Paper 2        [Structured and Free Response Questions]

                        Section A [Structured Questions]
1 Petrol...
(ii) Suggest why there may be future difficulties in this use of petroleum.

    Petroleum takes millions of years to form...
zone X


                                                     zone Y


                                            zone Z
...
The amount of heat produced is much lesser in the incomplete combustion of
         methane that produces soot when compar...
(a) Explain in terms of the bonds broken and made why this reaction is exothermic.

       The amount of energy released i...
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Organic Chem Hw 1

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Organic Chem Hw 1

  1. 1. Paper 2 [Structured and Free Response Questions] Section A [Structured Questions] 1 Petroleum (crude oil) is a mixture of hydrocarbons which is separated into useful fractions by fractional distillation. The table shows the percentages of various fractions obtained by fractional distillation (the supply) and the percentages required (the demand) for the petroleum from a Middle East oil field. percentage produced percentage required fraction (supply) (demand) Petroleum gases 4 4 Petrol 13 26 Naphtha 10 5 Kerosene 14 8 Diesel 13 20 Bitumen and other heavy 46 37 hydrocarbons (a) (i) What is meant by the term hydrocarbon? Hydrocarbons are organic compounds that contain the elements hydrogen and carbon only. (ii) What feature do the different hydrocarbons in petroleum have, which enables them to be separated by fractional distillation? The different hydrocarbons in petroleum have different boiling points. (b)(i) For which fraction does the demand exceed the supply by the greatest amount? Petrol (ii) For which fraction does the supply exceed the demand by the greatest amount? Bitumen and other heavy hydrocarbons (iii) Suggest how the demand for the fraction named in your answer in (i) can be satisfied. By cracking the large hydrocarbon molecules from fractions such as kerosene and bitumen into smaller hydrocarbon molecules so that they can be used as petrol for motor cars. (c) (i)Which fraction in the table is used to manufacture a wide range of organic chemicals? Naphtha 1
  2. 2. (ii) Suggest why there may be future difficulties in this use of petroleum. Petroleum takes millions of years to form and is a non-renewable resource. The demands for the use of petroleum is ever-increasing, and the finite reserves of the world’s petroleum means that it will be used up one day. 2 Natural gas consists mainly of methane (CH4). Hydrogen sulphide (H2S) is a common contaminant of natural gas and must be removed before natural gas is used as a fuel. (a) Construct the equation to show the complete combustion of methane in air. CH4 (g) + 2O2(g) → CO2(g) + 2H2O (l) (b) (i) Construct the equation to show the combustion of hydrogen sulphide in air to form sulphur dioxide and water. 2H2S(g) + 3O2(g) → 2SO2(g) + 2H2O(l) (ii)A 480 dm3 sample of natural gas, containing 10% hydrogen sulphide, is completely burnt in air. Calculate the mass of sulphur dioxide produced. Volume of hydrogen sulfide present in the sample = 10/100 × 480 dm3 = 48 dm3 Number of moles of hydrogen sulfide = 48/24 = 2 mol Number of moles of sulfur dioxide produced = 2 mol Mass of sulfur dioxide produced = 2 × [32 + 2(16)] = 128 g (iii) Suggest why it is necessary to remove the hydrogen sulphide before natural gas is used as a fuel. If the hydrogen sulfide is not removed, sulfur dioxide will be produced when the sample of natural gas is burnt in air. Sulfur dioxide is an air pollutant that dissolves in rain water to form acid rain. (c) State one environmental problem that can be caused by leakage of methane from natural gas pipelines. Methane is a greenhouse gas that causes an increase in global temperature. The effect of global warming will cause the polar ice caps to melt, resulting in a rise in sea level bringing floods to low lying areas. 3 The diagrams show the flames of a Bunsen burner, with the air-hole closed (diagram A) and with the air-hole open (diagram B). The Bunsen burner is using methane fuel. 2
  3. 3. zone X zone Y zone Z gas gas inlet inlet diagram A diagram B (a) What colour is the flame (i) in zone X? yellow/orange (ii) in zone Z? colourless (b) (i) Suggest an explanation for the colour of the flame in zone X. Incomplete combustion of methane forms carbon particles. The glowing specks of hot soot particles give the colour of the flame in zone X. (ii) Write an equation for the combustion of methane in zone X that supports your answer in (i) above. CH4 + O2 → C + 2H2O (c) State two reasons why burning methane to produce energy in the manner shown in diagram A is undesirable. Reason I: Carbon monoxide is also produced in the incomplete combustion of methane. Carbon monoxide is a poisonous gas that binds with the haemoglobin molecule, preventing it from carrying oxygen in the blood. Reason II: 3
  4. 4. The amount of heat produced is much lesser in the incomplete combustion of methane that produces soot when compared to the complete combustion of methane. (d) The equation for the complete combustion of methane in zone Y is CH4 + 2O2  CO2 + 2H2O Assuming that the Bunsen burner in diagram B is completely burning 120 cm3 of methane per second and that air contains 20% oxygen, calculate the volume of air required by the burner per second. Volume of oxygen required by burner per second = 2 × volume of methane = 2 × 120 cm3 = 240 cm3 Volume o air required by burner per second = 5 × 240 cm3 = 1200 cm3 Section B [Free Response Questions] 1 Methane is a fuel obtained from natural gas. Methane completely burns in air to form carbon dioxide and water. An equation for this reaction is shown below. H H C H + 2O O O C O + 2 O ;∆H = -890 kJ/mol H H H 4
  5. 5. (a) Explain in terms of the bonds broken and made why this reaction is exothermic. The amount of energy released in making new bonds in the products (i.e. the 2 C=O and 4 O-H bonds) is more than the amount of energy absorbed in breaking old bonds in the reactants (i.e. the 4 C-H and 2 O=O bonds). Thus, there is a net release of heat energy and the reaction is exothermic. (b) Calculate the energy released when 0.96 g of methane is completely burnt in air. Number of moles of methane = 0.96/[12 +4(1)] = 0.06 mol Amount of energy released = 0.06 × 890 kJ = 53.4 kJ (c) Name all the possible products in the incomplete combustion of methane. Explain how this incomplete combustion can be harmful. Carbon particles (soot), carbon monoxide and water. Carbon monoxide is a poisonous gas that binds with the haemoglobin molecule, preventing it from carrying oxygen in the blood. 2 Propane, C3H8, is a fuel obtained from petroleum. For the complete combustion of one mole of propane, ∆H = -2200 kJ. (a) State how propane can be obtained from petroleum. Propane can be obtained by the fractional distillation of petroleum. (b) Construct the equation for the complete combustion of propane. C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l) (c) Calculate the quantity of heat evolved from the complete combustion of 32 dm3 of propane, measured at room temperature and pressure. Amount of heat evolved = 32/24 × 2200 kJ = 2933.33 kJ (d) How can you tell from the equation that the reaction is exothermic? The overall enthalpy change, ∆H, for the reaction is negative. 5
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