KNEC KCSE Past Papers 2016 Chemistry paper 2 (233/2)
Kenya certificate of Secondary Education
2016 Chemistry paper 2
Questions and Answers
No. 1. Use the information in the table below to answer the questions that follow. The letters do not represent the actual symbols of the elements.
(a) Give reasons why the melting point of: (i) S is higher than that of R; (1 mark)
❖ Metalic bonds in S are stronger than in R
(ii) V is lower than that of U. (2 marks)
❖ V is monoatomic while U is diatomic
❖ The Van der Waals force in V are weaker than in U
(b) How does the reactivity of W with Chlorine compare with that of R with chlorine? (2 marks) Explain.
❖ W reacts more vigorously
❖ It is easier to lose the odur/valence electrons in W than in T
❖ W is more electropositive than R
❖ Reactivating of groups elements. Increases down the group
(c) Write an equation for the reaction between T and excess oxygen. (1 mark)
❖ 4T(s) + 502(g) -> 2T2O5(g)
(d) When 1.15g of R were reacted with water, 600cm3 of gas was produced. Determine the relative atomic mass of R. (Molar gas volume = 24 000 cm2). (3 marks)
(e) Give one use of element V. (1 mark)
❖ Used in florescent bulbs / lumbs
❖ Used in arc welding
❖ Used in fire extinguishers
❖ Used as a preservative in museum
No. 2 .Describe the process by which Nitrogen is obtained from air on a large scale. (4 marks)
❖ Filter the air/ electrostatic precipitation/ Purify the air
❖ Pass air through NaOH in KOH to remove CO2
❖ Cool to remove to remove water vapour
❖ Cool the remaining gases from a liquid air
❖ Perform fractural distillation of liquid air
❖ Nitrogen is collected at – 196° C
(a) (b) Study the flow chart below and answer the questions that follow.
(i) Identify gas J.(1 mark)
❖ Nitrogen II Oxide (NO)
(ii) Using oxidation numbers, show that ammonia is the reducing agent in step Tip (2 marks)
❖ 4NH3 (g) + 3 CUO —2N2 (g) + 3H20(1) + 3 Cu
❖ Oxidation no of N in ammonia increases from -3 to 0
❖ Oxidation number of reducing agent increases
❖ Oxidation number Cu decreases from + 2 to 0 hence an oxidizing agent Ammonia is a reducing agent
(iii) Write the equation for the reaction that occurs in step (V). (1 mark)
❖ NH4NO3(s) or (ag) —>N20 (g) + 2H20 (g or 1)
(iv) Give one use of ammonium nitrate. (1 mark)
❖ Fertilizer
❖ Explosive (c) The table below shows the observations made when aqueous ammonia was added to cations of elements E, F and G until in excess.
Cation of Addition of a few drops of Aqueous ammonia. Addition of excess aqueous ammonia E White precipitate Insoluble F No precipitate No precipitate G White precipitate Dissolves (i) Select the cation that is likely to be Zn2+ (1 mark)
❖ G or G2+
(ii) Given that the formula of the cation of element E is E 2+ , write the ionic equation for the reaction between E2+(aq) and aqueous ammonia. (1 mark)
E2+ + 20H (ag) -> E(OH)2(s)
No.3.(a) Methanol is manufactured from carbon (IV) oxide and hydrogen gas according to the equation: CO2 (g) + 3H2(g) CH3OH(g) + H20(g) The reaction is carried out in the presence of a chromium catalyst at 700K and 30kPa. Under these conditions, equilibrium is reached when 2% of the carbon (IV) oxide is converted to methanol (i)How does the rate of the forward reaction compare with that of the reverse reaction when 2% of the carbon (IV) oxide is converted to methanol?(1 mk)
❖ Rate of forward reaction is equal to tare of backward reaction
(ii)Explain how each of the following would affect the yield of methanol: I Reduction in pressure (2mks)
❖ Production will reduce since equilibrium will shift backward so as to raise the pressure.
II Using a more efficient catalyst (2mks)
❖ No change in amount of methanol since a catalyst will help reaction to come to equilibrium
(iii) If the reaction is carried out at 500K and 30IcPa, the percentage of carbon (IV) oxide converted to methanol is higher than 2% I what is the sign of ,H for the reaction? Give a reason (2mks)
❖ Negative: the reaction is exothermic since it requires low temperature to be fast.
II Explain why in practice the reaction is carried out at 700K but NOT at 500K (lmk)
❖ To ensure that the reacting particles posses more activation energy
(b)Hydrogen peroxide decomposes according to the following equation: 2H202(aq) ,2H20(1) + 02 (g) In an experiment, the rate of decomposition of hydrogen peroxide was found to be 6.0 x 10-8 mol dm-3 S-1.
(i)Calculate the number of moles per dm3 of hydrogen peroxide that had decomposed within the first 2 minutes (2mks)
❖ No. of seconds = 2 x 60 = 120 Sec Moles of H202 decomposed = 120 x 6.0 x las = 7.20 x 106
(ii) In another experiment, the rate of decomposition was found to be 1.8 x 10 – 7 mol dm -3S-1. The difference in the two rates could have been caused by addition of a catalyst. State, giving reasons, one other factor that may have caused the difference in two rates of decomposition (2 marks)
❖ Concentration of H202 may be higher since concentration increases the rate of reaction.
No.4.The set up below can be used to produce sodium hydroxide by electolysing brine.
(i) Identify gas Y. (1mk)
❖ Chlorine or Cl
(ii)Describe how aqueous sodium hydroxide is formed in setup above. (2mks)
❖ Na+ and H+ migrate to the cathode. The H+ ions are preferentially discharged liberating hydrogen gas. Cl– and OH- migrate to anode.
The Cl+are preferentially discharged liberating Cl2 gas. Na– migrate to anode through the membrane , they combine with OH to from NaOH
(iii)One of the uses of sodium hydroxide is in manufacture of soaps. State one other use of sodium hydroxide. (lmk)
❖ Glass making
❖ Separating components of air.
❖ Manufacture of soda lime, NaoCl and NaC103
❖ Purification of bauxite
❖ Used in science lab.
lb) Study the information given in the table below and answer the questions that follow.
(i) Construct an electrochemical cell that will produce the highest emf. (3mks)
(ii) Calculate the emf of the cell constructed in (i) above. (2mks)
❖ E = Ered – Eox = + 0.80- (-2087) = + 3.67v
(iii)Why is it not advisable to store a solution containing E+ ions in the container made of H? (2mks)
❖ H has a high tendancy to lose electrons hence will react with the solution by displacing E+ ions
No.5.The diagram below represents a set up of an electrolytic cell that can be used in the production of aluminium.
(b)Write the equation for the reaction at the anode.(1 mark)
❖ 202--> 02(g)+4e-
(c) Give a reason why the electrolytic process is not carried out below 950°C. (1 mark)
❖ Aluminium oxide would solidify hence the ions will not be free to conduct current.
(d)Give a reason why the production of aluminium is not carried out using reduction process (1 mark)
❖ The common reducing agents such as carbon cannot reduce aluminium oxide.
(e)Give two reasons why only the aluminium ions are discharged. (2 marks)
❖ They are in higher concentration than Na+ from cryolite
❖ Aluminium is lower than sodium in electrochemical series.
(f)State two properties of duralumin that makes it suitable for use in aircraft industry.(2 marks)
❖ It is stronger than pure aluminium
❖ It is light
(g)Name two environmental effects caused by extraction of aluminium. (2 marks)
❖ It causes land degradation due to mining the ores from deep down from the earth.
❖ Carbon (Iii Oxide produced cause global warming.
No.6.a)Draw the structural formula for all the isomers of C2H3CL3 (2marks)
b) Describe two chemical tests that can be used to distinguish between ethane and ethane. (4marks)
❖ Add a few drops of acidified potassium dichromate with ethane the solution changes from orange to green while in ethane the solution remains orange
❖ Add a few drops of acidified potassium manganate with ethane solution changes from purple to clourless while in ethane the solution remains purple.
c)The following scheme represents various reactions starting with propan-i-ol. Use it to answer the uestions that follow.
Add sodium carbonate i) Name one substance that can be used in step I. (1 mark)
iv) Calculate the mass of propan-I-ol which when burnt completely in air at room temperature and pressure would produce 18dm3 of gas. (C = 12.0; 0 = 16.0; H = 1.0; Molar gas volume = 24dm3) (3 marks)3
No. 7. (a) Write an equation to show the effect of heat on the nitrate of: (2 marks)(i) Potassium
❖ 2 KNO3 (s) heat
(ii) Silver
❖ 2 AgNO3(s) -> heat -> 2Ag + 2N02 + O2(g)
(b) The table below gives information about elements A1, A2, A3, and A4
i) In which period of the periodic table is element A2? Give a reason. (2 marks)
❖ Period 2, two energy levels
i) In which period of the periodic table is element A2? Give a reason. (2 marks)
❖ Period 2, two energy levels
(ii) Explain why the atomic radius of: I. Ai is greater than that of A2;
❖ Az has greater atomic number than Ai
❖ A2 has greater nucleus charge than A1
❖ A2 has more protons than Al
❖ Therefore across the period from left to right nuclear charge, exert greater pull on electrons hence reduction in size.
II. A4is smaller than its ionic radius. (2 marks)
❖ ila gains electrons, incoming electron is repelled by existing electrons then electrons cloud increases.
(iii) Select the element which is in the same group as A3.(1 mark)
• A2
(iv) Using dots (•) and crosses(x) to represent outermost electrons, draw a diagram to show the bonding in the compound formed when Ai reacts with A4. (1 mark)
