Chemistry Practicals OCR B

Weighing a Solid

Use of balance accurate to 2dp

  • Zero the balance
  • Place a weighing bottle on the balance and accurately weigh the mass of solid required
  • Record the mass of both the weighing bottle and the solid
  • Empty the solid into glassware for use
  • Reweigh the empty weighing bottle
  • Subtract the mass of both from the mass of the weighing bottle after use to find the accurate mass of solid used
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Volumetric Pipette

To measure a fixed volume of liquid

  • Ensure the pipette is clean by rinsing with water and then a small volume of the solution being pipetted
  • Dip the pipette into the solution and using a pipette filler draw enough liquid until the bottom on the meniscus is level with the line on the neck of the pipette at eye level
  • Run the liquid out  of the pipette until it stops into glassware for use
  • There will still be a drop in the pipette (this should be left)
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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 and pour the solution into the burette using a funnel. Fill the burette above the zero line
  • Use a clamp to hold the burette in place and allow some of solution to run through to fill the jet of the burette
  • Record the burette reading to the nearest 0.05cm^3
  • Carry out titration
  • Record the reading on the burette to the nearest 0.05cm^3. Subtract the original reading from the reading at end. This is the titre
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Measuring Volumes of Gases

Can be measured using a gas syringe or an inverted burette/measuring cyclinder

This apparatus needs to be gas tight

The volume of gas collected is the initial volume minus the final volume

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Heating Under Reflux

Ensures reactant/products dont escape whilst the reaction is in progess

  • Put the reactants into a pear shaped flask and add anti-bumping granules (reduce chance of boiling over)
  • Do not stopper the flask (or pressure build up will occur)
  • Attach a condenser vertically to the flask so water flows into condenser at bottom and out at top
  • Heat so that the reaction mixture boils gently using a bunsen burner.

Any vapours should condense back into liquid

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Purifying an Organic Liquid

  • When the prduct is mixed with another immiscible liquid the two layers can be seperated using a seperating funnel. The layers seperate, the denser liquid forming the lower layer .
  • Allow layers to settle and then run off and dispose of aqueous layer and run off the organic product layer into a clean conical flask
  • If acidic impurities are present, add sodium hydrogen carbonate solution and shake well
  • If crude product is alkaline and needs neutralising then add dilute acid until reaction is neutral
  • Dry the crude product by adding anhydrous sodium sulphate and swirling the mixture
  • The pure product can then be seperated by distillation
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Making Water Soluble Inorganic Salts

Reacting an acid and a soluble base:

  • Carry out acid-base titration to find out how much acid needed to neutralise volume of alkali
  • Transfer volume of alkali into conical flask and add correct amount of acid (without indicator)
  • Transfer to clean evaporating basin and heat gently to evaporate water 
  • Once crystals appear stop heating and leave to cool. Filter micture and wash solid residue with distilled water
  • Transfer to watchglass and heat in oven to dry
  • At regular intervals remove watchglass and weigh to constant mass. Leave to cool in desiccator

Reacting an acid and an insoluble base:

  • In a beaker, warm excess insoluble base in dilute acid. Continue to warm until solution is neutral (universal indicator), adding more solid base if needed
  • Leave to cool. Filter off the exess base and transfer the filtrate to a clean dry evaporating basin
  • Heat the evaporating basin until salt crystals appear on sides of basin
  • Then crystalise (as before)
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Making Water Insoluble Inorganic Salts

  • Add equal volume of desired salt solutions in a beaker for forn a precipiate of insoluble salt.
  • Filter the ppt. and wash several times with cold distilled water
  • Transfer the filtered washed ppt to a clean watchglass
  • Place in drying oven
  • Remove at regular intervals and weigh to constant mass
  • Cool in desiccator
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Distillation

Seperates mixtures of misciple liquids with different boiling points. By heating each pure compnent is vapourised, condensed and collected. Will evaporate in order of boiling points

Apparatus includes: thermometer to meaure temp of vapour, pear shaped flask, condenser (water in at bottom right and out at top left), anti-bumping granules and collecting beaker.

When vapour temp is 2 degrees below boiling pint put collecting beaker in place and collect until temp rises above boiling point. Stop heating.

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Paper Chromatography

Used to seperate small quantities, purify or follow progress of reaction of organic substances. Different organic compounds have different affinities for solvents. Paper uses paper and TLC uses a silica plate.

  • Spot the micture on pencil line 1cm from base
  • Suspend in sovent and cover to prevent solvent evaporating
  • Remove plate when solvent front near top and mark with pencil line
  • Allow to dry
  • Locate spot using iodine, ninhydrin or UV light
  • Match height or Rf values with known compound
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Recrystallisation

Used to purify solid organic products with impurities

  • Select a suitable hot solvent that the compound dissolves in when hot but recystalises when cool. Dissolve mixture in minimum quanitity of hot solvent
  • Filter insoluble impurities while hot
  • Leave filtrate to cool until cystals from
  • Collect crystals by vacuum filtration (filters soluble impurities)
  • Dry crystals in oven until constant mass
  • Allow crystals to form
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Vacuum Filtration

Used to seperate a solid from filtrate quickly

  • Connect a conical flask to a vacuum pump via the side arm. Do not switch pump on yet
  • Dampen a piece of filter paper and place it fla in te Buchner funnel
  • Switch the vacuum pump on and then carefully pour in the mixture to be filtered. The pump 'pulls' the filtrate through quickly
  • Disconnect the flask from the vacuum pump before turning pump off to avoid 'suckback'
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Determining Melting Points

Melting point can be used to test purity and identify an organic solid.

  • Seal the end of a glass melting point tube by heating it to melting with a bunsen burner
  • Tap the open end of tube into the solid so a small amount goes into the tube. Tap the tube so that the solid falls to the bottom of the sealed end.
  • Fix the tube in the melting point apparatus and heat surrounding liquid gently, stirring ensures even heating (temp should rise slowly)
  • Note the temp the solid starts and finishes melting. The difference between the highest and lowest temps is the melting range
  • Compare experimental values to published value. The wider the melting range the more impure (Pure compounds melt within 0.5 degrees of melting point)

Can also use a melting point determining machine

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Making a Standard Solution from Solid

  • Calcualte mass of solute required and weigh using a weighing bottle (measure mass of solid as before)
  • Pour destilled water into beaker and add solid
  • Stir to ensure fully dissolved and transfer into volumetric flask
  • Rinse beaker and glass rod with distilled water and add to flask
  • Add distilled water (swirling regularly) until level below mark on neck and then use dropper pipette until bottom of meniscus is level with mark at eye level
  • Insert stopper and invert to ensure mixing
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Making a Standard Solution by Dilution

The stock solution is diluted to form a diluted solution

  • Rinse a clean dry beaker with stock solution and then half fill it
  • Use pipette filler to rinse pipette with stock solution
  • Fill pipette with stock solution until bottom of meniscus is level with mark on neck at eye level and run into a volumetric flask
  • Add distilled water as before

Calculation:

conc diluted solution x volume diluted solution = conc stock solution x volume stock solution

(Number of moles are same for both)

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Acid-Base Titration

  • Rinse burette with acid solution and fill with acid. Run some through to fill the jet of the burette
  • Record intital reading on burette to nearest 0.05cm^3
  • Fill clean pipette with alkaline solution and run into conical flask. Add indicator and swirl
  • Run acid into flask and swirl continually until colour changes and record reading = rough titre
  • Refill burette and repeat steps 2 and 3
  • Run acid until 1cm before rough titre. Then add drop b drop until indicator changes
  • Repeat until 3 concordant results (within 0.1cm^3)
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Iodine-Thiosulphate Titration

  • Strong oxidising agent oxidises iodide ions to iodine when you add it to excess iodide ions and excess hydrogen ions (by adding sulphuric acid). The contents of conical flask will be brown.

Using a volumetric pipette follow as before. Use a measuring cylinder for those in excess.

  • You can titrate iodine produced with sodium thiosulphate soultion to work out the moles of iodine produced

Follow titration method as before (including completing a rough titre first)

  • Use a burette to add thiosulphate ions to solution
  • Near the end point when the solution reaches a pale straw colour add a few drops of starch solution. This will turn it black and continue adding thiosulphate until it turns colourless
  • Work out titre and repeat until there are 3 concordant results
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Redox Titration

Similar to acid-base titration but instead redox reaction is occuring.

Often no need for an indicator since one of the reactants or products is coloured e.g manganate (VII) ions in conical flask are reduced= purple to colourless OR if added to solution the end point is when first pale pink colour appears

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Colourimetry

The amount of light absorbed or transmitted can be measured. Absorbance is proportional to conc.

  • Select a filter with complementary colour to solution tested (Allows only those absorbed most strongly to pass through solution)
  • Make up a range of standard solutions above and below the conc of unknown
  • Zero colorimeter using a curvette of water
  • Measure absorbance of each conc and plot a calibration curve
  • Measure the absorbance of unknown and use curve to determine the conc

A visible spectrophotometer can give data for absorption and transmission for any given vakue of visible spectrum. A colorimeter is more limited by the filter used.

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Measuring Energy Transferred when Fuel Burns

Used to measure enthalpy change of combustion when a fuel is burnt

  • Using a measuring cylinder pour a known volume of water into a copper calorimeter and record initial temp
  • Weigh a spirit burner (with cap on to reduce loss of fuel by evaporation)
  • Support calorimeter over spirit burner containing fuel to be tested and surround with draught excluder to reduce energy loss
  • Remove the cap of spirit burner and light the wick
  • Use a thermometer to continually stir water and heat until temp risen by 15 or 20 degrees
  • Extinguish spirit burner and put cap back on. Keep stirring water and note the highest temp
  • Reweigh burner

The results can be used to calculate enthalpy change of combustion

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Measuring Energy Transfer of Reactions in Solution

This allows enthalpy change of reaction to be measured. E.g enthalpy change of neutralisation

  • Using a measuring cylinder add a known volume of a known conc of acid to a polystyrene cup and measure temp
  • Using measuring cylinder add a known volume of a known conc of alkali. Stir well to mix reactants
  • Place a lid with a hole in over cup and place thermometer through the hole
  • Record changes in temp every 30 secs until no further changes
  • Calculate maximum increase in temp

Could also use a known mass of solid reactant (should be in excess).

In order to calculate the max change in temp you will need to plot a graph of temp against time. Once a line of best fit it drawn you can estimate the temp rise immediately after mixing reagents.

The values recorded experimentally are too low because the reaction doesnt occur instantaneously but over a period of time. Also heat is lost to surroundings. The theoretical value must be found by extrapolation of the line a best fit back to when reactants were initially mixed.

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Electrolysis

During electrolysis of aqueous solution, an electrical current is passed through the electrolyte (an electrical circuit must be set up). The dc power supply is from batteries. The most common materials for electrodes is graphite. This conducts electricity, is cheap and relatively inert.

If products of electrolysis are gaseous they can be collected using inverted test tubes filled with water.

If electrolysis is carried out for purification of a metal, the anode will be made of the impure metal and the cathode should be made of the pure metal.

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Electrochemical Cells

Used to determine the potential of an electrochemical cell. Standard electrode potentials can be measured by connecting any half-cell to a standard hydrogen half-cell.

  • Construct the half-cell whose elctrode potential is to be measured

For metal ion/metal half cell the electrode will be the metal

For metal ion/metal ion half cell the electrode will be platinum

  • Connect the half-cell to a standard hydrogen half cell using a high resistance voltmeter and salt bridge
  • Connect the two electrodes to a high resistance voltmeter and check the reading is positive (if negative change the connections around)
  • Record the voltmeter reading 

Conditions- 298K, all solutions are 1moldm-3, 1 atm

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Measuring pH

Use a pH probe to take pH of solution. To ensure accuracy the electrode must be calibrated:

  • Wash the electrode with distilled water and transfer into buffer solution of pH 7.00. Check the bulb is fully immersed and wait for reading to stabilise . Ensure value reads 7.00.
  • To measure pH of acidic solutions, calibrate using an acidic buffer (usually pH 4.00)
  • To measure pH of alkaline solutions, calibrate using an alkaline buffer (usually pH 10.00)
  • To measure both acidic and alkaline solutions with a wide range of values, calibrate with acidic, alkaline and pH 7.00 buffer solutions

Once calibrated you can measure pH of a solution by rinsing the elctrode the immersing in solution to be measured.

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Cracking a Hydrocarbon Vapour

Used to break longer molecules down into smaller molecules

  • Set up apparatus and ensure space above Al2O3 catalyst to allow gases to pass freely over it
  • Place several test tubes in the water in the collection trough
  • Heat the catalyst strongly so temp is high enough for cracking to occur
  • Heat the alkane gently, collecting any gases that pass into collection tubes, changing and corking full tubes. Continue to heat while changing collection tubes to prevent suckback
  • Discard the first tube of gas as this will be displaced air rather than product
  • Continue heating the catalyst and alkane until you have collected several tubes of gas or until no more gas is produced
  • Remove the delivery tube from the collection through before stopping heating
  • Leave to cool and dismantle the apparatus
  • Test any liquid product from middle collection tube with bromine water (should remain yellow/brown)
  • Test the gas collected by shaking with bromine water (should decolourise)
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Testing for Unsaturation in Alkenes

The addition of bromine water to an alkene is a test for presence of C to C double bond

  • Add a few drops of bromine water to a few drops of unknown liquid or gas and shake well
  • If it is decolourised from brown to colourless then there is a C-C double bond
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Determining Ksp

The Ksp of any sparingly soluble salt is determined from a saturated solution of that salt.

Prepare a saturated solution in destilled water by:

  • Warm distilled water in a small conical flask and add the salt shaking frequently
  • Keep adding solute until no more dissolves (there is an undissolved solid)
  • Leave the mixture to cool to room temp
  • There should still be solid at bottom (if not repeat process)
  • Filter the mixture through filter paper and discard residue

Determining conc of ions in solution: (Note the temp as Ksp is temp dependent)

The conc of one ion is proportional to the conc of the other ion in solution

Method of determination for conc of an ion iclude:

  • Basic solutions (e.g. Ca(OH)2) the OH- ions can be titrated with HCl
  • Coloured solutions the coloured ion (e.g. Cu2+ or MnO4-) can be determined by colorimetry
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Determining Kc

  • Allow a mixture of reactants to reach equilibrium at a known temp
  • Determine the conc of one of the components (the other conc can be calculated from this value and the equation if initial concs are known). The method depends on substances and can include: redox titrations, acid-base titrations, colorimetry or pH measurements
  • Use a balance equation to write Kc and insert data calculated
  • Calculate Kc (inc. units) and note the temp
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Uncertainties

All measuring equipment used has an uncertainty associated with them

As general rule, the uncertainty is usually half a division on either side of the smallest unit on the scale you are using. You are normally given the absolute uncertainty or the smallest unit. 

Calculating % uncertainty:
% uncertainty= (uncertainty/ measured quantity) x 100

When multiple measurements

% uncertainty= (uncertainty x no of measurements / measured quantity) x 100

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