11.0 Alkane exam Q's

2. (a) Write two equations to show how chlorine radicals react with ozone molecules in the upper atmosphere. (b) Chloropentafluoroethane is a CFC that has been used as a refrigerant. Draw its displayed formula (c) Explain why 1,1,1-trifluoroethane does not lead to the depletion of the ozone in the upper atmosphere. d) One of the steps in the synthesis of 1,1,1-trifluoroethane (CF3CH3) is the reaction of 1,1-difluoroethane (CHF2CH3) with fluorine in a free-radical substitution reaction. Write two equations to represent the propagation steps in this conversion of CHF2CH3 into CF3CH3 (e) A refrigerator contains 1.41 kg of 1,1,1-trifluoroethane (CF₃CH₃). Calculate the number of molecules of 1,1,1-trifluoroethane in the refrigerator. Give your answer to an appropriate number of significant figures. (f) There are growing concerns about the use of 1,1,1-trifluoroethane as a refrigerant as it is a greenhouse gas that absorbs some of Earth's infrared radiation. Give one reason why bonds in molecules such as carbon dioxide and 1,1,1-trifluoroethane absorb infrared radiation.

(a) ●Cl + O₃ → ●ClO + O₂ ●ClO + O₃ → ●Cl + 2O₂ (c) Doesn't contain chlorine (d) CHF₂CH₃ + ●F → ●CF₂CH₃ + HF ●CF₂CH₃ + F₂ → CF₃CH₃ + ●F (e) Moles CF₃CH₃ = 1410/84= 16.79 Molecules = 16.79 × 6.022 × 10²³ = 1.01 × 10²⁵ (f) bonds vibrate

7. (d) How do the products of the reaction in part (c) show that the reaction is an example of thermal cracking? (e) Deduce the number of monochloro isomers formed by isooctane. Draw the structure of the monochloro isomer that exists as a pair of optical isomers.

(d) Mainly alkenes formed (e) 4 (monochloro isomers)

1. (e) Carbon monoxide is produced when incomplete combustion takes place in engines. Nitrogen monoxide is another pollutant produced in car engines. Write an equation to show how these pollutants react together in a catalytic converter. (f) Platinum, palladium and rhodium are metals used inside catalytic converters. A very thin layer of the metals is used on a honeycomb ceramic support. Explain why a thin layer is used in this way. (g) Oleic acid (C18H34O2) is a straight-chain fatty acid obtained from plant oils. Isooctane can be made from oleic acid. Identify a reagent that could be used in a chemical test to show that oleic acid is unsaturated. State what would be observed in this test.

(e)2CO + 2NO → N₂ + 2CO₂ (f) To reduce amount of metal needed and increases surface area for reactants to react on. (c) Bromine water, orange to colourless.

4. (b) Compound X is produced when an alkane is cracked. (i) Give the IUPAC name for compound X. (ii) One molecule of an alkane is cracked to produce one molecule of compound X, one molecule of octane and one molecule of ethene. Deduce the molecular formula of this alkane. (iii) Name the type of cracking that produces a high yield of compound X. Give two conditions required for this process.

(i) But-1-ene (ii) C₁₄H₃₀ (iii) Thermal cracking, High pressure / 7000kPa, High temperature 400-1000°C

4. (a) (i) Identify the homologous series to which octane belongs. (ii) An isomer of octane used to improve the performance of car engines is shown.Give the IUPAC name of this isomer.

i) Alkanes ii) 2,4,4-trimethly pentane

7. Isooctane (C₈H₁₈) is the common name for the branched-chain hydrocarbon that burns smoothly in car engines. The skeletal formula of isooctane is shown. (a) Give the IUPAC name for isooctane. (b) Deduce the number of peaks in the 13C NMR spectrum of isooctane. (c) Isooctane can be formed, together with propene and ethene, in a reaction in which one molecule of an alkane that contains 20 carbon atoms is cracked. Using molecular formulas, write an equation for this reaction.

(a) 2,2,4-trimethylpentane (b) 5

1. Octane and isooctane are structural isomers with the molecular formula C8H18 (a) Give the IUPAC name for isooctane. (b) Name and explain a laboratory technique that could be used to separate isooctane from a mixture of octane and isooctane. (c) Isooctane is added to petrol to increase its octane rating. Some high-performance engines require fuel with a higher octane rating. Write an equation for the complete combustion of isooctane. Use the molecular formula (C8H18) of isooctane in your equation. (d) Explain, in general terms, how a catalyst works.

(a) 2,2,4-trimethylpentane (b) Fractional distillation, Isooctane would boil first as it has the lower boiling point and then isooctane can be condensed and collected. (c)C₈H₁₈ + 12.5O₂ →8CO₂ + 9H₂O (d) Provide and alternative pathway for a reaction with a lower activation energy.

5. (b) Chlorotrifluoromethane (CClF3) is used as a refrigerant, but is being phased out due to concerns about ozone depletion in the upper atmosphere. In the upper atmosphere, CClF3 decomposes in the presence of UV light forming a reactive intermediate that catalyses the decomposition of ozone. (i) Write an equation to show how CClF3 decomposes to form the reactive intermediate. (ii) Write two equations to show how this reactive intermediate is involved in catalysing the decomposition of ozone.

(i) CClF₃ → ●CF₃ + ●Cl (ii) O₃ + ●Cl → ●ClO + O₂ ClO● + O₃ → ●Cl + 2O₂

3. Tetradecane (C14H30) is an alkane found in crude oil. When tetradecane is heated to a high temperature, one molecule of tetradecane decomposes to form one molecule of hexane and three more molecules. Which of the following could represent this reaction?

A

6 (i) Name the homologous series that both C2H4 and C4H8 belong to. Draw a functional group isomer of C4H8 that does not belong to this homologous series. (b) Identify compound X. (c) Name polymer Y. (d) Reaction 1 is an example of thermal cracking and is carried out at a temperature of 750 °C. State one other reaction condition needed.

i) Alkene b) C₆H₁₄ (c) Polybut-2-ene (d) High pressures : 7000Kpa

5. (a) Trichloromethane (CHCl₃) is a haloalkane that can be formed by heating a mixture of chloromethane (CH₃Cl) and chlorine. (i) Write an overall equation for the formation of trichloromethane by the reaction of chloromethane with chlorine. (ii) Name the mechanism for this formation of trichloromethane. (iii) Dichloromethane (CH₂Cl₂) is an intermediate in this formation of trichloromethane. Write an equation for each of the following steps in the mechanism for the reaction of dichloromethane with chlorine, which includes a termination step with the product C₂H₂Cl₄

i) CH₃Cl + 2Cl₂ → CHCl₃ + 2HCl ii) Free radical substitution iii) Initiation: Cl₂ → 2Cl● 1st propagation: Cl● + CH₂Cl₂ → ●CHCl₂ + HCl 2nd propagation ●CHCl₂ + Cl₂ → Cl●+ CHCl₃ Termination ●CHCl₂ + ●CHCl₂ → C₂H₂Cl₄

6. (e) Reaction 2 is exothermic. A typical compromise temperature of 200 °C is used industrially for this reaction. Explain the effect of a change of temperature on both the position of equilibrium and the rate of reaction, and justify why a compromise temperature is used industrially.

• Le Chatelier's principle predicts that equilibrium shifts to oppose any increase in temperature • Exothermic reaction, so equilibrium shifts in endothermic direction • So a Higher T will reduce yield • At higher temperature, more high energy molecules • More collisions have E>Ea • So rate of reaction increases / time to reach equilibrium decreases • Industrial conditions chosen to achieve (cost-effective) balance of suitable yield at reasonable rate