Techniques and Procedures

  • Created by: lovelyy
  • Created on: 29-05-18 21:05

Determining the Solubility Constant

  • Add distilled water to a flask with the salt, shaking
  • Keep adding solute until no more dissolves (it is saturated)
  • Leave to cool
  • Check to make sure there is still solid at the bottom of the flask
  • Filter the solution, discarding the residue
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Cracking Hydrocarbons

  • Set up as above
  • Heat aluminium oxide catalyst strongly
  • Heat alkane, if you need to change collecting tubes, keep heating to prevent '**** back'
  • The first tube of gas should be disgarded as it's just displaced air
  • Continue heating until no more gas produced
  • Test the liquid (yellow) and gas (colourless) with bromine water 
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Measuring the pH

  • Calibrate the pH electrode by washing it with distilled water and then putting into a buffered solution of pH 7
  • Wait for the reading to stabilise, ensuring it reads 7
  • To measure pH of acid, calibrate with pH 4, with an alkali, pH 10
  • To measure a range, calibrate with acid, alkali and neutral 
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Electrochemical Cells

  • Construct half cell with the salt bridge and all that jazz
  • Connect to hydrogen half cell or other reference by using a high resistence voltmeter
  • The reading should be positive- if it is, the half cell attached to the positive terminal is the positive elctrode
  • If the reading is negative, shange the connections around to give a positive one
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Electrolysis

(http://www.nuffieldfoundation.org/sites/default/files/images/quantitative-electrolysis-of-aq-sol-263.jpg)

  • Most commonly used is graphite, platinum is expensive

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Colourimetry

Determines concentration of a coloured solution. Amount of light absorbed/transmitted is measured and is directly proportional to the concentration of the solution

  • Select a filter that's complementary in colour to the solution being tested(only allows those wavelengths absorbed most strongly by solution to pass through sample)
  • Make a series of standard solutions
  • Zero it with water
  • Measure absorbance of each solutiona and plot absorbance against concentration
  • Measure absorbance of unkown 
  • Use calibration curve to determine concentration of unkown

A visible spectrophotometer does the same but gives data for absorption/transmission for any value of the VL spectrum

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Reflux

Ensures volatile reactants don't escape during heating

  • Put reactants in pear shaped flask
  • Add anti-bumping granules to prevent bubbling over
  • Don't stopper the flask- it would casue pressure to build up and smash glass
  • Attach the condenser so it's always full of water

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Purifying an Organic Liquid Product

  • Use a separating funnel to separate the layers
  • Allow the layers to settle then run off and dispose of the aqueous layer, run the organic product into a flask
  • If acidic impuritites are present, add sodium hydrogen carbonate and shake well
  • If the product is slightly alkaline, add a dilute acid
  • Dry with anhydrous sodium sulfate and swirl
  • Distil
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Making a Soluble Salt With an Acid-Base Titration

  • Carry out acid-base titration to work out how much acid is needed to neutralise
  • Transfer alkaline to a conical flask
  • Add correct acid to neutralise (no indicator)
  • Transfer to an evaporating basin and heat to evaporate water
  • Stop heating when crytals have formed
  • Cool
  • Filter
  • Wash with distilled water
  • Transfer to a watch glass and heat in an oven to dry (oven temp must be below the mp of salt)
  • At regular intervals, remoe, cool and weigh
  • Stop heating when the mass is constant
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Acid and Insoluble Base

  • Warm excess insoluble base in dilute acid
  • Continue to warm until aolution is neutral
  • Cool
  • Filter off exces base
  • Heat filtrate until crystals appear
  • Cool 
  • Filter, discarding filtrate
  • Wash solid residue with distilled water
  • Transfer to a watch glass and heat in an oven (oven temp should be below mp of salt)
  • At regular intervals, remove, cool and weigh
  • Stop heating at a constant mass
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Making Insoluble Salt By Reacting Soluble Salts

  • Add equal volumes of salt solutions to form a precipitate os insoluble salt
  • Filter
  • Wash with cold, distilled water
  • Transfer to oven
  • Heat until consatnt mass
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Distillation

Separates mixture of miscible liquids with different boiling points

Lowest boiling point will evaporate first

  • Set up apparatus
  • Heat until a gentle boil
  • When the vapour temperature is two degrees below bp of liquid we want to collect, put a collecting beaker in place
  • Collect distilled liquid until temperature of the vapour rises above the boiling point of the liquid we are collecting, stop heating

(http://nptel.ac.in/courses/102103016/module4/lec34/images/1.png)

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Recrystallisation

Purifies solid crude organic products with small amounts of impurities

Hot solvent must be chosen so that only the deisred compound dissolves in it at high temps. The smaller amount used, the better the yield

  • Filter to remove insoluble impurities 
  • Allow crystals to form
  • Collect crystals by vacuum filtration
  • Dry
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Vacuum Filtration

  • Connect conical flask to vacuum pump, don't switch on yet
  • Dampen filter paper and place in Buchner funnel
  • Switch on pumpand pour mixture into it
  • [Pump creates a partial vacuum to filtrate is pulled through quicker]
  • Disconnect flask from pump before turning pump off to avoid '**** back'
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Determining the Melting Point

By finding out melting point, you can identify the product and its purity

  • Seal the end of a glass melting point tube by melting end in the flame
  • Tap open end of tube into solid so that a small amount of solid goes up tube
  • Tap tube so solid falls to sealed end
  • Fix melting point tube to the apparatus and heat surrounding liquid gently, stirring to ensure even heating throughout, temp should rise slowly
  • Note the temperature when solid starts and finishes melting- the differnece between highest and lowest is the melting point range
  • Wider the range, more impure the substance
  • A pure compound will melt between 0.5 degrees of published value
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Making a Standard Solution From A Solid

Impurities will lower the melting point, to make more accurate, solution should be purified by recrystallisation

  • Weigh mass of solute and weighing boat accurately
  • Pour 100cm3 of deionised water into a 250cm3 beaker- carefully add weighed solute 
  • Re-weigh weighing boat. The difference between the starta dn after transfered is the weight of the solute 
  • Stir to ensure complete dissolving
  • Transfer to a clean 250cm3 volumetric flask
  • Rinse beaker and stirring rod with deionised water to ensure all washings enter the flask
  • Add deionised water to the solution, swirling to mix, until level is near the mark on neck of volumetric flask
  • Add with a dropping pipette to make sure meniscus is level with mark when looking at eye level
  • Insert stopper and invert 
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Making a Standard Solution By Dilution

Solution to be diluted is called stock solution

This example is diluting by a factor of 10

  • Rinse beaker withs tock solution then half fill it
  • Use a pipette filler to rinse a 25cm3 pipette with stock solution
  • Fill burette with bottom of meniscus on 25cm3 mark
  • Run solution unti a 250cm3 volumetric flask
  • Add deionised water to solution, swirling, until lelvel is near mark 
  • Use a dropping pipette to add deionised water so meniscus is level with 250cm3 mark at eye level
  • Insert stopper and invert
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Iodine-Thiosulfate Titration 1

Used to find the concentration of a chemical that's a strong enough oxidising agent to oxidise iodide ions to iodine. 

The iodine is then titrated with thiosulphate with starch as an indicator

STEP JUAN (e.g. sodium II chlorate):

  • Rinse a 25cm3 volumetric pipette with water, and then the sodium solution (so pipette is clean but not diluted)
  • Use the volumetric pipette with a filler to transfer an aliquot to a conical flask (meniscus, dry outside with a paper towel and after emptying tap tap of pipette into conical flask. This ensures all of the solution is transferred)
  • Add excess iodide ions by using a measuing cylinder to transfer potassium iodide to flask (not preicse because the amount of iodine liberated is determined by the amount of chlorate used)
  • Add excess hydrogen ions by using a measuring cylinder to transfer sulfuric acid
  • WILL BE BROWN DUE TO IODIDE
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Iodine-Thiosulfate Titration 2

STEP DOS:

  • Wash burette with water and then sodium thiosulfate solution
  • Fill burette with sodium thiosulfate, making sure the jet is full to aid accuracy 
  • Put conical flask on white tile
  • Record initial burette reading then do rough titration
  • Pale straw colour near end point, now add starch
  • Will turn blue-black (starch makes it easier to see when end-point is reached as it will go colourless)
  • Calculate rough titre
  • Accurate run
  • Add dropwise within 1cm3 of rough titre (so you don't overshoot endpoint)
  • Wash conical flask and end of burette regularly with distilled water to ensure all reactants are in conical flask
  • Continue untik concordant results
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Practical Skills Terms

Accuracy is a measure of closeness of agreement between an individual test result and accepted reference value

Precision is a measure of closeness of agreement between independent measurements obtained under the same conditions 

Error is the difference between an individual measurement and the true value of a quantity measured

Uncertainty is a an estimate attached to a measurement that characterises the range of values within which the true value is asserted to lie

Reliability is low if uncertainty is high

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