Chemistry Paper 2 (SCHOOL CANDIDATES) 2019

Question 1

 

  1. State the conditions necessary for the cracking of long-chain hydrocarbons to produce more gasoline. [2 marks]
  2. State two reasons why metallic objects are electroplated [2 marks]
    1. Explain briefly why calcium oxide cannot be used to dry hydrogen chloride gas.
    2. State one drying agent for hydrogen chloride gas. [3 marks]
  3. Concentrated trioxonitrate (V) acid was added to a solution of iron (II) tetraoxosulphate (VI) and the mixture heated. The mixture turned from pale green to yellow with the evolution of a brown gas. Explain briefly these observations. [3 marks]
    1. Write the equation for the reaction between zinc oxide and
      (I) dilute tetraoxosulphate (VI) acid,
      (II) sodium hydroxide solution.
    2. State which property of zinc oxide is shown by the reaction in 1(e)(i). [5 marks]
  4. Two isotopes of chlorine are 35 Cl and Cl. State one:
    1. similarity;
    2. difference between the isotopes.
  5. State the two products formed when chlorine water is exposed to sunlight. [2 marks]
  6. Consider the reaction represented by the following equation:

    State the:
    1. species that is undergoing oxidation;
    2. oxidizing agent. [2 marks]
  7. What is meant by carbon-12 scale? [2 marks]
  8. State two properties of a chemical system in equilibrium. [2 marks]
                                  

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:


    • high temperatures / stated temperature of 500oC or more
    • catalyst
    • 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
    1. Because calcium oxide is basic and hydrogen chloride is acidic ,they will react to form salt and water.
      • Fused CaCl2
      • (Conc) H2SO4 (do not accept dil H2SO4)
      • Silica gel
      • Phosphorus(V) oxide/P2O5
  1. 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.
    1. I. ZnO(s) + H2SO4(aq) → ZnSO4(aq) + H2O(l)
      II. ZnO(s) + 2NaOH(aq) → Na2Zn(OH) /
      OR
      ZnO + 2NaOH→ Na2ZnO2+ H2O
    2. Amphoterism / Amphoteric properties
    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
    • HCl / hydrochloric acid
    • O2 / Oxygen
    1. H2S
    2. SO2
  2. 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.
    • 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