AQA A LEVEL CHEM PRACTICALS - PAPER 3 Questions and Answers

AQA A LEVEL CHEM PRACTICALS - PAPER 3 Questions and Answers measuring e.m.f. of electrochemical cells The standard cell potential is the e.m.f. between two half cells making up the cell, measured under standard conditions. clean piece of each metal, e.g. copper and zinc. degrease using propanone, place copper in a beaker with around 50cm₃ 1moldm⁻³ copper sulfate solution, place zinc in a seperate beaker of same concentration and volume zinc sulfate solution join the two beakers with filter paper coated in NaCl/potassium nitrate - acts as a salt bridge connect one end to positive terminal of voltmeter and the other electrode to the negative terminal, ensuring the reading is positive - if it is negative, reverse the connections to the electrodes record the reading of the voltmeter, writing which electrode is connected to the positive and negative terminals standard solution - place a known mass of a sample in a beaker, add 100cm³m of distilled water to beaker to dissolve sample, record mass to 2 d.p. - use a glass rod to stir - pour this solution into a graduated flask via a funnel - rinse beaker and funnel using distilled water and add washings to volumetric flask - make up to the mark with distilled water so that the meniscus sits on the line - invert flask several times to ensure uniform solution Titration - known concentration in burette, unknown concentration in conical flask - rinse burette with deionised water and then with the solution you're using - fill the burette past the mark (using funnel) and empty the burette until meniscus is on the line - removal funnel so no more solution can enter - rinse pipette with deionised water and then with the solution you're using. when filling pipette go past the line then empty until meniscus is on the line -rinse conical flask with deionised water - pipette 25cm³ of required solution into conical flask - add required indicator - swirl to mix - titrate the solution from the burette, about 1cm³ at a time, until indicator changes colour *any drops or splashes up the side of the flask should be washed with deionised water *repeat titration adding solution dropwise when reaching end point - record all to 2.d.p but only ending in 0 or .5 repeat until results are concordant (within .1) What may go wrong with titration? may lose some solution before it starts don't remove funnel so some drops fall during titration may rinse conical flask with solution so you have added more solution to the flask may not include all of the washings Which indicator should be used? strong acid - strong base = either methyl orange or phenolphthalein ( pink to colourless) weak acid - strong base = phenylphthalein, strong acid - weak base = methyl orange ( yellow to red/orange) Indicator must change sharply at the equivilance point, so the pH range of the indicator should fall within the 'vertical section' (region of rapid pH change) What happens in an acid-base titration solution of alkali of known concentration added from a burette to a measured amount of an acid. indicator used to find end point - from volume of base required, concentration of acid can be calculated How can number of molecules of water of crystallisation be determined in a practical? titrate the aq solution, with known volume in a conical flask and acid in burette, repeat titration until concordant results found. Use added volume to work out moles using molar ratio. calculate mr using known mass and number of moles found - then calculate 'x' using found mr - mr of each molecule in substance, divided by 18 to find number of H2O molecules Acid-base titration - pipette 25cm³ acid and add indicatir, place beaker on a magnetic stirrer and place magnet in the beaker, fill burette with base - rinse electrode with distilled water as readings may have changed over time, dip it into the acid in the beaker, recording pH -switch on stirrer and ensure magnet doesnt strike the electrode. - add 5cm³ base and take readings of pH and indicator - continue adding base taking 5cm³ readings each time then 1cm³ around end point, then 5cm³ util most base has been added - plot a graph of pH (Y axis) against volume of base added Using hess's law to determine enthalpy change of a reaction - calculating ∆1 This is an exothermic reaction so a cooling curve should be drawn. - weigh a known mass of anhydrous copper sulfate (sample) - using measuring cylinder, place 25cm³ deionised water into polystyrene cup, place this in a beaker -use thermometer to record temperature of deionised water every minute for 15 minutes, stirring constantly (use stopwatch) - at FOURTH minute, add anyhdrous sample rapidly and continue to stir, record temperature at 5 minutes then every minute until 15 mins. plot graph of temp on Y against time, draw two seperate best fit lines and use the graph to determine the temperature change at the fourth minute (rise in temp) use this value in q=mc∆t Using hess's law to determine enthalpy change of a reaction - calculating ∆2 this is an ENDOTHERMIC reaction so max and min temperatures should be recorded - weigh out mass of hydrated copper sulfate, using measuring cylinder place 25cm³ of deionised warer in a polystyrene cup (INSULATION) and this in a beaker for support and additional insulation - record temp of deionised water - add hydrated sample to cup and stir, watch thermometer and record the minimum temperature use temperature fall in q=mc∆t to find enthalpy change for ∆h2 ∆H3 = ∆H1 - ∆H2 Investigating how rate changes with temperature - mark black cross on piece of white paper - fill 250cm³ beaker one third full with hot water, place over black cross and measuer temp using thermometer (should be around 60°C) - Using a measuring cylinder put 10cm³ of 0.05moldm⁻³ sodium thiosulfate into boiling tube and heat - add 1cm³ HCl to test tube and placein water bath, allow reactants to heat - record temp of boiling tube and add acid to the sodium thiosulfate, look down through test tube and record the time for the cross to dissapear from view (ppt of sulfur forms) - record temp when reaction ends. - repeat experiment at a lower temperature until 5 different temperatures have been looked at Investigating how rate changes with temperature - data calculate mean temp of each reaction mixture from initial and final temps - calculate 1/t for 3 s.f. plot a graph of 1/t (rate) against mean temp or against 1/T and work out arrhenious Test tube reactions to identify cations and anions NH₄⁺ - add ammonium chloride and sodium hydroxide, red litmus paper turns blue as alkali gas is formed Ca²⁺ - add NaOH - thick white ppt, Sulfuric acid - colourless solution Ba²⁺ - NaOH - thick white ppt, sulfuric acid - white ppt Flame tests for group 2 cations - dip wooden splint in small amout of the ion, place this in blue bunsen flame and record the colour of the flame Ca²⁺ = brick red Sr²⁺ = red Ba²⁺ = pale green (etc) synthesis of a halogenoalkane - part 1, preparation preparation of a halogenoalkane from an alcohol - weigh the alcohol, pour into a separating funnel, measure out about 20cm³ of HCl and gradually add this to the alcohol in the separating funnel over around 2 minutes place the stopper in the separating funnel and gently shake the mixture every few minutes for around 10 minutes, after shaking CAREFULLY release any pressure synthesis of a halogenoalkane - part 2, separation allow separating funnel to stand until two layers have separated remove stopper and open the tap so that the lower aq layer runs into a clean conical flask Add 10cm³ of sodium hydrogencarbonate to the separating funnel place stopper and shake contents gently, carefully releasing any pressure once the two layers have separated, run the aq lower layer, allow the organic layer to run into a clean 100cm³ flask - this is the product Add a drying agent to the organic liquid (e.g. anhydrous sodium sulfate) - add small quantities and swirl between each addition until the liquid is clear and drying agent no longer forms clumps synthesis of a halogenoalkane - part 3, distillation transfer impure organic product to round bottom flask add anti bumping granuals to promote even heating and prevent large bubbles from forming use distillation apparatus!! heat the round bottom flask gently with a small flame or spirit burner when the temperature (use thermometer) is close to that of the boiling point of the product, use a sample tube to collect the liquid until temp is a couple degrees above the boiling point weigh to find final mass of product collected distillation separating funnel When to use distillation apparatus/why? Used to purify liquids with different boiling points; liquids heated in round bottom flask, as each liquids boiling point is reached it vapourises, passes into the condenser and is condensed back into liquid form - water must go in at the bottom and come out at the top to ensure efficient cooling of the vapours identifying organic functional groups 1 - alkene test, add few drops of 0.01moldm3 bromine water, record any colour change (orange to colourless) 2 - primary/secondary alcohol. add 1cm³ acidified potassium dichromate and warm. positive test = orange to green (alcohol is oxidised to an aldehyde) 3 - test for aldehydes, mix equal amounts of fehlings 1 & 2. add few drops to sample and warm gently, positive test = blue to brick red (aldehyde oxidised to carboxylic acid) 4 - test for carboxylic acids, add Na₂CO₃ and warm, positive test = effervesence (CO₂ produced) Measuring rate of reaction by initial rates method/ CLOCK REACTION method make up a mixture of sulfuric acid and propanone and water for 4 different concentrations to be produced. use a burette to measure volumes accurately, keep iodine concentration the same - concentrations of each reactant are changed in turn to see the individual effects - stand the flask on a white time, start the reaction by adding iodine to the contents, swirling to mix. immediately start the clock and measure the time in seconds for the iodine colour to dissapear - rate is equal to 1/t - compare rates for each concentration and see effect of concentration on rate to find order with respect to each reactant Measuring rate of reaction by continuous monitoring method finding respect to iodine (example) - set up colorimeter and select blue filter - press and release kinetics button and select absorbtion mode - insert cuvette with distilled water and check absorbance reading (should be zero) - mix acid, ketone and add iodine, swirl to mix and as quickly as possible fill a second cuvette with this mixture - take readings every 30 seconds for 7 minutes to continually monitor the absorbance reading - plot a graph of absorbance (Y) against time (X) and draw a line of best fit. shape of the graph will indicate which the order of the reaction is with respect to iodine - if gradient is constant, zero order, if half is constant, first order, if half life increases this is second order Examples of continuous monitoring methods gas volume, change in pH, loss of mass, colour via colorimeter to see concentration changes Examples of initial rate methods time taken for pH to change, time taken for set vol of gas to be evolved, time taken for change in colour to occur in terms of electrons, state what is happening at the negative electrode negative electrode is supplying the electrons to the solution which are transferred to the positive electrode. This is where oxidation occurs, this metal is the more reactive metal. preparing an organic solid and testing its purity (e.g. aspirin) weigh out known mass of 2-hydroxybenzoic acid and react this with 4cm³ ethanoic anhydride in a conical flask add 5 drops of concentrated sulfuric acid and swirl for around 10 minutes until a crystalline mush forms, stir in 4cm³ conc. ethanoic acid and cool the mixture by placing the flask in a water bath of ice speration of organic solid product (Aspirin) filter off the crystals that form using suction filtration and wash once with a small amount of ice cold water Recrystallisation of organic solid product (Aspirin) -transfer crystals to 100cm³ beaker and dissolve this impure compound in the minimum volume of hot solvent (an appropriate solvent that dissolves both the impurities and the pure solid in hot, and only the impurities when cold. minimum volume used to obtrain saturated solution and enable crystals to form on cooling) - filter the solution (hot) by filtration paper to remove any insoluble impurities and prevent crystals reforming during filtration - cool filtered solution in ice bath (crystals reform but soluble impurities remain in solution - ice increases yield of crystals) - suction filtrate with a buchner funnel to separate out crystals (water pump of buchner funnel reduces pressure and speeds up filtration) - wash the crystals with cold distilled water (wash away any soluble impurities) - dry the crystals between absorbent paper Determining purity of the organic product (aspirin) melting point of product formed can be established by placing a small amount of the product in a capillary tube and heating the tube slowly near the melting point. compare against databook value for the compound. - pure sample has sharp melting point at the same point of the data book, if impurities are present, the melting point will be lower and will melt over a range of several °C * To increase purity, re-crystallise more than once * to increase percentage yield, take more care in transfer process to reduce mechanical losses and use a more pure starting product preparing a pure organic liquid using reflux - reflux used when heating an organic product for a long period of time; condenser prevents organic vapours from escaping by condensing them back to liquids - never seal the end of the condenser as this will cause a build up of gas pressure, apparatus may explode - anti-bumping granules are used to prevent vigorous uneven boiling by making small bubbles rather than large bubbles reflux apparatus Test tube reaction to identify transition metal ions in solution - measure 3 samples of each unknown metal ion - to the first test tube add NaOH solution, record observations, add excess NaOH and again record onservations - to the second test tube, add a few drops of ammonia solution, record observation, add excess ammonia solution until in excess - to the third test tube add a few drops of sodium carbonate solution and record any observations