AQA A Level Chemistry - Paper 3 Experiments Questions and Answers

AQA A Level Chemistry - Paper 3 Experiments Questions and Answers Make Up a Volumetric Solution 1. Weigh dry weighing boat 2. Place mass of solid into the boat and weigh accurately 3. Pour solid into a beaker. Reweigh empty weighing boat and calculate how much was transferred 4. Add deionised water and stir with glass rod until dissolved 5. Funnel into a volumetric flask 6. Make up to mark with deionised water until bottom of meniscus is on the mark 7. Stopper and invert to mix Acid-Base Titration 1. Fill a burette 2. Use a pipette to transfer 25cm³ of the other solution into the conical flask 3. Add 2-3 drops of indicator 4. Start adding from the burette, swirling constantly, until the indicator changes colour. Record how much solution was added (rough titre) 5. Repeat to get concordant results and calculate a mean titre Measuring Enthalpy Change of Combustion 1. Measure 100cm³ of water into a calorimeter 2. Weigh the spirit burner 3. Measure initial temperature of the water 4. Clamp calorimeter above spirit burner 5. Light the spirit burner 6. Heat until the temperature rises by 15°C. Stir and record final temperature 7. Reweigh spirit burner 8. Calculate temperature change and heat energy change (q=mcAT) 9. Calculate mass of fuel used and moles of fuel used. Calculate energy change per mole. Measuring Enthalpy Change of Neutralisation 1. Place polystyrene cup in glass beaker 2. Measure out 25cm³ of 1moldm³ HCl and transfer to polystyrene cup 3. Stir and record temperature 4. Measure out 25cm³ of 1moldm³ NaOH into another measuring cylinder. 5. Add NaOH to HCl, stir and record the highest temperature reached 6. Calculate temperature change and q=mcAT 7. Calculate moles of acid used and water formed, and the enthalpy of neutralisation Investigating Effect of Temperature on Rate of Reaction 1. Measure out 50cm³ of aqueous solution A and pour into a clean dry 250cm³ conical flask 2. Measure out 5cm³ of aqueous solution B and pour into test tube 3. Place conical flask on a piece of laminated paper with a cross. Add the acid and start the stopwatch 4. Gently stir and record initial temperature 5. Watch over the top and time how long it takes for the precipitate to form and cross to disappear 6. Note the final temperature and calculate the initial and final temperatures e.g sodium thiosulfate and acid: Na2S2O3 + 2HCl -- S + SO2 + H2O + 2NaCl Preparing an Organic Liquid Heat under reflux. Most organic liquids are flammable = water bath. Anti-Bumping Granules Small, rough pieces of silica that are added to the mixture before reflux. They provide a rough surface on which small gas bubbles can grow, avoiding bumping. They promote smooth, even boiling. Separating a Crude Product Distillation apparatus. Solvent Extraction (Purifying Product) 1. Place organic liquid in separating funnel and add a portion of aqueous solution 2. Stopper + shake, releasing the pressure by inverting and opening the tap 3. Allow separating funnel to stand until layers settle and separate 4. Remove stopper and open tap to run off bottom layer into a beaker. Close the tap partially to slow the flow towards the end. Run the second layer into a separate beaker. 5. Discard aqueous layer 6. Place organic layer back into funnel and repeat Drying an Organic Liquid 1. Add a spatula of drying liquid (e.g. anhydrous CaCl2 or MgSO4) to the organic liquid 2. Swirl 3. Add more of the drying agent until the liquid changes from cloudy to clear 4. Filter or decant off the liquid into a clean, dry flask Measuring Rates: Continuous Monitoring 1. Set up gas syringe attached to conical flask 2. Measure 50cm³ of HCl + add to flask 3. Add a 6cm strip of magnesium ribbon to the conical flask, place the bung firmly onto the top and start the timer 4. Record volume of hydrogen in the syringe every 15 seconds for 2.5 minutes 5. Repeat at different concentrations Measuring Rates: Initial Rates 1. Fill burette with potassium iodide 2. Transfer 10cm³ of hydrogen peroxide into a 100cm³ beaker 3. Add 25cm³ to a 250cm³ beaker 4. Add 20cm³ deionised water into the beaker 5. Add 1cm³ of starch solution 6. Add 5cm³ potassium iodide from the burette 7. Add 10cm³ sodium thiosulfate solution 8. Add the hydrogen peroxide and start the timer 9. Stir 10. Stop the timer when the solution turns blue black 11. Repeat using different volumes of water and potassium iodide H2O2 + 2H+ + 2I- -- I2 + 2H2O Measuring EMF of an Electrochemical Cell 1. Construct the electrochemical cell. First, clean the electrodes with emery before use. 2. Place one electrode into a 100 cm3 beaker with about 50 cm3 of 1 mol dm-3 (electrode) solution. 3. Place the other electrode into a 100 cm3 beaker with about 50 cm3 of 1 mol dm-3 (electrode) solution. 4. Use a strip of filter paper soaked in saturated potassium nitrate solution for the salt bridge 5. Connect the half cells by connecting the metals using the crocodile clips and leads provided to the voltmeter 6. Record measured voltage 7. Set up another cell using new solutions and a new salt bridge 8. Record voltage How pH Changes When A Weak Acid Reacts With A Strong Base 1. Rinse pH probe thoroughly with deionised water and shake gently. Place it in the standard pH 7 buffer solution to calibrate. 2. Fill burette with 0.1M NaOH solution 3. Use pipette to transfer excactly 20cm³ of 0.1M ethanoic acid to a clean 100cm³ beaker 4. Rinse pH probe with deionised water and clamp it into the ethanoic acid solution in the beaker. Stir gently with a rod and record the pH 5. Add the NaOH from the burette in 2cm³ intervals, stirring and measuring the pH after each interval until 18cm³ is added 6. Add NaOH in 0.2cm³ intervals until 22cm³ is added, recording pH after each interval 7. Add NaOH in 2cm³ intervals until 40cm³ is added 8. Plot pH (y) against volume of NaOH added (x) 9. Repeat with a strong acid and strong base Preparing an Organic Solid 1. Put a few anti-bumping granules in a 50cm³ pear shaped flask 2. In a fume cupboard, add 10cm³ ethanol, 12cm³ glacial ethanoic acid and 15 drops of conc sulphuric acid to the flask 3. Place a 250cm³ beaker with water on a tripod and gauze over a Bunsen burner 4. Clamp pear shaped flask in the beaker of water so the reaction mixture is below the water line 5. Add condenser and set up for reflux 6. Bring water bath to a gentle boil and continue for 15 mins. Turn off the bunsen and cool the mixture by removing the water bath. 7. Solid is separated by suction filtration Recrystallisation (Pure Organic Solids) 1. Dissolve impure crystals in minimum of hot solvent to ensure crystals form on cooling 2. Filter hot solution by gravity filtration, using a hot funnel and fluted filter paper to remove solid impurities 3. Allow solution to cool + crystallise. It might help to scratch the glass 4. Filter off crystals using suction filtration 5. Wash with ice cold solvent to remove aqueous impurities 6. Dry with air in a Buchner flask and place in low temperature oven Checking Purity Of An Organic Solid 1. Place some solid in a melting point tube sealed at one end 2. Place in m.p. apparatus and heat slowly 3. Record the melting temperature and the temperature when it stops melting 4. Repeat and average the remperatures 5. Compare m.p. to data book values M.p. range of + or - 2°C = pure Thin Layer Chromatography 1. Draw pencil line 1.5cm from bottom of TLC plate + place 2 pencil crosses on the line 2. Place a drop of the purified solid on a watch glass + dissolve in a few drops of solvent 3. Use a capillary tube to place a spot of the solvent on a pencil cross. Allow spot to dry and repeat 3-4 times, ensuring diameter is no more than 0.5cm. This gives a concentrated spot 4. Repeat for pure solid 5. Place solvent in beaker at depth of 1cm 6. Place TLC plate in beaker and cover with lid 7. Allow solvent to run up the plate until it has almost reached the top 8. Remove from beaker + mark the line of the solvent front with pencil 9. Place plate in a fume cupboard until all of the solvent has evaporated and the plate is dry 10. View colourless spot by UV light or with ninhydrin 11. Measure distance from pencil line to spot and from pencil line to solvent front. Calculate Rf values Extrapolation (Enthalpy Changes) 1. Measure 50g of water into polystyrene cup held in a beaker 2. Start stop clock and record temp of water every minute for 3 minutes 3. On minute 4, add the 5g solid and stir vigorously 4. Read temp of solution every min for 15 mins 5. Plot graph of temp vs time 6. Extrapolate temp readings back from min 15 to min 4 7. Draw best fit line to give mean start temp from min 1 to min 3 8. Calculate temperature change at minute 4 Distillation NB, oxidation: Dilute acid and less K2Cr2O7 than needed. 1. Heat liquid to boil and vaporise the most volatile component in the mixture. Anti-bumping granules are used. 2. The vapour passes up from the flask and down into the condenser, where it is cooled by cold water and ice, and condenses back to a liquid (the distillate) which is collected in the flask. 3. Any dissolved solids are left in solution because they have too high a boiling point to be distilled over .