Section 5 exam questions and answers

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Published on March 9, 2014

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Section 5 Exam Questions and Answers 2002 Question 8 The following hydrocarbons can all be used as fuels. methane (CH4) butane (C4H10) 2,2,4-trimethylpentane (C8H18) (a) Butane is a major component of LPG. What do the letters LPG stand for? (5) Draw two structural isomers of butane. (6) (b) Methane is a major component of natural gas. Why are mercaptans often added to natural gas? What environmental change or effect is associated with the release of methane to the atmosphere? Apart from leaking gas pipes, name a major source from which methane is released to the atmosphere. (9) (c) What structural feature of 2,2,4-trimethylpentane results in it having a high octane rating? Give one other structural feature which increases the octane number of a hydrocarbon. (6) (d) Define heat of combustion of a compound. (6) (e) The combustion of butane is described by the following balanced equation. 2C4H10(g) +13O2(g) → 8CO2(g) + 10H2O(l) Calculate the heat of combustion of butane given that the heats of formation of butane, carbon dioxide and water are – 125, –394 and –286 kJ mol-1, respectively. (18) 2002 Question 8 (a) LPG: liquefied (liquid) petroleum gas (5) (6) [Accept sticks without Hs put in] (3) [Accept sticks without Hs put in] (3) WHY: to give an odour (smell) / to detect leaks (3) WHAT: greenhouse effect / global warming / reducing ozone damage (3) SOURCE: fossil fuels / oil / coal / marshes (bogs, paddy fields) / animals (cows, sheep, etc.) /dumps / slurry / anaerobic decay / compost (3) FEATURE: branching / short chain OTHER: ring / branching (unless used above) / aromatic / short chain (if not used above) DEFINE: heat when 1 mole (3) is burned completely / burned in excess oxygen –1 CALCUL: – 2881 kJ mol (18) ΔH = ΔHformation(products) - ΔHformation(reactants) ΔH = [4 (3) ( – 394) (3)+ 5 (3) (– 286) (3) – (– 125) (3)] = – 2881 kJ mol–1 (3) OR [ – 1576 (6) – 1430 (6) + 125 (3)] = – 2881 kJ mol–1 (3) (b) (c) (d) 2003 (3) (3) (3) (18) Question 10. (a) Define heat of combustion. (7) Propane may be used in gas cylinders for cooking appliances. Propane burns according to the equation C3H8 + 5O2  3CO2 + 4H2O (i) The heats of formation of propane, carbon dioxide and water are –104, –394 and –286 kJ mol–1 respectively. Calculate the heat of combustion of propane. (12) (ii) If 500 kJ of energy are needed to boil a kettle of water what mass of propane gas must be burned to generate this amount of heat? Express your answer to the nearest gram. (6) 2003 Question 10. (a) (a) (i) (ii) Define: heat when 1 mole (4) is burned completely / burned in excess oxygen –1 Calc: ΔH = – 2222 kJ mol Σ ΔH formation(products) – Σ ΔH formation(reactants) = ΔH –1 3 × – 394 or – 1182 (3) 4 × – 286 or – 1144 (3) – (–104) /or + 104 / 104 (3) = – 2222 kJ mol (3) 10 g 1 × 500 = 0.225 mol (3) × 44 = 10 (3) OR 44 × 500 (3) = 10 (3) Note: penalty (– 1) if answer 2222 2222 not rounded off to 10. (3) (12) (6)

2004 Question 6 (a) Define (i) heat of formation of a substance, (ii) octane number of a fuel. (11) (b) The combustion of methane is described by the following balanced equation. CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) ΔH = − 890.4 kJ mol-1 The standard heats of formation of carbon dioxide and water are −394 and −286 kJ mol-1 respectively. Calculate the heat of formation of methane. (12) (c) Methane is an excellent fuel. Give two properties of methane which account for its usefulness as a fuel. Natural gas is a rich source of methane. Why are mercaptans often added to natural gas? (9) (d) Methane is often found in gas fields which occur in association with crude oil deposits. Crude oil is fractionated in order to obtain more useful products. Outline clearly how the fractionation process is carried out. (12) (e) Identify two structural features of a hydrocarbon fuel which affect its octane number. (6) 2004 (a) (b) (c) (d) (e) Question 6 (i) (ii) heat (energy) when 1 mole //of compound formed from its elements measure of tendency to auto-ignite (knock) or number representing ability of fuel to resist auto-igniting (knocking) –1 CALC.: – 75.6 kJ mol ΔH(reaction) = Σ ΔHf(products) – Σ ΔHf(reactants) – 890.4 (3) = [ – 394 (3) – 572 (3)] – [ΔHf(methane) + 0] –1 – ΔHf(methane) = – 394 – 572 + 890.4 = – 75.6 kJ mol (3) [Allow 3 marks only for +75.6 kJ mol 1 ] PROPS: high kilogram cal. value (high heat of combustion) / clean (non- polluting) / non-toxic / relatively cheap/ can be piped Do not accept „easily distributed in tanks‟ ANY TWO: (2 x 3)] WHY: to give an odour (smell) / to detect leaks / to make safe OUTLINE: crude oil heated / crude added continuously at bottom / vapour passes up through column / fractionating tower with trays fractions condense at different levels depending on their boiling points / high b.p. fractions at bottom / low higher up /heavier at bottom / lighter at top / or named exemplar to indicate this [The first three points can be got from a diagram; the last point must be specified in words or clearly written on the diagram.] ANY THREE: (6 + 2 x 3) IDENTIFY: chain length / branching / cyclic / aromatics ANY TWO: (2 x 3) (3+2) (6) (12) (6) (3) (12) (6) 2005 Question 6 (a) The octane number of a fuel is described as a measure of the tendency of the fuel to cause knocking, or as a measure of the tendency of the fuel to resist auto-ignition. This number is found by comparing the combustion of the fuel with the combustion of a mixture of two reference hydrocarbons using the same standard engine. (i) Name both of the reference hydrocarbons present in the mixture used when measuring octane number by this comparison method. (8) (ii) State two structural features of a hydrocarbon molecule which contribute to it having a high octane number. (6) (iii) Lead compounds were used in the past to increase the octane number of fuels. Why are lead compounds unsuitable as additives for petrol used in modern cars? (3) (iv) Identify one additive or type of additive, other than a compound of lead, used to increase the octane number of fuels. (3) (b) There are three structural isomers of the hydrocarbon of formula C5H12. In the case of each of these isomers, draw the structure of the molecule and give its systematic IUPAC name. (18) (c) The combustion of liquid benzene is described by the following equation: 2C6H6(l) + 15O2(g)  12 CO2(g) + 6H2O(l) Given that the heats of formation of carbon dioxide gas, liquid water and liquid benzene are –394,–286 and 49 kJ mol-1 respectively, calculate the heat of combustion of liquid benzene. (12)

2005 (a) Question 6 (i) (ii) (iii) (iv) (b) (c) NAME: 2,2,4-trimethylpentane (isooctane) // heptane (n-heptane) (2 x 4) STATE: short chain length / branching / ring (cyclic) / aromatic ANY TWO: (2 x3) WHY: catalyst poison / destroys catalytic converter IDENTIFY: oxygenate or methanol or ethanol / methyl-t-butyl ether (MTBE) CH3CH2CH2CH2CH3 / CH3(CH2) 3CH3 (3) pentane (3) (CH3) 2CHCH2CH3 / CH3CH2CH(CH3) 2 (3) 2-methylbutane (3) (CH3) 4C / (CH3) 3CCH3 / (CH3) 2C(CH3) 2 (3) 2,2-dimethylpropane (3) [Matching of names and formulae are required. In expanded structures, bonds without Hs are acceptable. Numbers are not required for the methyl branches but, if incorrect numbers are offered (e.g. 1,2-dimethylpropane), then no marks should be awarded.] ΔH = Σ ΔH – Σ ΔH f(products) f(reactants) (8) (6) (3) (3) (6) (6) (6) (12) ΔH = 12 x –394 or – 4728 (3) + 6 x –286 or –1716 (3) – {2 x 49 or 98 (3) + 0} -1 = – 6542 => ΔHc = – 3271 (3) [Allow 3 marks only for + 3271 kJ mo ] 2006 Question 6 (a) The table shows the octane numbers of four hydrocarbons. (i) What is meant by the octane number of a fuel? (8) (ii) Hexane has the lowest octane number of the four compounds listed. What structural feature of the molecule contributes to this? (3) (iii) In the case of each of the other three compounds, identify the structural feature of its molecules which contributes to it having a high octane number. (9) (iv) Name the process carried out in an oil refinery that converts hexane to compounds such as cyclohexane and benzene. Why is the use of benzene in petrol strictly controlled? (6) (b) (i) Give two reasons why oxygenates such as MTBE are added to petrol. (ii) Give two reasons why the addition of lead to petrol has been discontinued. (12) (c) The combustion of cyclohexane may be described by the following balanced equation: C6H12(l) + 9O2(g)  6CO2(g) + 6H2O(l) Given that the heats of formation of cyclohexane, carbon dioxide and water are –156, –394 and –286 kJ mol–1, respectively, calculate the heat of combustion of cyclohexane. (12) 2006 (a) Question 6 (i) (ii) (iii) (iv) (b) (i) (ii) (c) WHAT: measure of tendency to auto-ignite (knock) or number representing ability of fuel to resist auto-igniting (knocking) WHAT: straight chain / unbranched IDENTIFY: cyclohexane: ring / cyclic benzene: aromatic [Accept ring / cyclic] 2,2,4-trimethylpentane: branched PROCESS: dehydrocyclisation WHY: benzene is carcinogenic or benzene is toxic (poisonous) GIVE: high octane rating (number) / reduces knocking produce clean products / reduce pollution / more complete oxidation / less carbon monoxide produced /do not poison catalyst in converter (2 × 3) GIVE: it poisons (destroys) the catalyst in catalytic converter // lead emission presents a health hazard / toxic (poisonous) to living things (2 × 3) [Do not accept „lead is a pollutant‟ or „it damages the environment‟] –1 – 3924 kJ mol (12) ΔH = Σ ΔH – Σ ΔH f(products) (8) (3) (3) (3) (3) (3) (3) (6) (6) (12) f(reactants) ΔH = 6 × –394 or –2364 (3) + 6 × –286 or –1716 (3) – {1 × –156 or –156 (3) + 0} => ΔHc = – 3924 (3)

2007 Question 6 Useful hydrocarbons are obtained by the fractional distillation of crude oil, which itself has little or no direct use. Hydrocarbons are excellent fuels. (a) In which fraction of crude oil do pentane and its isomers occur? (5) Give the systematic (IUPAC) name of each of the structural isomers of pentane shown below. (9) Which of these isomers would you predict to have the lowest octane number? Justify your choice in terms of the structural features of the molecules. (9) Write a balanced equation for the combustion of pentane (C5H12) in excess oxygen. (6) (b) Naphtha and gas oil are two of the hydrocarbon fractions obtained from the fractional distillation of crude oil. How do the molecules of the naphtha fraction differ from the molecules of the gas oil fraction? (3) Explain with the aid of a labelled diagram how naphtha (b.p. approximately 100 ºC) is separated from gas oil (b.p. approximately 300 ºC) in the fractional distillation of crude oil. (9) Bitumen is a residue fraction obtained from crude oil. Give one use for bitumen. (3) (c) What is catalytic cracking? What is its economic importance in oil refining? (6) 2007 (a) (b) (c) Question 6 WHICH: light gasoline (petrol) [Accept “petroleum”] [Allow “second highest fraction” or “from C5 to C10(C11)”] GIVE: pentane // 2-methylbutane // 2,2-dimethylpropane (3 x 3) [Numbers are not required as the structures are unambiguous but no marks should be awarded if incorrect numbers are used] WHICH: pentane or the one on the left JUSTIFY: pentane is a straight (unbranched) chain molecule [“longest chain” or “not highly branched” are not acceptable.] WRITE: C5H12 + 8O2 → 5CO2 + 6H2O FORMULAS: (3) BALANCING: (3) HOW: naphtha (they) have shorter (smaller, less carbon atoms, smaller mass, lighter) chains / gas oil have longer (bigger, more carbon atoms, bigger mass, heavier) chains EXPL: diagram with one correct label (3) [layers or outlets must be shown; outlets may be shown by tubes (pipes), holes, gaps, lines, arrows (→)] heat (boil) or pass vapour up tower (column) or temperature gradient shown (3) naphtha condenses higher up or gas oil comes off lower down (3) GIVE: road surfacing / roofing / waterproofing (3) WHAT: splitting (breaking) of long chain molecules by heat and catalyst(s) [Accept “hydrocarbons” for “molecules”] ECON: more demand for products / products used as feedstock for chemical industry / gives higher octane numbers (5) (9) (3) (6) (6) (3) (12) (3) (3) 2008 Question 6 (a) The hydrocarbon molecules in petrol typically contain carbon chains with between five and ten carbon atoms. The most widely used petrol in Ireland has an octane number of 95. (i) What is meant by the octane number of a fuel? (5) (ii) The two hydrocarbons used as references when establishing the octane number of a fuel are heptane and 2,2,4-trimethylpentane. Draw the structure of each of these molecules. (6) (iii) Crude oil is separated into a number of fractions in oil refining. Name the two fractions which contain molecules with the carbon chain lengths needed for petrol. (6) (iv) Dehydrocyclisation is one of the processes used to increase the octane numbers of hydrocarbons. What two changes to the hydrocarbon molecules occur during this process? (6) (v) Ethanol is an example of an oxygenate. Give another example of an oxygenate. Give two reasons why oxygenates are added to petrol. (9)

(b) Write a balanced chemical equation for the combustion of ethanol, C2H5OH. Given that the heats of formation of ethanol, carbon dioxide and water are –278, –394 and –286 kJ mol–1, respectively, calculate the heat of combustion of ethanol. (18) 2008 (a) Question 6 (i) (ii) (iii) (iv) (v) (b) (c) 2008 WHAT: measure of tendency to auto-ignite (knock,) / number representing ability to resist autoignition (knocking, etc.) / DRAW: heptane: CH3CH2CH2CH2CH2CH2CH3 / CH3(CH2)5CH3 / (5) NAME: light gasoline / petroleum (3) naphtha (3) WHAT: removal (loss) of hydrogen [Accept “hydrogen produced”.] (3) ring (aromatic, cyclic) formation (3) EXAMPLE: methanol or methyl-t-butyl ether (MTBE) [Accept correct formula] (3) GIVE: raise octane number / decrease knocking (3) less pollution (CO) produced or more environmentally friendly or alternatives for lead (3) [Accept “less harmful gases”, “less harmful to environment”, but not “less harmful”.] WRITE: C2H5OH + 3O2 → 2CO2 + 3H2O FORMULAS: (3) BALANCING (3) –1 CALC: – 1368 kJ mol ΔH = Σ ΔHf(products) – Σ ΔHf(reactants) ΔH = 2 × – 394 or – 788 (3) + 3 × – 286 or – 858 (3) – {–278 (3) + 0} ΔH = – 1368 (3) (6) (6) (6) (9) (6) (12) Question 11 (a) Alcohols can be obtained by the reduction of aldehydes and ketones using hydrogen and a suitable catalyst. (i) Name a suitable catalyst for these reduction reactions. (4) (ii) Name the alcohol produced when propanal (C2H5CHO) is reduced. (3) (iii) Draw the structure of the alcohol produced when propanone (CH3COCH3) is reduced. To which class (primary, secondary or tertiary) of alcohols does it belong? (6) (iv) Which of the two compounds, propanal or propanone, would be oxidised by warm Fehling’s solution? Give the name and structure of the organic product of the oxidation reaction. (9) (v) Give one common use for propanone. (3) 2008 (b) Question 11 (i) (ii) (iv) MASS: 1144 g (6) 143 x 8 = 1144 (6) MOLES: 26 mol 1144 ÷ 44 (3) = 26 (3) To be accepted as a slip, some work must be shown in these calculations. VOLUME: 624 litres 26 x 24 (3) = 624 (3) (6) SUV: 528 litres (6) [Note: subtraction step (3); other step(s) (4)] [In part (iii), using 22.4 for 24 loses the 3 (4) marks for that step but the candidate is penalised once only. The same applies to the use of PV = nRT except in cases where the correct answer is obtained.] (6) (6)

2009 Question 6 (a) Define (i) hydrocarbons, (ii) structural isomers. (8) (b) Give a use for the kerosene fraction obtained when crude oil is fractionated. Explain why some of the kerosene produced in oil refining is subjected to catalytic cracking. (9) (c) Straight chain molecules of C13H28 occur in the kerosene fraction. Upon cracking a molecule of C13H28, a C2H4 molecule, a C4H8 molecule and an unbranched alkane molecule are obtained. Identify this unbranched alkane molecule and state its octane number. Draw structures for three of the isomers of C4H8. (15) (d) Name two other processes carried out in oil refineries to modify hydrocarbon structures. (6) (e) The combustion of one of the C4H8 isomers is described by the following balanced equation. C4H8 + 6O2  4CO2 + 4H2O ΔH = –2710 kJ mol–1 The standard heats of formation of water and carbon dioxide are –286 and –394 kJ mol–1, respectively. Calculate the heat of formation of this C4H8 isomer. (12) 2009 (a) Question 6 (i) (ii) (b) (c) (d) (e) compounds of* carbon (C) and hydrogen (H) only (4) [*if “containing” is used, then “only” has to accompany it] (ii) compounds with same molecular* formula but different structural formulas ([*not „chemical‟] (4) GIVE: home heating / used as paraffin oil / jet (aviation, aircraft) fuel / bus fuel / rocket fuel / storing reactive elements (metals e.g. alkali metals; non-metals e.g. white phosphorus) / solvent / lubricant / lighting /(paraffin lamps) / cooking / produce petrol (gasoline) / camping stoves [Note: medicinal liquid paraffin and paraffin wax (candles) are not kerosene derivatives.] EXPLAIN: greater demand for shorter chains (smaller molecules, kerosene chains too long) // useful products // lighter fractions // increased octane number // production of raw materials (alkenes – or named alkene, monomers) for making polymers (plastics, petrochemicals) (6 + 3) [The first correct point from GIVE or EXPLAIN gets (6)]. IDENTIFY: heptane or C7H16 or CH3(CH2)5CH3 (3) STATE: octane number: zero) [IDENTIFY & STATE are linked] (3) NAME: isomerisation // reforming (dehydrocyclisation) (2 x 3) –1 CALCULATE: – 10 kJ mol (12) –1 C4H8 + 6O2 = 4CO2 + 4H2O ΔH = – 2710 kJ mol ΔH(reaction) = ΔHf (products) – ΔHf (reactants) – 2710 = 4 x –394 or –1576 + 4 x –286 or –1144 – ΔHf(compound) * ΔHf(compound) = 4 x –394 or –1576 (3) + 4 x –286 or–1144 (3) + 2710** (3) –1 = – 10 kJ mol (3) *Give 3 for – 2710 only if the full equation in this line is given correctly (8) (9) (15) (6) (12)

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