Unit 2 Fuels

HIGHER CHEMISTRY REVISION. Unit 2:- Fuels Unleaded petrol uses hydrocarbons with a high degree of molecular branching in order to improve the efficiency of burning. The structure of one such hydrocarbon is shown.      (a) Give the systematic name for this hydrocarbon. (b) Name one other type of hydrocarbon that is used in petrol for the same reason. 2,3,4-trimethylpentane. (b) Cyclic hydrocarbons or aromatic hydrocarbons.

2. Mordenite is a porous material with a surface area of over 500 m2 g-1. It is used in an isomerisation reaction, part of a sequence which converts pentane into 2-methylbutane for blending into petrol. pentane pent-2-ene 2-methylbut-2-ene 2-methylbutane (a) Draw a structural formula for 2-methylbut-2-ene. (b) What role does mordenite play in the isomerisation reaction? (c) Why is 2-methylbutane a more suitable component than pentane when used in unleaded petrol? CH3 CH3 C=CHCH3 (b) Catalyst. (c) 2-methylbutane is a branched chain molecule which is not as likely to ignite prematurely as the straight chain pentane molecule in a car engine. Higher Chemistry

3. Petrol is produced by the reforming of a fraction obtained from crude oil. One such reforming reaction is:- CH3–CH2–CH2–CH2–CH2–CH2–CH2–CH3 CH3–C–CH2–CH2–CH3 octane Compound A (a) Which crude oil fraction is reformed to make petrol? (b) Give the systematic name for compound A. (c) If petrol burned in a car engine contains straight-chain alkanes. like octane, a process called ‘knocking’ takes place. Why does the presence of straight-chain alkanes result in ‘knocking’? CH3 CH3 Naphtha. 2,2-dimethylpentane. Straight chain compound tend to undergo premature ignition in a petrol engine.

4. Carbon monoxide and oxides of nitrogen are pollutants present in the exhaust gases from petrol engines. Explain how (i) carbon monoxide forms in a petrol engine. (ii) oxides of nitrogen form in a petrol engine. A catalytic converter fitted to a car exhaust will turn these pollutant gases into less harmful gases. Name the gases formed from the (i) carbon monoxide. (ii) oxides of nitrogen. (c) Name a metal used as a catalyst in a catalytic converter. (a) (i) Incomplete combustion of the hydrocarbon fuel produces carbon monoxide. (ii) The high energy spark that ignites the petrol and mixture provides enough energy for nitrogen and oxygen (from air) to combine to form oxides of nitrogen. (i) carbon dioxide. (ii) nitrogen and oxygen. (c) Platinum.

5. Under anaerobic conditions, carbohydrates, like glucose, can be used to produce biogas. The main constituent of biogas is methane which is a useful fuel. State one advantage of using biogas as a fuel rather than natural gas. Biogas is formed from a renewable source unlike natural gas which is in finite supply.