Techniques and Procedures

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  • Created by: LBCW0502
  • Created on: 05-06-16 11:58

Measuring volumes of gases

  • Can be measured either using a gas syringe or an inverted burette
  • The latter is called 'collecting gas over water'
  • In order for as much as possible of the gas to be collected the system needs to be gas tight
  • Volume of gas collected in an inverted burette is the inital volume minus the final volume of gas
  • If the gas is soluble in water you need to use the gas syringe methods as some of the gas would dissolve before reaching the burette in the second method
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Burette

  • Clean the burette by rinsing out with water and then a small volume of liquid of the solution to be used
  • Make sure the burette tap is closed. Pour the solution into the burette using a small funnel. Fill the burette above the zero line
  • Use a clamp to hold your burette in place and allow some of the solution to run into a beaker until there are no air bubbles in the jet of the burette. Record the burette reading to the nearest 0.05cm3
  • Carry out titration to the end point
  • Record the reading on the burette to the nearest 0.05cm3
  • Subtract the reading taken at the beginning of the titration from this reading taken at the end. This is known as the titre
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Weighing a solid

  • Use an accurate weighing balance that records to two or three decimal places
  • Zero the balance (tare)
  • Place a weighing bottle (container) onto the balance and add in approximately the required mass of solid
  • Accurately weigh the mass of solid and the weighing bottle and record this information 
  • Empty the solid into the glassware where you will be using it
  • Accurately reweight the empty weighing bottle
  • Subtract the recorded mass from the empty weighing bottle from the mass recorded for the solid and the weighing bottle
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Heating under reflux

  • Used for reactions involving volatile liquids 
  • Ensures that reactants/products do not escape whilst the reaction is in progress
  • Put the reactants into a pear shaped or round bottomed flask and add a few anti-bumping granules (these granules burst the bubbles into the boiling mixture and reduce the chance of boiling over)
  • Do not stopper the flask - doing this would cause pressure to build up and the glassware could crack or the stopper could fly out. In either case a serious accident could result
  • Attach a condenser vertically to the flask so that water flows into the condenser at the bottom and out of the condenser at the top. This ensure tht the condenser is always full of cold water
  • Heat so that the reaction mixture boils gently, using a Bunsen flame or heating mantle. When refluxing correctly, any vapours should reach no more than half way up the condenser before condensing back into a liquid. The liquid should drip back into the reaction flask steadily
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Reacting an acid and a insoluble base

  • In a beaker warm excess insoluble base in dilute acid
  • Continue to warm (but not boil) until the solution is neutral (use universal indicator paper for this step) adding more solid base if needed
  • Leave to cool
  • Filter off the excess base and transfer the filtrate to a clean, dry evaporating basin
  • Heat the evaporating basin until salt crystals begin to appear on the sides of the basin
  • Cool the basin and contents
  • Filter the mixture and discard the filtrate
  • Wash the solid residue with cold distilled water
  • Transfer the residue to a watch glass and heat in an oven to dry the solid. Ensure the oven is set at a temperature below the melting point of the salt you have prepared
  • At regular intervals remove the watch glass and solid, cool in a desiccator and weigh. Once the solid has dried to a constant mass (mass between the readings does not change) stop heating the solid salt and leave to cool in a desiccator
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Pipette

  • Ensure the pipette is completely clean by rinsing out with water and then a small volume of the solution to be pipetted
  • Dip the pipette into the solution to be pipetted and using a pipette filler, draw enough liquid into the pipett until it is exactly the right volume - when the bottom of the meniscus is level with the len on the neck of the pipette when viewed at eye level
  • Run the liquid out of the pipette into the piece of glassware the solution is being transferred to
  • When all the liquid has run out allow the liquid to run out of the pipette until it stops. Touch the end of the pipette on the side of the conical flask and remove. There will still be a drop in the pipette (this is how it should be). The precise volume you require will be dispensed
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Purifying an organic liquid

  • Organic liquid products have to be purified after it has been synthesised
  • When the organic product is mixed with anoter immiscible liquid (aqueous liquid) the two layers can be separated using a separating funnel
  • The layers separate with the denser liquid forming the lower layer 
  • Upper layer (cyclohexane), lower layer (water/impurities)
  • Allow the layers to settle and then run off and dispose the aqueous layer 
  • If acidic impurities are present, add sodium hydrogencarbonate solution to the organic liquid in the separating funnel and shake well (to remove acidic impurities) - if the crude product is alkaline and needs neutralising then add dilute acid until the mixture is neutral
  • Whilst shaking the separating funnel, invert the apparatus (make sure the stopper is in place) and open the tap to release any carbon dioxide produce (decreases the pressure in the separating funnel)
  • Place the apparatus back into the clamp and stand
  • Remove the stopper and run the organic liquid into a clean conical flask 
  • Dry the crude product by adding anhydrous sodium sulfate and swirl the mixture (drying agent - could also use calcium chloride) to dry the compound
  • The pure product can then be separated by distillation
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Vacuum filtration

  • Used to separate a solid from a filtrate rapidly
  • Connect a conical flask to a vacuum pump via the side arm (do not switch the pump on yet)
  • Dampen a piece of filter paper and place it flat in the Buchner funnel
  • Switch the vacuum pump on and then carefully pour in the mixture to be filtered. The pump creates a partial vacuum so that the filtrate gets 'pulled through' quickly
  • Disconnect the flask from the vacuum pump before turning the pump off 
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Determining melting points

  • Technique used to determine melting point of organic solids. Used as evidence of a product's identity and purity

1) Seal the end of a glass melting point tube by heating it to melting in a Bunsen flame

2) Tap open the end of the tube into the solid so a small amount goes into the tube. Tab the tube so that the solid falls to the bottom of the sealed end

3) Fix the tube in the melting point apparatus and heat the surrounding liquid gently, stirring to ensure even heating throughout. The temperature should rise very slowly

4) Note the temperature at which the solid starts and finishes melting. The difference between the highest and lowest temperatures recorded is known as the melting range

5) Compare the experimental value to the published value for the melting point. The wide the melting range, the more impure the substance. A pure compound will melt within 0.5 degrees Celsius of the true melting point

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Reacting an acid and a soluble base (alkali)

  •  Carry out an acid-base titration to find out houw much acid solution is needed to neutralise 25 cm3 of the alkaline solution
  • Transfer 25 cm3 of the alkaline solution to a clean conical flask
  • Using a burette, add the correct amount of acid to neutralise the alkali. Do not add any indicator
  • Transfer the neutralised solution to a clean evaporating basin and heat over a Bunsen Burner flame to evaporate the water. Take care not to heat too strongly, in order to avoid spitting. Once crystals of solid start to appear stop heating
  • Leave the mixture to cool in the evaporating basin. Filter the mixture. Wash the solid residue with cold distilled water
  • Transfer the residue to a watch glass and heat in an oven to dry the solid. Ensure the oven is set at a temperature below the melting point of the salt you have prepared
  • At regular intervals, remove the watch glass and solid, cool in a desiccator and weigh. Once the solid has dried to a constant mass (mass between two readings does not change) stop heating the solid salt and leave to cool in a desiccator 
  • (a desiccator allows materials to cool in a dry atmosphere, so preventing the reabsorption of moisture) 
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Thin layer chromatography

  • Used to separate small quantities of organic compounds (follow the progress of a reaction over time)
  • Method relies on the fact that different organic compounds have different affinites for a particular solvent (carried out through chromatography medium at different rates)
  • Thin layer chromatography carried out using a silica plate

1) Spot the test mixture and reference samples on a pencil line 1cm from the base of the thin layer chromatography plate. Pencil is used because it will not run into the solvent

2) Suspend the plate in a beaker containing the solvent and cover the beaker with a watch glass to prevent the solvent from evaporating 

3) Remove the plate when the solvent front is near the top. Mark how far the solvent has reached. Allow the plate to dry

4) Locate the spots with iodine, ninhydrin or under an ultraviolet lamp

5) Match the heights or Rf values with those of known compounds musing the same chromatography solvent mix 

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Recrystallisation

  • Used to purify solid crude organic products with small amounts of impurities 
  • A suitable hot solvent is chosen - only the desired compound dissolves to an appreciable extent. When cooled, the pure organic compound will drop out of solution (recrystallise). 
  • Any other soluble impurities stay in solution

1) Select a solvent in which the desired substance is very soluble at higher termpatures and insoluble, or nealy so at lower temperatures

2) Siddolve the mixture in the minimum quantity of hot solvent - the smaller the amount of solvent used the better the yield of the desired substance

3) Filter to remove any insoluble imourities and retain the filtrate. It is best to preheat the filter funnel and conical fask to prevent any sold crystalling out at this stage

4) Leave the filtrate to cool until crystals form

5) Collect the crystals by vacuum filtration

6) Dry the crytals in an oven or leaving them in the open, covered by an inverted filter funnel

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Simple distillation

  • Used to separate a mixture of miscible liquids with unique boiling points 
  • Each pure component is vaporised, condensed and collected when heated
  • Components will evaporate in order of their boiling points - the one with the lowest boiling point will evaporate first
  • Quickfit glassware is commonly used for distillation
  • It has ground glass joints that can be sealed using grease to prevent the loss of reagents (other small scale systems are available)
  • Put the mixture into a pear-shaped flask and add a few anti-bumping granules. Set up the distillation apparatus. Position of the thermometer must be important (gives an accurate reading of the vapour temperature)
  • Heat the mixture until it boils gently, using a Bunsen flame or heating mantle. Heating mantles are safer to use when heating flammable liquids as they do not have a naked flame
  • When the vapour temperature is approximately two degrees below the boiling point of the liquid you are about to collect, put the collecting beaker in place. Collecting the distilled liquid until the temperature of the vapour rises above the boiling point of the liquid you are collecting. Stop heating
  • If another compound is required of a higher boiling point, repeat the previous step using a clean collecting beaker
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Making a standard solution from a solid

  • Standard solution: the concentration is accurately known and can be used to determine the concentration of another unknown solution/purity of a solid
  • Use Analar reagents - pure reagents
  • Calculate the mass of solute required. In a weighing bottle, weigh out this amount accurately to the nearest 0.1g. Make a note of the mass of the weighing bottle and solute
  • Pour 100cm3 of deionised water into a 250cm3 beaker. Carefully transfer the weighed solute into the water from the weighing bottle
  • Reweigh the weighing bottle. The difference between the mass of the weighing bottle and solute and the mass of just the weighing bottle is the mass of solute transferred
  • Stir the mixture in the beaker to ensure complete disolving of the solute
  • Transfer the solution to a clean 250cm3 volumetric flask. Rinse the beaker and stirring rod well with deionised water, making sure that all the washings go into the volumetric flask
  • Using a dropping pipette, add deionised water so that the bottom of the meniscus is level with the mark on the neck of the flask when looking at it at eye level
  • Insert the stopper in the flask and invert it, shaking thoroughly to ensure complete mixing
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Making a standard solution by dilution

  • If the concentration of an existing solution is too high it can be adjusted 
  • Existing solution is called the stock solution 
  • E.g. concentration is diluted by a factor of 10 to produce 250 cm3 of a 0.1 moldm-3 solution from a standard solution of 1.0 mol dm-3
  • Rinse a clean dry beaker with the stock solution and then half fill it
  • Use a pipette filler to rinse a clean 25cm3 pipette with some of the stock solution. Fill the pipette to the 25 cm3 mark - with the bottom of the meniscus exactly on the mark
  • Run the solution into a 250 cm3 volumetric flask
  • Add deionised water to the solution, swirling at intervals to mix the contents, until the level is within about 1cm of the mark on the neck of the volumetric flask
  • Using a dropper pipette, add deionised water so that the bottom of the meniscus is level with the mark on the neck of the flask
  • Insert the stopper in the flask and invert it, shaking thoroughly to ensure complete mixing, You now have your new diluted standard solution
  • C1 x V1 = C2 x V2
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Acid-base titration

  • Used to determine concentration of an acid/alkali
  • Assumes that you are titrating an alkali of a known concentration against an acid to calculate the concentration of the acid
  • Rinse a burette with some acid solution then fill it with acid. Run a little of the acid through the burette reading to the nearest 0.05 cm3
  • Fill a clean 25 cm3 pipette with some of the alkaline solution
  • Run the alkaline solution into a clean 250 cm3 conical flask. Add two or three drops of suitable indicator and swirl to mix. Run the acid from the burette into the flask
  • Swirl the flask continually and watch for the first hint of the solution changing colour. The first titration should be used as a trial run to give a rough indication of the amount of acid required. Record the final burette reading - volume of acid used is called the titre
  • Using a pipette, transfer 25 cm3 of the alkakine solution to a clean conical flask. Add two to three drops of the indicator and swirl to mix
  • Run in the acid solution to 1cm3 below the rough titre. Then add the acid dropwise, swirling after each drop until the colour of the indicator changes
  • Repeat steps 6, 7 and 8 until there are three concordant results (within 0.1 cm3 of each other)
  • The results can be used to calculate the concentration of the acid
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Iodine-thiosulfate titration

  • Involves redox reactions
  • Used to find concentration of chemical that is strong enough oxidisng agent to oxidise iodide ions to iodine 
  • Starch is used as an indicator 
  • Chlorate (I) ions - strong enough oxidising agents to reduce iodide ions to iodine
  • Pour some of the chlorate (I) solution into a clean dry beaker
  • Rinse a volumetric pipette with water then the chlorate (I) solution - ensures that there is no dilution 
  • Transfer 25 cm3 aliquot of the chlorate (I) solution to a conical flask using a volumetric pipette and filler
  • Add excess iodide ions using a measuring cylinder to transfer 15 cm3 of 0.5 mol dm-3 potassium iodide to the conical flask
  • Add excess hydrogen ions using a measuring cylinder to transfer 20 cm3 of 1 mol dm-3 sulfuric acid
  • Contents of the flask will be brown due to the iodine produced 
  • Titrate iodine produced with sodium thiosulfate
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Using a colorimeter

  • Used to determine the concentration of a coloured solution 
  • Select a filter with the complementary colour to the solution being tested (e.g. a purple solution absorbs orange light so you would choose an orange filter) - this allows wavelengths absorbed most strongly by the solution to pass through to the sample
  • Make up a range of standard solutions of the test solution. There should be solutions both above and below the concentration of the unknown solution
  • Zero the colorimeter using a cuvette of pure solvent - water
  • Measure the absorbance of each of the standard solutions and plot a calibration curve of concentration against absorbance 
  • Measure the absorbance of the unknown sample and use the calibration curve to determine the concentration of the unknown solution
  • By determining the concentrations of a reactant at different time intervals in a reaction you can follow the progress of that reaction
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Measuring the energy transfer when a fuel burns

  • Used to determine enthalpy change of combustion
  • Using a measuring cylinder, pour a known volume of water into a copper calorimeter. Record its temperature
  • Weigh a spirit burner - keep the cap on the burner to reduce the loss of the fuel by evaporation
  • Support the calorimeter over a spirit burner containing the fuel to be tested. Surround it with a draught excluder to help to reduce energy losses
  • Remove the cap of the spirit burner and light the wick
  • Use the thermometer to stir the water all the time it is being heated - carry on heating until the temperature has rise by 15 to 20 degrees Celsius
  • Extinguish the spirit burner and put the cap back in place. Keep stirring the water and make a note of the highest temperature reached
  • Weigh the burner again
  • Results can be used to calculate the enthalpy change of combustion
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Measuring the energy transferred (solution)

  • Allows the enthalpy change of reaction to be calculated through the measurement of changes in temperature when known quantities of reactants react together e.g. enthalpy change of neutralisation 
  • Using a measuring cylinder, add a known volume of a known concentration of acid to an insulated vessel and take the temperature
  • Using ameasuring cylunder, add a known volume of a known concentration of the alkali. Stir well to mix the reactants
  • Top the vessel with a lid with a hole in 
  • Place the thermometer through the hole in the lid and record the changes in temperature every 30 seconds until there are no further changes in temperature 
  • Calculate the maximum increase in temperature 
  • Reactions take place in insulate vessels to minimise the transfer of thermal energy to the surroundings.
  • These reactions may involve two solutions or solids reacting with solutions
  • The results can be used to calculate the enthalpy change of neutralisation
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Reactions of solids and solutions

  • Using a measuring cylinder add a known volume of a known concentration of the reactant solution into an insulated vessel. Take the temperature
  • Add a known mass of solid reactant. This should be in excess
  • Top the vessel with a lid with a hole in 
  • Place the thermometer through the hole in the lid and record changes in temperature every 30 seconds until there are no further changes in temperature
  • In order to calculate the maximum change in temperature you will need to plot a graph of temperature against time
  • Values recorded experimentally are too low - reaction occurs over a period of time
  • Theoretical maximum temperature change is never achieved experimentally and must be found by extending the line of best fit back to the time at which the reactants were mixed
  • Extrapolation
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Carrying out the electrolysis of aqueous solutions

  • Electric current is passed through the electrolyte
  • DC power supply may be a powerpack or batteries
  • Most common material for electrodes is graphite
  • This conducts electricity, is cheap and relatively inert
  • Platinum may be used but is expensive
  • Gaseous products can be collected in inverted test tubes
  • Test tubes are filled with water at the start of the reaction
  • Gaseous products displace the water and are collected in the test tubes
  • Once the powerpack is switched on the electrolysis will proceed
  • If electrolysis is to be carried out for the purpose of purification of a metal, the anode will need to be made of the impure metal, the electrolyte must contain ions of the metal and the cathode should be made of pure metal
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Cracking a hydrocarbon vapour (catalyst)

  • Set up apparatus, ensuring there is space above the catalyst (aluminium oxide) to allow gases to pass freely over it. Place several tubes in the water in the collection trough
  • Heat the catalyst strongly. This ensures that when the alkane vapour passes over it the temperature is high enough for the cracking reactions to take place
  • Heat the alkane gently, collecting any gases that pass into the collection tubes, changing and corking full tubes. Continue to heat whilst changnig the collection tubes. 
  • Discard the first tube of gas. This will be just displaced air, rather than product
  • Continue heating both the catalyst and the alkane mixture to be cracked untli you have collected several tubes of gas or until no more gas produced
  • Remove the delivery tube from the collection trough before stopping heating catalyst and alkane mixture. 
  • Leave to cool then dismantle the apparatus
  • Test any liquid product from the middle collection tube with bromine water. The bromine water should remain yellow/brown
  • Test the gas collected in tubes by shaking with bromine water. This bromine water should decolourise
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