This question was not popular with the candidates but the performance of those who attempted it was fair.
In (a)(i), most candidates gave an incomplete definition of acidic oxide by stating that they are oxides of non metals without stating that they dissolve in water to form acidic solutions hence lost the marks. However, they correctly gave one example each of acidic oxide, basic oxide, amphoteric oxide and neutral oxide as follows:
- Acidic oxides: SO2/CO2/ NO2, etc.
- Basic oxides: CaO/Na2O/K2O, etc.
- Amphoteric oxides: Al2O3/ZnO/PbO, etc.
- Neutral oxides: CO/N2O, etc.
In (b)(i), most candidates correctly defined each term thus:
(I) Heat of combustion is the heat change when one mole of a substance is completely burnt in oxygen/burnt in excess oxygen.
(II) Heat of neutralization is the heat change when one mole of water is produced as a result of a reaction between an acid and alkali in dilute solution/the heat change when one mole of H+ from an acid reacts with one mole of OH- from an alkali to form one mole of H2O.
In (b)(ii) majority of the candidates could not write an equation to illustrate each of the terms hence lost the marks. The correct equation were as follows:
I. C(s) + O2(g) → CO2 (g) ∆HcӨ
II. NaOH (aq) + HCI(aq) → NaCI(aq) + H2O(1) ∆HnӨ
In (b)(iii) candidates correctly calculated the standard heat of combustion of butane thus:
Molar mass of = 12 x 4 + 1 x 10 = 48 + 10 = 58gmol-1
58 g of C4H10 5877kJmol-1
∴ 14.5 of C4H10 = 5877 x 14.5 kJmol-1
58
= + 1469.25 kJ
In (c)(i) and (ii) candidates correctly named two allotropes of sulphur and stated one difference between the two allotropes.
(1) Rhombic
Monoclinic
(II)
Rhombic |
Monoclinic |
Yellow translucent crystal |
Transparent amber crystal |
Stable at temperature below 900 |
Unstable at temperature below 900 |
Higher density than monoclinic |
Lower density than rhombic |
Lower melting point |
Higher melting point |
Octahedral shaped |
Needle shaped. |
In (d)(i), candidates correctly gave two characteristics of noble gases from the following:
- they are monoatomic gases
- they are chemically inert or unreactive
- have low melting/boiling points.
However in (d)(ii), candidates could not correctly give one use of each of Helium and argon.
The expected answers from candidates were
He – filling balloons
Ar – arc welding
In (e), most candidates correctly stated that ammonia gas was given off on warming ammonium trioxonitrate (V) with sodium hydroxide.