Chemistry Paper 2 (Essay) ,May/June 2011

Questions:		1	2	3	4	5	6	7	8		Main

General Comments
Weakness/Remedies
Strength

Question 7
(a) A compound X reacts with excess HNO3(aq) to give carbon (IV) oxide and another compound Y. A solution of Y reacts with NaOH(aq) to form a white precipitate which is insoluble in excess NaOH(aq). Identify X and Y. (b) (i) Write a balanced equation to illustrate the reducing property of ammonia in its reaction with CuO. (ii) Explain why it is not advisable to heat ammonium dioxonitrate (III) directly. (iii) Give two uses of nitrogen. (c) Give the reason why (i) dilute H2SO4 is not suitable for the preparation of CO2(g) from CaCO3(s). (ii) concentrate H2SO4 cannot be used to dry ammonia gas. (d) State two: (i) physical properties; (ii) chemical properties of metals. (e) What is the oxidation number of (i) chlorine in I. Cl2; II. CLO3-; (ii) vanadium in V2O5. (f) (i) Explain the term half-life. (ii) Two radioactive elements, P and Q have half-life of 1200 seconds and 3600 seconds respectively. I. Which of the elements is more stable? II. Give a reason for your answer.
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OBSERVATION
This was an unpopular question as it involves mainly comprehension and application processes. In (a), most candidates could not identify X and Y as required. Part (b), only few candidates were able to write balanced equation to illustrate the reducing the property of NH3 in its reaction with CuO. Few candidates were able to state that heating ammonium diononitrate (III) could be explosive but could not explain why. Most candidates wrongly gave the use of nitrogen as in the manufacture of fertilizer (which is not a direct use). Parts (c),(d) and (e) were poorly attempted by most responding candidates. Part (f), most candidates thought that decomposition is synonymous with decay disintegration in use when explaining the term half-life. Most candidates could not also state with reasons which of P and Q is more stable. The expected responses from candidates were as follows: (a) X – CaCO3 Y – Ca(N)3)2 (b) (i) 2NH3(g) + 3CuO(s) → 3Cu(s) + 3H2O(g) (ii) explosion may occur because ammonium dioxonitrate (III) is unstable and decomposes exothermically. (iii) - manufacture of NH3 etc. liquid nitrogen as cooling agent gas carrier in gas chromatography as preventive against oxidation (c) (i) Dilute H2SO4 reacts with CaCO3(s) to form an insoluble salt. CaSO4, which covers the CaCO3. Thus preventing further reaction or production of CO2(g) after a short time. (ii) conc. H2SO4 reacts with ammonia to form a salt (NH4)2 SO4 (d) (i) Physical Properties of metals - malleability/ductility - good conductor of heat/electricity - sonorous - lustrous - relatively high density - high melting/boiling points (ii) Chemical Properties of metals ionize by electron loss/reducing agent form basic oxides (some) displace hydrogen from dilute acids a more reactive metal displaces a less reactive metal from a solution of its salt (Accept displacement reaction). (e) (i) I. oxidation number of Cl = O II. oxidation number of Cl = +5 (ii) oxidation number of V = +5 (f) (i) Half-life is the time taken for half of the total number of atoms in a given sample of the element to decay. (ii) I. Q is more stable II. stability is dependent on half life, the longer it takes substance to decay the more stable it is.