Past papers questions chemistry - November 2020 AS paper 1- 2017 paper 2 A level paper 1 2, Exams of Chemistry

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Past papers questions chemistry - November

2020 AS paper 1- 2017 paper 2 A level paper

1 2019 A level

This question is about atomic structure. There is a general trend for an increase in ionization energy across Period 3. Give one example of an element that deviates from this trend. Explain why this deviation occurs. - Correct Answer Aluminum / Al (Outer) electron in (3) p orbital / sub- shell (level) (3p) higher in energy / slightly more shielded (than 3s) / slightly further away (than 3s) Give an equation, including state symbols, to represent the process that occurs when the third ionization energy of sodium is measured. - Correct Answer Na2+ (g) →Na3+ (g) + e− This question is about a titration. A student dissolves an unknown mass of sodium hydroxide in water to make 200 cm of an aqueous solution. A 25.0 cm3 sample of this sodium hydroxide solution is placed in a conical flask and is titrated with 0.150 moll dm-3 sulfuric acid. The equation for this reaction is shown. 2NaOH (as) + H2SO4 (as) → Na2SO4 (as) + 2H2O (l) Table 1 shows the results of the titrations. (check paper for table - November 2020 as paper 1) Calculate the mass of sodium hydroxide used to make the original solution. - Correct Answer Volume of H2SO4= (19.60 + 19.55) / 2 = (19.575 cm3 / 19.58 cm3) moles of H2SO4 = concentration x volume = 0.150 x (19.575 / 1000) (= 2.936 x 10-3 moll) Moles of Noah in 25 cm3 = 2.936 x 10-3 x 2 = 5.87 x 10^-3mol moles of Noah in original 200cm3 = 5.87 x 10^-3 x 8 = (= 0.04698 moll) Mass of Noah = Mr. x moles = 40.0 x 0.04698 = 1.88 g (1.9 g) The student uses a funnel to fill the burette with sulfuric acid before starting the titration. After filling, the student forgets to remove the funnel from the top of the burette. Suggest why this might affect the titer volume recorded. - Correct Answer Additional drops of solution could have entered the burette from the funnel, (making the value on the burette lower).

State one advantage of using a conical flask rather than a beaker for the titration. - Correct Answer Less chance of splashing/losing any solution using a conical flask (when swirling) In a TOF mass spectrometer, ions are accelerated to the same kinetic energy (KE). The kinetic energy of an ion is given by the equation KE = 12 mv Where’ve = kinetic energy / J m = mass / keg = speed / ms- In a TOF mass spectrometer, each 84Kr+ ion is accelerated to a kinetic energy of 4. × 10−16 J and the time of flight is 1.72 × 10−5 s Calculate the length, in meters, of the TOF flight tube. The Avogadro constant, L = 6.022 × 1023 mol−1 - Correct Answer m = (84/1000)/6.022 x 10^23 = 1.395 x 10^-25kg v^2 = 2ke/m =2x (4.83×10−16)/ (1.395x10-25) v = √ (6924731183) = 83214. d =v x t= 83214.97 x 1.72 × 10−5 = 1.43 (m) A student determines the enthalpy change for the reaction between calcium carbonate and hydrochloric acid. CaCO3(s) + 2 HCl (as) → CaCl2 (as) + CO2 (g) + H2O (l) The student follows this method: measure out 50 cm3 of 1.00 moll dm-3 aqueous hydrochloric acid using a measuring cylinder and pour the acid into a 100 cm3 glass beaker weigh out 2.50 g of solid calcium carbonate on a watch glass and tip the solid into the acid stir the mixture with a thermometer record the maximum temperature reached. The student uses the data to determine a value for the enthalpy change. Explain how the experimental method and use of apparatus can be improved to provide more accurate data. Describe how this data from the improved method can be used to determine an accurate value for the temperature change. - Correct Answer Stage 1: Apparatus1a. Use a burette/pipette (instead of a measuring cylinder) 1b. Use a polystyrene cup (instead of a beaker) / insulate beaker1c. Reweigh the watch glass after adding the solid 1d: Use powdered solid Stage 2: Temperature Measurements2a. Measure and record the initial temperature of the solution for a few minutes before addition2b. Measure and record the temperature after the addition at regular intervals (egg each minute) for 8+ minutes/until a trend is observed Stage 3: Temperature Determination3a. Plot a graph of temperature against time 3b. Extrapolate to the point of addition3c. Determine ΔT at the point of addition Suggest how, without changing the apparatus, the experiment in Question 04.3 could be improved to reduce the percentage uncertainty in the temperature change - Correct Answer Increase the concentration of the solutions This question is about Group 2 elements and their compounds. Explain why the melting point of magnesium is higher than the melting point of

sodium. - Correct Answer Mg2+ has a higher charge than Na+, Mg2+ ions are smaller and so Stronger attraction to delocalized sea of electrons Give an equation to show how magnesium is used as the reducing agent in the extraction of titanium. Explain, in terms of oxidation states, why magnesium is the reducing agent. - Correct Answer 2Mg + TiCl4 → 2MgCl2 + Tim changes oxidation state from 0 to +2 so electrons are lost / Ti changes oxidation state from +4 to 0, so gains electrons State what is observed when dilute aqueous sodium hydroxide is added to separate solutions of magnesium chloride and barium chloride. - Correct Answer Observation with MgCl2: (slight) white pot Observation with BaCl2: no (visible) change / colorless solution / no reaction Give an equation for the reaction of chlorine with cold water. - Correct Answer Cl +H2O⇌HCl+HClO Identify one other sulfur-containing reduction product formed when solid sodium iodide reacts with concentrated sulfuric acid. - Correct Answer SO2 or H2S Silver nitrate is added to the solution. Suggest why an excess is used. - Correct Answer To ensure that all the halide ions (chloride and iodide) are removed from the solution Calculate, using your answer to Question 07.5, the percentage by mass of sodium chloride in the mixture. - Correct Answer mass of Nalco = 600 - 201 = 399mg %Nalco = 399/600 x 100 = 66.5 %( 66.5 - 68.3) The student noticed that some of the liquid injected into the gas syringe did not vaporise.Explain the effect that this has on the Mr. calculated by the student. - Correct Answer Calculated Mr. value would be greater than actual A lower volume would have been recorded / mass evaporated less than mass of liquid / lower moles calculated / mass recorded higher than mass of gas / mass recorded would be too high Each reading on the balance used to record the mass of the fine needle syringe and contents had an uncertainty of ±0.001 g Calculate the percentage uncertainty in the mass of liquid An injected in this experiment. - Correct Answer % uncertainty = (uncertainty / mass added) x 100 = ((2 x 0.001) / 0.460} x 100 = 0.435% Which compound has hydrogen bonding? A Noah B NH C HI

D SiH4 - Correct Answer B NO2− ions can be reduced in acidic solution to NO How many electrons are gained when each NO2− ion is reduced? A 1 B 2 C 3 D 4 - Correct Answer A Which represents the correct order of increasing radius of the ions? An F- O2- Li+ Be2+ B Li+ Be2+ O2- F- C Be2+ Li+ F- O2- D O2- F- Li+ Be2+ - Correct Answer C Which compound contains a co-ordinate bond? A HF B NH C CHCl D NH4Cl - Correct Answer D Calculate the mean titer using the concordant results. Give your answer to the appropriate number of significant figures. - check paper for the table (as chemistry paper 2 2018) - Correct Answer use of titrations 1 & 3 only = 23.95 cm The total uncertainty when using the burette is ±0.15 cm3. This is the combination of uncertainties in the start reading, final reading and the determination of the end point. Use your answer to Question 02.3 to calculate the percentage uncertainty for the use of the burette in this experiment. - Correct Answer (0.15 x 100) = 0.63% State the general formula of an alkane containing n carbon atoms. Deduce an expression for the relative molecular mass (Mr.) of an alkane in terms of n. - Correct Answer 14.0n+2.0 or 14n+ CnH2n+ A hydrocarbon contains 87.8% by mass of carbon and has a relative molecular mass (Mr.) of 82. The hydrocarbon decolorizes bromine water. Determine the empirical and molecular formulae of the hydrocarbon. Suggest two possible structures for the hydrocarbon. Name the type of reaction taking place when bromine water reacts with the hydrocarbon. - Correct Answer C: H = 7.3: 12.2 seen converting C: H - 7.3:12.2 - 3:5 C3H molecular formula: C6H cyclic compounds with a membered C ring with one double bond or any dynes with 6C atoms or any molecule with a triple bond electrophilic addition

Compound A has a relative molecular mass (Mr.) of 134. The main isotope of hydrogen is 1H The main isotope of carbon is 12C 35 37 35 Chlorine consists of two common isotopes, Cal and Cal, of which 75% is Cal The mass spectrum of A was recorded when A was ionized by electron impact to form A+ ions. Draw, on Figure 3, the peaks for the main molecular ions in the mass spectrum of A. - Correct Answer lines at 134 and 136 line at 134 to be three times higher than line at 136 Compound D reacts with dilute aqueous sodium hydroxide in a similar way to A to form alcohol B. (check AS paper 2 2018 for the compounds) Explain why D reacts more quickly than A with dilute aqueous sodium hydroxide at the same temperature. - Correct Answer C-Br is weaker than C-Cal Four compounds, all colorless liquids, are

• butan-2-ol
• butane
• butanone
• 2-methylpropan-2-ol Two of these compounds can be identified using different test-tube reactions. Describe these two test-tube reactions by giving reagents and observations in each case. Suggest how the results of a spectroscopic technique could be used to distinguish between the other two compounds. - Correct Answer Stage 1 Carries out a test-tube reaction to identify a compound (or to split the compounds into two groups). 1a reagent 1b observation with correct deduction Stage 2 Carries out a second test-tube reaction to identify a second compound. 2a reagent 2b observation with correct deduction Stage 3 Uses spectroscopy to distinguish two compounds. 3a suitable technique 3b data that will distinguish compounds A student has a 10 cm3 sample of 1.00 × 10−2 moll dm−3 mechanic acid solution. The student is asked to dilute the mechanic acid solution to a concentration of 2.00 × 10− moll dm−3 by adding distilled water. Which volume of water should be added?

A 200 cm B 490 cm C 500 cm D 510 cm3 - Correct Answer B Which statement is correct about the fractional distillation of crude oil? A zeolite catalyst is used. B Each fraction contains a mixture of hydrocarbons. C Gaseous fractions are formed by breaking covalent bonds. D The fractionating column is hottest at the top - Correct Answer D How many structural isomers with an unranked carbon chain have the molecular? formula C4H8Br2? A 4 B 5 C 6 D 7 - Correct Answer C The Maxwell-Boltzmann distribution of molecular energies in a sample of gas at a fixed temperature is shown. Which letter represents the mean energy of the molecules? check AS paper 2 2018 for graph A B C D - Correct Answer D Ethanol can be made from glucose by fermentation. C6H12O6 → 2C2H5OH + 2CO Which letter represents the mean energy of the molecules? [1 mark] In an experiment, 268 g of ethanol (Mr. = 46.0) were made from 1.44 kg of glucose (Mr. = 180.0). What is the percentage yield? An 18.6% B 36.4% C 51.1% D 72.8% - Correct Answer B What is the enthalpy of formation of buta-1, 3-diene, C4H6 (g)? check 2018 paper 2 AS for the table A +112 kJ mol- B -112 kJ mol- C +746 kJ mol- D -746 kJ mol-1 - Correct Answer A

Name the mechanism for Reaction 2 and give an essential condition used to ensure that CH3CH2CH2NH2 is the major product. - Correct Answer Nucleophile substitution Excess NH Calculate the mass, in grams, of CH3CH2CH2NH2 produced from 25.2 g of CH3CH2CH2Br in Reaction 2 assuming a 75.0% yield. Give your answer to the appropriate number of significant figures. (1:1 molar ratio) - Correct Answer Amount of CH3CH2CH2Br - 25.2/122.9 (=0.205) (moll) Amount of CH3CH2CH2NH2 - M1 x 0.75 (= 0.154) (moll) Mass CH3CH2CH2NH2 - M2 x 59.0 = 9.07g When Reaction 2 is carried out under different conditions, a compound with molecular formula C9H21N is produced. Draw the skeletal formula of the compound. Identify the functional group in the compound including its classification. Skeletal formula - Correct Answer going to have to go on the 2017 paper 2 a level mark scheme for the skeletal formula tertiary amine (only award if a tertiary amine shown) Identify the reagent and conditions used in Reaction 3. (2017 paper 2 a level) - Correct Answer Noah/ ethanol or KOH / ethanol (both required) The experiment was repeated at the same temperature and with the same initial concentration of B but with a different initial concentration of A. The new initial rate was 1.7 times greater than in the original experiment. Calculate the new initial concentration of A. - Correct Answer [A] increases by √1. new [A] = 1.30 X 0.50 = 0.65moldm^- new [A] 2 = 1.7 × (0.50)2 = 0. new [A] = 0.65mol^- An equation that relates the rate constant, k, to the activation energy, Ea. and the temperature, T, is link=−Ea. +ln RT Use this equation and your answer from Question 3.1 to calculate a value, in kJ mol−1, for the activation energy of this reaction at 25 °C. For this reaction ln A = 16.9The gas constant R = 8.31 J K−1 mol− (If you were unable to complete Question 3.1 you should use the value of 3.2 × 10−3 for the rate constant. This is not the correct value.) - Correct Answer link = ln2.8×10-2 (=-3.58) Ea. =RT (lnA-lnk) OR -Ea. =RT (lnk-lnA) Ea. = 8.31 × 298 (16.9 + 3.58) (= 50716 J mol-1) Ea. = 51 kJ mol- Describe how you would distinguish between separate samples of the two

stereoisomers of CH3CH2CH2CH2CH (OH) CN - Correct Answer (Plane) polarized light Enantiomers would rotate light in opposite directions Explain why the reaction produces a racemic mixture. - Correct Answer planar carbonyl group Attack from either side With equal probability OR produces equal amounts (of the two isomers/enantiomers Draw a structural formula for the dieter C6H10O4 - Correct Answer check mark scheme for 2017 paper 2 a level for structural formula Write an expression for the equilibrium constant, KCl, for the reaction. The total volume of the mixture does not need to be measured to allow a correct value for KCl to be calculated. justify this statement. - Correct Answer there are equal no of moles on each side of the equation Draw the structures of both dipeptides formed when phenylalanine reacts with serine. In each structure show all the atoms and bonds in the amide link. - Correct Answer check 2017 a level paper 2 mark scheme An amide link is also formed when an acyl chloride reacts with a primary amine. Name and outline a mechanism for the reaction between CH3CH2COCl and CH3CH2NH Give the IUPAC name of the organic product. - Correct Answer N-ethylpropanamide You are provided with samples of each of the four compounds. Describe how you could distinguish between all four compounds using the minimum number of tests on each compound. You should describe what would be observed in each test. (check 2017 paper 2 for compounds) - Correct Answer Stage 1: An initial test to separate into two groups (2 groups of 2 OR 1 group of 3 and 1 group of 1) Stage 2: A second test to distinguish within a group or to separate into two further groups Stage 3: A third test leads to a set of results/observations which distinguishes between all 4 compounds Tests must include reagent and observation which identifies compound(s) -COOH a) NaHCO3 / Na2CO3 (or correct alternative) b) effervescence /gas turns limewater milky c) K and /or M but not L and/or N -OH and -CHO d) acidified K2Cr2O e) solution turns green f) K and/or L and/or N but not M -CHO

g) Fehling’s OR Tolle’s h) red pot OR silver mirror i) N only but not K and/or L and/or M -Br j) Silver nitrate k) cream pot l) L and/or N but not K and/or M Isolated tests on individual compounds - max LEVEL 2 Isolated tests not linked to any compound - max LEVEL 1 Penalize observation if deduction wrong, but allow observation if deduction incomplete Nitrobenzene reacts when heated with a mixture of concentrated nitric acid and concentrated sulfuric acid to form a mixture of three isomeric dinitrobenzenes. Write an equation for the reaction of concentrated nitric acid with concentrated sulfuric acid to form the species that reacts with nitrobenzene. - Correct Answer HNO3 + 2H2SO4 → NO2+ + H3O+ + 2HSO4- State in general terms what determines the distance travelled by a spot in TLC. - Correct Answer (Balance between) solubility in moving phase and retention by stationary phase To obtain the chromatogram, the TLC plate was held by the edges and placed in the solvent in the beaker in the fume cupboard. The lid was then replaced on the beaker. Give one other practical requirement when placing the plate in the beaker. - Correct Answer Solvent depth must be below start line A second TLC experiment was carried out using 1, 2-dinitrobenzene and 1, 4- dinitrobenzene. An identical plate to that in Question 8.3 was used under the same conditions with the same solvent. In this experiment, the Fry value of 1, 4- dinitrobenzene was found to be greater than that of 1, 2-dinitrobenzene. Deduce the relative polarities of the 1, 2-dinitrobenzene and 1, 4-dinitrobenzene and explain why 1, 4-dinitrobenzene has the greater Fry value. - Correct Answer 1,2- is more polar OR 1,4- is less polar OR 1,2 is polar, 1,4- is non-polar 1, 4- (or Less/non polar is) less attracted to (polar) plate / stationary phase / solid OR (Less/non polar is) more attracted to / more soluble in (non-polar) solvent / mobile phase / hexane A third TLC experiment was carried out using 1, 2-dinitrobenzene. An identical plate to that in Question 8.3 was used under the same conditions, but the solvent used contained a mixture of hexane and ethyl ethanoate. A student stated that the Fry value of 1, 2-dinitrobenzene in this third experiment would be greater than that of 1, 2-dinitrobenzene in the experiment in Question 8. Is the student correct? Justify your answer. - Correct Answer No CE = 0 Yes - mark on but there is NO MARK FOR YES Solvent (more) polar or ethyl ethanoate is polar

Polar isomer more attracted to / more soluble in / stronger affinity to the solvent (than before) Name the types of group attached to 2-deoxyribose at positions X and Y. check 2017 a level paper 2 for diagram - Correct Answer X - base - phosphate (group) Complete the diagram below to show the other molecule and the hydrogen bonds between it and adenine - Correct Answer check paper mark scheme for answer. Give the full IUPAC name of isomer P. (check paper) - Correct Answer Z-2-methylpent- 2-en (-1- ) oil acid A sample of P was mixed with an excess of oxygen and the mixture ignited. After cooling to the original temperature, the total volume of gas remaining was 335 cm When this gas mixture was passed through aqueous sodium hydroxide, the carbon dioxide reacted and the volume of gas decreased to 155 cm Both gas volumes were measured at 25 °C and 105 kappas Write an equation for the combustion of P in an excess of oxygen and calculate the mass, in mg, of P used. The gas constant R = 8.31 J K−1 mol−1 - Correct Answer C6H10O2+71⁄2O26CO +5H2O Volume of CO2 formed = 180 cm Moll carbon dioxide = uP/RT = 105000 × (180 × 10-6) 8.31 × 298 = 7.632 × 10- Moll P, C6H10O2 used = 7.632 × 10-3 / 6 = 1.272 × 10- Mass P used = 1.272 × 10-3 × 114(.0) g = 145mg Isomer Q (C6H10O2) is a cyclic compound. The infrared spectrum of Q is shown in Figure 4 and the 13C NMR spectrum of Q is shown in Figure 5. Use these spectra and Tables A and C in the Data Booklet to deduce the structure of Quinn your answer, state one piece of evidence you have used from each spectrum. - Correct Answer OH (acid) peak (2500-3000cm-1) present 13C NMR 4 peaks so 4 (non-equivalent) environments Or Peak at 160-185 (show C=O) in (esters or) acids Or Peak at 40-50 (show R-CO-CH) presence of carbonyl Although the 13C spectra of R and S both show the same number of peaks, the spectra can be used to distinguish between the isomers. Justify this statement using Table C from the Data Booklet. Give the number of peaks for each isomer. - Correct Answer R has 4 C next to C=O S has 2 C next to C=O in range ppm = 20- R has two peaks and S only one peak in this range Or R has more peaks (allowed if no numbers given) Although the 1H spectra of R and S both show the same number of peaks, the spectra can be used to distinguish between the isomers. Justify this statement using the splitting patterns of the peaks. Give the number of peaks for each isomer. - Correct Answer R Both singles

S has triplet and a quartet The action of heat on 5-hydroxyhexanoic acid can lead to two different products. On gentle heating, 5-hydroxyhexanoic acid loses water to form a cyclic compound, T (C6H10O2). Under different conditions, 5-hydroxyhexanoic acid forms polyester. Draw the structure of T. Draw the repeating unit of the polyester and name the type of polymerization. - Correct Answer check mark scheme (2017 paper 2 a level) AminesE, FandGareweakbases. Explain the difference in base strength of the three amines and give the order of increasing base strength. - check paper for diagrams - Correct Answer (Strength depends on availability of) lone pair on N (atom) E N (next to ring): (lap) delocalized into ring (lap) less available (to donate to or to accept an H+) F or G: N (next to alkyl): (positive) inductive effect/electrons (lap) more available (to donate to or to accept an H+) pushed to N order of increasing base strength E<G<F Amine F can be prepared in a three-step synthesis starting from methylbenzene. Suggest the structures of the two intermediate compounds. For each step, give reagents and conditions only. Equations and mechanisms are not required. - Correct Answer Intermediate compounds Product of step 1 C6H5CH2Cl Product of step 2 C6H5CH2CN Reagents/conditions step 1 Cl2 and UV step 2 alcoholic KCN and as (both needed) step 3 H2/Ni or Pt. or Pad