14. Hydrocarbons
A section of Chemistry, 9701
Listing 10 of 313 questions
Use Electrophoresis can be used to separate amino acids which are produced by the hydrolysis of a polypeptide. Using glycine as an example, explain why the result of electrophoresis depends on pH. The diagram below shows the results of electrophoresis in neutral solution. At the start of the experiment a spot of a solution containing a mixture of amino acids P, Q, R and S was placed in the middle of the plate. Following electrophoresis the amino acids had moved to the positions shown in the lower diagram. before + – after + – P Q R S Which amino acid existed mainly as a zwitterion in the buffer solution? Explain your answer. Assuming amino acids R and S carry the same charge when in this buffer solution, which is likely to be the larger molecule? Explain your answer. Amino acids may also be separated by using two-dimensional paper chromatography. This involves putting a spot of the mixture on the corner of a piece of chromatography paper and allowing a solvent to soak up the paper. The paper is then dried, turned through 90° and placed in a second solvent. This method gives better separation than a one solvent method. Paper chromatography relies on partition between the solvent applied and another phase. What is this second phase? Examiner’s Use The table below shows the Rf values for some amino acids in two different solvents. amino acid Rf solvent 1 Rf solvent 2 A 0.1 0.2 B 0.0 0.4 C 0.3 0.0 D 0.8 0.9 E 0.6 0.5 Use the grid below to plot the positions of the amino acids after two-dimensional paper chromatography using solvent 1 followed by solvent 2. solvent 2 solvent 1 solvent front solvent front spot applied here Which amino acid travelled fastest in both solvents? Which amino acid did not move at all in solvent 2?
9701_s07_qp_4
THEORY
2007
Paper 4, Variant 0
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Use Both ethene and benzene react with bromine, but the mechanisms and the types of products of the two reactions are different. Br2 Br BrCH2CH2Br H2C CH2 + HBr + reaction I no heat, no light, no catalyst needed Br2 + reaction II heat and catalyst needed State the type of reaction undergone in each of reactions I and II. reaction I reaction II Examiner’s Use In each of reactions I and II, the intermediate is a bromine-containing cation. In each of the following boxes, draw the intermediate and use curly arrows to show how it is converted into the product. reaction I product intermediate reaction II product intermediate Why do ethene and benzene differ in their reaction with bromine?
9701_s08_qp_4
THEORY
2008
Paper 4, Variant 0
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Although standard electrode potentials are measured for solutions where the concentrations of ions are 1.0 mol dm–3, cells used as sources of battery power tend to operate with more concentrated solutions. This question concerns the electrode reactions involved in the hydrogen-oxygen fuel cell and the lead-acid car battery. In the hydrogen-oxygen fuel cell, H2and O2are fed onto two inert electrodes dipping into NaOH. V hydrogen oxygen NaOHThe following reactions take place. left hand electrode : H2+ 2OH–2H2O+ 2e– right hand electrode : O2+ 2H2O+ 4e– 4OH–Use the Data Booklet to calculate E o–– cell for this reaction. Construct an equation for the overall reaction. By using one of the phrases more positive, more negative or no change, deduce the effect of increasing [OH–] on the electrode potential of • the left hand electrode • the right hand electrode Hence deduce whether the overall Ecell is likely to increase, decrease or remain the same, when [OH–] increases. Explain your answer. Suggest one other reason why a high is used in the fuel cell. In the cells of a lead-acid car battery the following reactions take place. cathode: PbPb2++ 2e– anode: PbO2+ 4H++ 2e– Pb2++ 2H2OUse the Data Booklet to calculate E o–– cell for this reaction. Construct an equation for the overall reaction. The electrolyte in a lead-acid cell is H2SO4. Most of the Pb2+ions that are produced at the electrodes are precipitated as the highly insoluble PbSO4. Construct an equation for the overall cell reaction in the presence of H2SO4. By considering the effect of decreasing [Pb2+] on the electrode potentials of the cathode and the anode, deduce the effect of the presence of H2SO4in the electrolyte on the overall Ecell. State whether the Ecell will increase, decrease or remain the same. Overall Ecell will . Explain your answer.
9701_s10_qp_41
THEORY
2010
Paper 4, Variant 1
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Briefly explain why the benzene molecule is planar. Briefly explain why all the carbon-carbon bonds in benzene are the same length. Benzene can be nitrated by warming it with a mixture of concentrated sulfuric and nitric acids. By means of an equation, illustrate the initial role of the sulfuric acid in this reaction. Name the type of reaction and describe the mechanism for the nitration reaction, including curly arrows showing the movement of electrons and all charges. type of reaction mechanism State the reagents and conditions needed to convert benzene into chlorobenzene. Nitrobenzene undergoes further substitution considerably more slowly than chlorobenzene. In nitrobenzene the incoming group joins to the benzene ring in the 3-position, whereas in chlorobenzene the incoming group joins to the benzene ring in the 4-position. Use these ideas to suggest the structures of the intermediate compounds Y and Z in the following synthesis of 4-chlorophenylamine. I II Z Y III NH2 Cl Suggest the reagents and conditions needed for reaction III in the above synthesis. Suggest the structural formulae of the products A, B, C and D of the following reactions. If no reaction occurs write “no reaction” in the relevant box. B A D C NH2 Br2OH –NaNO2 + HCl CH3COCl Cl
9701_s10_qp_42
THEORY
2010
Paper 4, Variant 2
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Briefly explain why the benzene molecule is planar. Briefly explain why all the carbon-carbon bonds in benzene are the same length. Benzene can be nitrated by warming it with a mixture of concentrated sulfuric and nitric acids. By means of an equation, illustrate the initial role of the sulfuric acid in this reaction. Name the type of reaction and describe the mechanism for the nitration reaction, including curly arrows showing the movement of electrons and all charges. type of reaction mechanism State the reagents and conditions needed to convert benzene into chlorobenzene. Nitrobenzene undergoes further substitution considerably more slowly than chlorobenzene. In nitrobenzene the incoming group joins to the benzene ring in the 3-position, whereas in chlorobenzene the incoming group joins to the benzene ring in the 4-position. Use these ideas to suggest the structures of the intermediate compounds Y and Z in the following synthesis of 4-chlorophenylamine. I II Z Y III NH2 Cl Suggest the reagents and conditions needed for reaction III in the above synthesis. Suggest the structural formulae of the products A, B, C and D of the following reactions. If no reaction occurs write “no reaction” in the relevant box. B A D C NH2 Br2OH –NaNO2 + HCl CH3COCl Cl
9701_s10_qp_43
THEORY
2010
Paper 4, Variant 3
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One method of preparing ethene in a school or college laboratory is from ethanol by using the apparatus shown below. mineral wool and ethanol pumice water ethene strong heat Write a balanced equation for this reaction. What type of reaction is this? Give the chemical name of a reagent other than pumice that could be used to carry out this reaction. It is not necessary to use the same apparatus. Ethene is bubbled into two separate test-tubes, one containing aqueous hydrogen bromide and the other containing cold, dilute acidifi ed potassium manganate(. In each case, describe any colour changes you would see and give the structural formula of the organic product. aqueous hydrogen bromide cold, dilute acidifi ed potassium manganate(colour at start colour after reaction structural formula of organic product Cyclohexene has the following structural formula. What is the molecular formula of cyclohexene? Draw the structural formula of the compound formed when cyclohexene is reacted with bromine. State as fully as you can what type of reaction this is. Draw the structural formula of the compound formed when cyclohexene is reacted with hot concentrated acidifi ed potassium manganate(.
9701_s12_qp_21
THEORY
2012
Paper 2, Variant 1
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The structure of H is shown. C C H CH2OH CH3 CH3 CH3 H reacts with both cold, dilute, acidifi ed potassium manganate(and with hot, concentrated, acidifi ed potassium manganate(. Give the structure of the organic product of the reaction of H with cold, dilute, acidifi ed potassium manganate(. Give the structures of the organic products of the reaction of H with hot, concentrated, acidifi ed potassium manganate(. Complete the reaction scheme to show the mechanism of the reaction of H with bromine to form J. Include all necessary curly arrows, lone pairs and charges. C C CH2OH CH3 Br δ+ Br δ– CH3 CH3 C CH3 C J CH3 Br CH3 CH2OH Br Explain the origin of the dipole on the bromine molecule. J is formed as an equimolar mixture of isomers. State the type of isomerism shown by J. Draw the structures of the two isomers of J.
9701_s15_qp_23
THEORY
2015
Paper 2, Variant 3
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Proteins are formed by the polymerisation of amino acids. State the type of chemical reaction used to form these polymer chains. The amino acids serine and valine can combine together to form a dipeptide. serine, ser OH O H2N OH valine, val H2N OH O Draw the skeletal structure of the dipeptide ‘val-ser’. Suggest how the type of amino acids in a protein determines its three-dimensional structure. Using labelled diagrams or words as appropriate, explain why a particular enzyme may only catalyse a specifi c reaction on a specifi c substrate, how non-competitive inhibition of an enzyme-catalysed reaction can occur.
9701_s15_qp_41
THEORY
2015
Paper 4, Variant 1
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Proteins are formed by the polymerisation of amino acids. State the type of chemical reaction used to form these polymer chains. The amino acids serine and valine can combine together to form a dipeptide. serine, ser OH O H2N OH valine, val H2N OH O Draw the skeletal structure of the dipeptide ‘val-ser’. Suggest how the type of amino acids in a protein determines its three-dimensional structure. Using labelled diagrams or words as appropriate, explain why a particular enzyme may only catalyse a specifi c reaction on a specifi c substrate, how non-competitive inhibition of an enzyme-catalysed reaction can occur.
9701_s15_qp_43
THEORY
2015
Paper 4, Variant 3
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Questions Discovered
313