Common exam questions A level chemistry unit 3 Questions and Answers

Common exam questions A level chemistry unit 3 Questions and Answers What is meant by an unsaturated hydrocarbon? one which contains one or more carbon-carbon double bonds What is meant by the term hydrocarbon? a molecule that contains the elements hydrogen and carbon only Recall the reagents and conditions needed to produce ethanol from bromoethane. aqueous NaOH reflux Name the organic product and mechanism when 2-bromopropane reacts with ethanolic KOH. prop-1-ene elimination Name the organic product and mechanism when chloroethane and an excess of ammonia react together. ethyl amine nucleophilic substitution Name the organic product and mechanism when 1-chloropropane and KCN react together. butane nitrile nucleophilic substitution Name the organic product and mechanism when but-2-ene and bromine react together. 2,3-dibromobutane electrophilic addition State the type of structural isomerism shown by propanal and propanone. (1) functional group isomerism Name the type of mechnaism in the reaction of chlorine with methane. (1) free radical substitution To which homologous series does penta-1,3-diene belong to? (1) alkene 2,2-dimethylpropane is a structural isomer of pentane. What type of structural isomerism is this? (1) chain isomerism Define structural isomerism. (1) same molecular formula, different structural formula What is meant by stereoisomerism? (1) same structural formula but the atoms are arranged differently in space What conditions need to be fulfilled in order for a compound to exhibit E-Z isomerism? (2) carbon-carbon double bond the carbon atoms each side of the double bond both need to have 2 different groups attached Explain the term homolytic fission. (1) the bond is broken equally so both of the atoms get one of the electrons from the bond Explain the term free radical. (1) a substance with an unpaired electron Explain what is meant by the term electrophile and the term addition. (1) electrophile = an electron pair acceptor addition = a reaction where 2 molecules form one molecule/a double bond is converted to a single bond State the role of potassium hydroxide in an elimination reaction. (1) base Why does 3-chloropentane react more slowly than 3-bromopentane? (1) the C-Br bond is weaker than the C-Cl bond Describe 4 test tube reactions that could be used to identify these compounds: propan-1-ol, propanal, propanoic acid and 1-chloropropane. (8) propan-1-ol = add sodium metal resulting in effervescence OR add acidified potassium dichromate and it will turn from orange to green propanal = add fehlings solution to give a brick red precipitate OR Tollen's reagent to give a silver mirror propanoic acid = sodium carbonate to give effervescence 1-chloropropane = add NaOH then silver nitrate to give a white precipitate Suggest the chemical process that would cause a sample of propan-2-ol to become contaminated with propanone. (1) oxidisation of the alcohol by oxygen in the air State 1 condition necessary for the initiation of a free radical substitution reaction. (1) UV light Identify a suitable catalyst and condition for catalytic cracking. (2) zeolite catalyst moderate pressure/high temperature Why can a single chlorine free radical cause the decomposition of many molecules of ozone? (1) chlorine is regenerated, causing a chain reaction Explain why one product of an electrophilic addition reaction may be formed more than the other. (2) different carbocations have different levels of stability tertiary more stable than secondary more stable than primary more stable carbocations are more likely to be formed Give 1 reason why 2,3,3,3-tetrafluoropropene is a more environmentally acceptable refrigerant than chlorodifluoromethane. (1) it doesn't contain chlorine so no chlorine radicals can be made Name the straight chain hydrocarbon with the molecular formula C9H20. (1) nonane Name the type of bond fission in the initiation stage of free radical substitution. (1) homolytic State the meaning of the term carbon neutral. (1) no net carbon dioxide emissions Give 2 conditions essential to ensure a good yield of ethanol from fermentation and name the process used to create a more concentrated solution of ethanol. (3) anaerobic, yeast fractional distillation State a process used to separate an alkane from a mixture of alkanes. (1) fractional distillation Explain why the complete combustion of butane may contribute to environmental problems. (1) it releases carbon dioxide which is a green house gas Identify a compound used to react with sulphur dioxide in the products of combustion before they enter the atmosphere. Give 1 reason why the compound reacts with sulphur dioxide. (2) calcium oxide/calcium carbonate it neutralises the sulphur dioxide as it is a base Explain the meaning of the term hydrolysis. (1) splitting molecules using water Explain why halogenoalkane is attacked by the nucleophile in this reaction. (3) the halogen is more electronegative than carbon - large difference in electronegativity carbon is partial positive lone pair on nucleophile donated to the partial positive carbon State the order in which the five fractions are removed in fractional distillation as the column is ascended and give 2 reasons why the fractions are collected at different levels in the fractionating column. (3) mineral oil, gas oil, kerosene, naphtha, petrol temperature of fractionating column decreases as you go up it larger molecules condense further down the column Explain how CO and NO are removed from exhaust gases and why the removal of each of them is desirable. (5) NO forms acid rain/respiratory problems 2NO + O2 - 2NO2 CO is poisonous 2NO + 2CO - N2 + 2CO2 catalytic converter uses Pt/Rh/Pd/Ir provides active sites/reduces activation energy Give 1 reason why bonds in molecules such as CO2 and 1,1,1-trifluoroethane absorb infrared radiation. (1) the bonds are polar Suggest, with a reason, which of C4H10 and C3H8 is liquified more easily. (1) C4H10 because it has a boiling point Suggest the type of cracking that produces a high proportion of ethene and give the 2 conditions for this process. (2) thermal cracking high temperature, high pressure Explain how oxides of nitrogen are formed. (2) nitrogen in the air reacts with oxygen in the air at high temperatures Explain, with reference to intermolecular forces, why distillation allows propanal to be separated from a mixture with propanol and propanoic acid. (3) propanol and propanoic acid have hydrogen bonding propanal has permanent dipole-dipole forces hydrogen bonding is the strongest intermolecular force Give 2 ways of maximising the yield of propanal obtained by distillation from a mixture of propanol and propanoic acid. (2) keep the temperature of the reaction mixture below the boiling point of propanol cool the distillate/collecting vessel Describe how you would carry out a simple test tube reaction to confirm a sample of propanal does not contain any propanoic acid. (2) add sodium carbonate effervescence would confirm the presence of acid Explain how you could measure out 50g of water without using a balance. (2) measure out 50cm3 of water water has a known density of 1gcm-3 State 2 general features of a homologous series of compounds. (2) same functional group they react in the same way, using the same mechanisms Name the type of mechanism for the reaction of chlorine with methane. (1) free radical substitution Outline the essential features of fractional distillation of crude oil that enable the crude oil to the separated into fractions. (4) fractionating column has a temperature gradient - hotter at the bottom and cooler at the top longer chain hydrocarbons have higher boiling points so condense at the bottom shorter chain hydrocarbons have lower boiling points so condense at the top of the column Explain the term cracking and why oil companies need to do this. (2) cracking = heating a long chain hydrocarbon to break a carbon-carbon bond and splitting it into multiple shorter chain hydrocarbons shorter chain fractions are in higher demand so more of these need to be made State why water produced in exhaust gases may contribute to global warming. (1) it can act as a greenhouse gas Inside catalytic converters, a very thin layer of the catalyst metals is used on a honeycomb ceramic support. Explain why a thin layer is used in this way. (2) provides a large surface area (to speed up the rate of reaction) reduces the amount of metals needed Explain why the melting point of dodecane is higher than the melting point of the straight-chain alkane produced by cracking dodecane. (2) dodecane is a larger molecule so the intermolecular forces (VDW) between molecules are stronger and take more energy to overcome. Suggest how the student could check a mixture had reached equilibrium after 1 week. (2) test several samples check result has not changed Anti-bumping granules are placed in the flask when refluxing. Suggest why these granules prevent bumping. (1) they form smaller bubbles/prevent large bubbles Explain how stereoisomers arise. (2) the c=c double bond cannot rotate each carbon atom in the c=c double bond has 2 different groups attached A student used a wash bottle containing deionised water when approaching the end point of a titration to rinse the inside of the conical flask. Explain why this improved the accuracy of the titration. (1) ensures all the reactants are in the mixture as some may have been on the side of the flask Give the meaning of the term concordant titres. (1) titres that are within 0.1cm3 of each other Explain whether you would expect to find a plasticiser in the PVC used to insulate electrical cables. (2) you would expect this because the PVC would need to be flexible Suggest one possible danger when a metal carbonate reacts with an acid in a sealed flask. (1) the pressure will cause the flask to explode Give the definition of the term carbon neutral. (1) no net carbon emissions What is the UV light used for in the reaction of ethane with chlorine? provides the energy to break the covalent bond in chlorine What type of bond fission occurs during thermal cracking? Name the chemical species produced by this and say what feature all members of this species have. (3) homolysis free radicals unpaired electrons What does the term non-polar mean? (1) electrical charge is evenly distributed so the molecule does not have an overall charge