2019 KCSE Marking Scheme

2019 KCSE chemistry paper 1 marking scheme

5. a) Mg(s) + H2O(g) ➔ MgO(s) + H2(g)

b) To prevent suck-back of water which would crack the boiling tube.

c) Sodium is a very reactive metal hence reacts explosively with steam.

6. Add calcium oxide /lime to raise the soil pH. Calcium oxide is a basic oxide hence dissolves in water in the soil to form alkaline solution that reacts with acidic soil raising soil pH.

8 .

(a) (i) Ionic/ electrovalent.

(iii) Covalent (Van der Waals)

(b) Chloride ofD giant ionic; when in molten state the ions are mobile hence conducts electric current. E is giant molecular and therefore does not
have mobile ions to carry electric current.

21. a) Atomic radii decreases across the period. Across the period the number of protons increases increasing the nuclear attraction for the outermost electrons contributing to decrease in atomic radii.

b) AlC13 is molecular/ covalent. It differs from other metal chlorides
because it exits as a dimmer. Two molecules of AlC13 pair through co­ordinate bonds while the other metal chlorides are ionic.
OR
AlC13 hydrolyzes in water while the other chlorides do not.

2019 KCSE chemistry paper 2 marking scheme

1. (a) (ii) Compounds made of carbon and hydrogen only and where the bonds ( carbon to carbon) are single covalent /every carbon atom contains its maximum number of hydrogen atoms.

4 (b)-Add excess aqueous ammonia to the water,
formation of white precipitate shows presence of Al3+ or Pb2+ ions

To another sample of water add dilute H2S04 / HCl
formation of colouress solution shows presence of Al3+ / add potassium iodide if a yellow precipitate does not form it confirms presence of Al3+ ions.

(c) (i) – Heat strongly and weigh.
– re-heating the substance and re-weighing. If mass does not change constant mass has been attained.

2019 KCSE chemistry paper 3 marking scheme