Edexcel A level chemistry practicals Questions and Answers

Edexcel A level chemistry practicals Questions and Answers 8)b. measure enthalpy change of decomposition of KHCO3- key points and errors This practical combines two neutralisation reactions: 1. K2CO3 + 2HCL -- 2KCL + H2O +CO2 2. 2KHCO3 + 2HCL -- 2KCL +2H20 +2CO2 The desired reaction is: 2KHCO3 -- 2K2CO3 +CO2+ H2O use Q=mcΔT and ΔH=-Q/n Errors- assume specific heat capacity is the same as water. Heat lost to surroundings. Polystyrene is more insulating than glass so this can reduce the heat lost 9)a finding Ka for a weak acid- method method- 1 titrate 25cm^3 of the acid being tested again NaOH. Use phenolphthalein indicator. 2 After titration, add another 25cm^3 of acid into the same flask. This is equal to half-equivalence point. 3 Use pH meter to measure pH of solution. The pH = pKa (since half-equivalence point). Ka = 10^pKa 9)b finding Ka for a weak acid- using a pH meter and errors calibrate with deionised water and buffer solutions of know pH. Wash in between readings to remove ions. Are good bc: read to 2 d.p and unlike indicators it doesn't add or remove ions. errors- burette readings, over-shooting end point (subjective decision) 10)a investigation of some electrochemical cells- methods 1 For each half cell, clean electrodes with sandpaper. 2 for salt bridge: soak strip of filter paper in KNO3 ( use k+ and NO3- as are soluable salts so prevent ppt forming) 3 connect circuit to high resistance voltmeter 10)b investigation of some electrochemical cells- key points and safety key points: standard conditions- 1moldm^-3, 100kPa, 298 K for thermodynamically feadible process EMF has to be positive. Where EMF = Eright(reduction)-Eleft(oxidation) safety: some too dangeroid at 1moldm^-3 (e.g. silver nitrate is highly oxidising). ZnSO4 and FeSO4 need to be disposed of safely as harmful to environment. Electrodes made of inert substance. Differences between experimental and theoretical values arise from conditions not being standard 11)a Redox titration:Find percentage of iron in tablet- prepare iron (II) solution 1 Use mortar and pestle for crushing the tablets to a powder. Weigh by the difference in mass to have an idea for how many grams of powder is being added 2 add H2SO4 and stir to dissolve 3 filter solution into a volumetric flask to remove any undissolved solids 4 Wash beaker and the filter paper with distilled water to get remaining solution into the volumetric flask. 5 Dilute the solution in the volumetric flask by adding sulphuric acid to the 250cm^3 mark. 11)b Redox titration:Find percentage of iron in tablet- titration 1 Prepare the titration equipment. Add KMnO4 to burette (use burette with white markings as solution is deep purple) 2 Add known volume of iron solution to conical flask. No indicator is needed as KMnO4 is self indicating. End point is when you first see a permanent pale pink colour. 3 Titrate solution and repeat until you get concordant titres. Calculate mean from concordant titres. 11)c Redox titration:Find percentage of iron in tablet- calculations and errors ionic equation: 5Fe2+ + MnO4- + 8H+ -- 5Fe3+ + Mn2+ + 4H2O From results calculate moles of MnO4-. 1:5 ratio so moles of Fe2+ = 5 x moles of MnO4-. Multiply by 10 to find moles of Fe2+ in 250cm^3. Rearrange n=n/M -- m= n x M. This gives the mass of Fe in the tablet. Compare to value on box. Errors: ensure as much of the powder is added and dissolved, warming may help. Add in washings from the containers to get as much iron as possible. Use a white tile to see the end point clearly. 12)a prepare a transition metal complex- method 1 Weigh out the mass of copper sulphate accurately and dissolve in distilled water (can weigh by difference). 2 In fume cupboard add conc ammonia. 3 Stir the mixture and pour into ethanol. Then, cool the mixture in ice baths. Crystals will form. 4 Set up vacuum filtration apparatus with buchner funnel. Collect the crystals in the funnel. Wash tube with cold ethanol and filter again, then wash the crystals with cold ethanol. 5 Leave crystals on funnel to dry. Then use two filter papers to dry crystals even more. 6 record mass and work out percentage yield (relative to the hydrated copper sulphate) 12)b prepare a transition metal complex- safety and errors hydrated copper sulphate is harmful to aquatic life, so use small quantities. errors: losses can be from reaction not going to completion (not crystallising out). gains can be from impure or wet crystals. 13a)a following rate of iodine- propanone reaction by titrimetric method ( continuous monitoring)- method 1 Mix large excess of propanone and sulphuric acid so that their concentrations remain effectively constant during the reaction. 2 Using a pipette add a sample of the mixture to excess sodium hydrogencarbonate immediately after removing it from the flask to quench the reaction. 3 start stop watch. 4 Withdraw samples every three minutes and repeat process. 5 titrate these samples to find their concentrations against sodium thiosulphate adding starch indicator. will turn blue black to colourless 6 use results to plot a graph of conc against time. order can be determined with w.r.t to iodine. should be straight( zero order) 13a)b following rate of iodine- propanone reaction by titrimetric method ( continuous monitoring)- safety propanone is irritant and flammable. Keep away from flames. Sodium thiosulphate releases sulphur dioxide when it reacts so keep room well ventilated. 13b)a iodine clock reaction- method total volumes are the kept the same by topping up with water if needed. This allows you to alter concentrations of the iodine or thiosulfate. 1 Add persulfate to test tube containing potassium iodine, sodium thiosulfate and starch indicator 2 start a stopwatch. 3 when the solution goes blue black stop the clock. 4 repeat for different concentrations. 13b)b iodine clock reaction- results and errors Plot graphs of of rate (1/time) against conc to determine order. errors: inaccurate timing of colour change. two students can both record then find average. adding starch slightly increases the volume which affect conc of reactants so rate is slightly changed. 14) Finding the activation energy of a reaction 1 Mix bromates, bromides an some sulfuric acid to produce bromide. BrO3- + 5Br- + 6H+ -- 3Br2 + 3H2O 2 Add phenol: C6H5OH + 3Br2 -- C6H3Br3OH + 3HBr 3 self indicating as when bromine reacts with the phenol the solution decolourises. 4 record the time to reach this point at different temperatures. Plot results with of lnk against 1/t or ln(t) against 1/T (as 1/t is proportional to k). gradient= Ea/R so activation energy can be determined. 15)a the preparation of aspirin 1 weigh out 2-hydroxybenzonic acid and transfer to pear shape flask. 2 Add acetic anhydride and 8 drops of conc H3PO4. 3 warm the mixture under reflux for 5 minutes 4 add cold deionised water to the solution and stand the flask in ice bath until ppt forms. 5 Filter the mixture under reduced pressure. 6 Dissolve impure product in minimum amount of warm ethanol. 7 add warm water 8 place the boiling tube into ice water for 15 mins. 9 filter the purified derivative under reduced pressure 10 Dry the purified product using filter paper. 11 Collect your sample in a dry, pre-weighed vial and calculate the mass of the product. use this to calculate the percentage yield 12. measure melting point 15)b the preparation of aspirin-errors and safety when washing with ice cold water don't use too much to minimise product loss ethanoic anhydride and conc H2SO4 are corrosive so use gloves 16) Identifying unknown compounds- Inorganic flame tests Li+= red Na+=orange/yellow K+= lilac Rb+=Red-violet Cs+= Blue Be2+= None Mg2+=none Ca2+=Yellow/red Sr2+=red Ba2+=pale green use conc HCL and nichrome wire 16) Identifying unknown compounds- Inorganic NH4+ test Add aqueous NaOH and gently warm the mixture. The ammonia gas will turn moist pH paper blue. Pungent smell given off 16) Identifying unknown compounds- Inorganic anion tests CO3- and HCO3- ions: Add acid, bubbles of CO2 will be observed. Delivery tube to limewater will turn cloudy as CaCO3 ppt forms. SO4- ions: Add HNO3 to remove carbonates. Add BaCl and White ppt of BaSO4 forms if SO4- are present. Halides: Add HNO3 and Add AgNO3. Cl- form white ppt that is soluble in dilute NH3. Br- forms cream ppt which is solutable in conc NH3. I- forms yellow ppt which is insoluble in dilute and conc NH3. can also be tested by reacting with conc H2SO4 Cl produces steamy fumes of HCL. Br produces steam fumes of HBr and brown vapour of Br2 and choking smell of SO2 I produces steamy fumes of HI, purple vapour and black solid of I2 and eggy smell of H2S and a yellow solid of S 16) Identifying unknown compounds- metal cations using NaOH All group 1 hydroxides are soluble. Mg2+ and Ca2+ white suspension ca2+ more soluable. Sr2+, Ba2+ soluble hydroxides. Zn2+ white ppt soluable in NH3. ppt dissolves with more NaOH. Al3+ white ppt, dissolves in more NaOH Ag+ Dark brown ppt, soluable in NH3 colourless solution. ppt doesn't dissolve in more NaOH Cu2+ blue ppt soluble in excess NH3 to for deep blue solution. ppt doesn't dissolve in more NaOH Fe3+ brown ppt, doesn't dissolve in more NaOH Fe2+ pale green ppt, slowly turning brown on standing. ppt doesn't dissolve in more NaOH Cr3+ blue green ppt, soluble in NH3 violet solution. dissolves in more NaOH green solution Co2+ blueish ppt, soluble in NH3-violet solution. Ppt doesn't dissolve in more NaOH Ni2+ Green ppt, soluble in NH3- blue solution. doesn't dissolve in more NaOH Mn2+ off white ppt turns brown when standing. ppt doesn't dissolve in more NaOH 16) Identifying unknown compounds- organic: alkenes, haloalkanes and alcohols alkenes: add bromine water and shake. decolourisation occurs if c=c is present haloalkanes: same as hydrolysis experiment/test for halides Alcohols: warm with acidified K2Cr2O7, colour change orange to green OH group (not phenols): add PCl5, misty fumes of HCl forms add small piece of Na, effervescence is seen. 16) Identifying unknown compounds- organic: carboxylic acids,aldehydes and ketones carboxylic acid: litmus paper turns red. add carbonate, effervescence of CO2 is produced. Aldehyde and ketones: add 2,4-DNP, orange ppt froms if C=O is present. add tollen's reagent (ammonical AgNO3), aldehyde produces silver mirror ketones don't. Add fehlings solution, aldehyde produces brick red ppt 16) Identifying unknown compounds- organic: iodoform reaction, esterfication test and solubility test iodoform: warm with iodine and NaOH, yellow ppt, antiseptic smell. postive test with methylketones, ethanal, ethanol and methyl secondary alcohols. esterfication test: Gently warm alcohol and carboxylic acid in presence of conc H2SO4. Product should have characteristic smell solubility test: compounds that can form hydrogen bonds may be soluble in water. some carboxylic acid may be soluble in alkaline solution. some amines may be soluble in acidic solution.