Observation

This question is a popular question, and majority of the candidates responded to it.

In part (a), majority of the candidates stated increase in temperature as a condition necessary for the cracking of long-chain hydrocarbon instead of high temperature. Also, few of them wrote temperature.

In part (b), majority of the candidates could not state two reasons why metallic objects are electroplated.

In part (c), majority of the candidates knew that CaO and HCl could react, but could not satisfactorily explain why calcium oxide could not be used to dry hydrogen chloride gas.

In part (d), majority of the candidates did not mention concentrated HNO₃ as an oxidizing agent, but got marks for writing Fe₂₊ to Fe₃₊.

In part (e), majority of the candidates were able to write the equation for the reaction between zinc oxide and dilute tetraoxosulphate (VI) acid, but could not write the reaction between zinc oxide and sodium hydroxide solution.

In part (f), majority of the candidates were able to state one similarity and one difference between the isotopes of chlorine.

In part (g), majority of the candidates were able to state the products formed when chlorine water is exposed to sunlight.

In part (h), few candidates were able to state the species that undergone oxidation in the reaction equation.

In part (i), majority of the candidates could not describe the term carbon-12 scale.

In part (j), A fair attempt was made at stating the properties of a chemical system in equilibrium.

The expected answers include:

1. • high temperatures / stated temperature of 500oC or more
   • catalyst
2. • To prevent corrosion/rusting/tarnishing
   • To make them shiny/for decoration purpose/beautify/attractive
   • To make surface harder/thicker
   • To make more resistant to chemicals
   • To make it durable
   • To reduce friction/abrasion
3. 1. Because calcium oxide is basic and hydrogen chloride is acidic ,they will react to form salt and water.
   2. • Fused CaCl₂
      • (Conc) H₂SO₄ (do not accept dil H₂SO₄)
      • Silica gel
      • Phosphorus(V) oxide/P₂O₅
4. Conc. HNO₃ is a strong oxidizing agent. It oxidizes pale green Fe²⁺ ions to yellow Fe³⁺ ions (1) and itself reduced to nitrogen (IV) oxide.
5. 1. I. ZnO_(s) + H₂SO_4(aq) → ZnSO_4(aq) + H2O_(l)
      II. ZnO_(s) + 2NaOH_(aq) → Na₂Zn(OH) /
      OR
      ZnO + 2NaOH→ Na₂ZnO₂+ H₂O
   2. Amphoterism / Amphoteric properties
6. 1. Same number (17) of protons / same number (17) of electrons/same atomic number
   2. Difference in mass numbers / different number of neutrons / 35Cl has 18 neutrons and 37Cl has 20 neutrons
7. • HCl / hydrochloric acid
   • O₂ / Oxygen
8. 1. H₂S
   2. SO₂
9. It is the mass reference scale in which the atom of the ¹²C isotope is taken to have twelve units of atomic mass
   OR
   It is a scale that uses the ¹²C isotope as the standard for comparing the atomic masses of other elements.
10. • The rate of forward reaction equals the rate of backward reaction / The system is dynamic
    • There is no net change in the concentration of each substance
    • The equilibrium compositions of the substances can be approached from either reactants or products
    • Gibb’s free energy/∆G is zero