Chemistry Paper 2 (SC),2019

Question 3

In the Solvay process, explain briefly with equations the functions of the following substances:
1. limestone;
2. ammonia.
[7 marks]
1. Write a chemical equation for the fermentation of glucose.
2. Explain briefly why a tightly-corked glass bottle filled to the brim with fresh palm-wine shatters on standing for some time.
[5 marks]
Consider the following metals: Na, Fe, K and Cu
1. Arrange the metals in order of increasing reactivity.
2. Which of the metals will react with cold water?
3. Which of the metals could form coloured salts?
[3 marks]
1. What is a redox reaction ?
2. Identify which of the following reaction equations are redox.
  I. 2Na + Cl₂ → 2NaCl
  II. AgCl + 2NH₃ → [Ag(NH₃)₂]Cl
  III. C₂H₂ + H₂ → C₂H₄
  IV. HCl + KOH → KCl + H₂O
  V. 2FeCl₃ + 2KI → 2FeCl₂ + 2KCl + I₂
3. Give a reason for each of the answers in 3(d)(ii).
4. Write balanced equations of the half reactions for any two of the redox reactions in 3(d)(ii).

Observation

Majority of the candidates avoided this question.

In part (a), majority of the candidates did not display adequate understanding of the solvay process and the relevant equations.

In part (b), majority of the candidates were able to write a chemical equation for the fermentation of glucose.

In part (c), a fair attempt was made at arranging metals in reactivity series and their reactions with cold metal.

In part (d), redox reaction was not satisfactorily explained, but a fair attempt was made at identifying redox reactions.

In part (e), majority of the candidates could not write balanced equations for half reactions.

The expected answers include:

1. - Source of CO₂ for the carbonating towers
  - Source of CaO for the recovery of ammonia
  - CaCO_3(g) ⟶ CaO_(s) + CO_2(g)
2. Ammoniation of brine /formation of ammoniacal brine/to make the brine alkaline
  NaCl_(aq) + NH_3(aq) + H₂O_(l) + CO_2(g) ⟶ NaHCO_3(s) + NH₄Cl_(aq)
  OR
  NH₃ + H₂O + CO₂ ⟶ NH₄ HCO₃
  NH₄ HCO₃ + NaCl ⟶ NH₄Cl + NaHCO₃
  Candidate must write both equations to score
1. C₆H₁₂O_6(aq) ⟶ 2C₂H₅OH_(l) + 2CO_2(g)
2. On standing for some time, there is a build-up of CO₂ released by the fermentation process causing the pressure in the glass bottle to build-up which eventually shatters the bottle.
1. A redox reaction is one that involves both oxidation and a reduction of species, one being oxidized (by losing electrons and the other reduced by gaining the electrons /change in oxidation number of species/transfer of electrons/ addition or removal of hydrogen or oxygen
2. I. 2Na + Cl₂ ⟶ 2NaCl
  
  III. C₂H₂ + H₂ ⟶ C₂H₄
  
  V. 2FeCl₃ + 2KI ⟶ 2FeCl₂ + 2KCl + I₂
  
  OR
  
  I, III, and V
3. I. Oxidation number of Na increases from 0 to +1 while oxidation number of Cl decreases from 0 to -1. /Na transfers electron to Cl and Cl accepts electrons from Na
  II. Oxidation number of H₂ decreases from 0 to -1 while that of carbon increases from +1 to +2. Carbon transfers electrons to hydrogen.
  III. I- transfers electron to Fe³⁺, while Fe³⁺ accepts electron from I-.
  (Explanation must include oxidation and reduction to score)

Chemistry Paper 2 (SCHOOL CANDIDATES) 2019

Question 3

Observation