Question 4
(a) Describe briefly how pure crystals of Zinc tetraoxosulphate (VI) can be prepared in the laboratory from zinc granules and dilute tetraoxosulphate (VI) acid. [6 marks]
(b) (i) State two substances produced when coal is heated in the absence of air.
(ii) Name the process that occurs in 4(b)(i).
(iii) State the major product(s) when carbon is heated with concentrated trioxonitrate (V) acid. [5 marks]
(c) (i) The production of nitrogen from air involves removal of other constituents of air namely: carbon (IV) oxide, oxygen and water.
I. State the reagents used to remove each of the constituents listed.
II. Identify the chemical or physical property of each of the reagents which allows it to be used.
(ii) What is meant by the statement nitrogen is an inert gas?
(iii) Using a chemical equation show that trioxonitrate (V) acid is an oxidizing agent. [9 marks]
(d) (i) List the two allotropes of carbon.
(ii) Explain briefly why one of the allotropes of carbon conducts electricity.
[5 marks]
Observation
Few candidates responded to this question and their performance was below average.
In part (a), majority of the candidates could not describe how pure crystals of zinc tetraoxosulphate (VI) can be prepared in the laboratory from zinc granules and dilute tetraoxosulphate (VI) acid.
In part (b), majority of the candidates stated two substances produced when coal is heated in the absence of air.
In part (c), majority of the candidates could not state the reagents that could be used to remove other constituents of air during the production of nitrogen from air.
In part (d), majority of the candidates listed the two allotropes of carbon.
The expected answers include:
(a) Excess zinc granules are added to dilute tetraoxosulphate (VI) acid in a flask
Effervescence is generated, filter the mixture when the effervescence ceases. The mixture is heated to concentrate the filtrate and allow to cool and then the crystals is filtered off washed with distilled water and dried.
(b) (i) - coal gas
- coal tar
- ammoniacal liquor
- coke
(ii) - destructive distillation of coal
(iii) - nitrogen (IV) oxide / NO2
- Carbon (IV) oxide / CO2
(c) (i)
I.
Carbon (IV) oxide - KOH / NaOH
Oxygen - Iron filing / heated Cu turnings / Alkaline pyrogallol
Water - Conc. H2SO4 / CaCl2 / KOH / NaOH
II. - KOH / NaOH is an alkali
- Fe / Cu turnings / alkaline pyrogallol is a reducing agent
- CaCl2 / Conc. H2SO4 / KOH / NaOH absorbs water hygroscopic / deliquescent
(ii) Nitrogen does not react under ordinary temperature and pressure / reaction takes place at high temperatures / strong triple bond between atoms of nitrogen
(iii)
3Cu + 8HNO3 → 3Cu(NO3)2 + 4H2O + 2NO
I2 + 10HNO3 →2HIO3 + 4H2O + 10NO2
Cu + 4HNO3 → Cu(NO3)2 + 2H2O + 2NO2
Sn + 4HNO3 → SnO2.H2O + H2O + 4NO2
C + 4HNO3 → CO2 + 2H2O + 4NO2
S + 6HNO3 → H2SO4 + 2H2O + 6NO2
P + 5HNO3 → H3PO4 + 5NO2 + H2O
(d) (i)
- diamond
- graphite
- coal
- coke
- charcoal
- lamp black
(ii) Graphite has delocalized electrons in the structure and therefore conducts electricity
Diamond does not have any delocalized electrons and hence does not conduct electricity.