the atomic size.
Across the period, effective nuclear charge increases and the outermost electrons are drawn more towards the nucleus.
OR
Number of shells remains the same while nuclear charge increases.
(c) - variable oxidation states
- formation of coloured ions
- complex ion formation
- paramagnetism/magnetic property
- catalytic ability
(d) (i) 2KClO3 → 2KCl + 3O2
(ii) MnO2 acts as catalyst
(iii) M(KClO3) = 39 + 35.5 + (3 x 16)
= 122.5 gmol-1
M(KCl) = 39 + 35.5
= 74.5 gmol-1
(2 x 122.5g) of KClO3 gives (2 x 74.5g) of KCl
15g of KClO3 = 2 x 74.5 x 15
2 x 122.5
= 9.12 g
