Question 7

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:

1. Acidic oxides:  SO2/CO2/ NO2, etc.
2. Basic oxides:    CaO/Na2O/K2O, etc.
3. Amphoteric oxides: Al2O3/ZnO/PbO, etc.
4. 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:

1. they are monoatomic gases
2. they are chemically inert or unreactive
3. 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.