OCR Salters B Chemistry- Paper 3 Practical Skills

Weighing a solid

1. Zero the balance

2. Place weighing bottle/boat onto balance and add in the approximate required mass of solid

3. Accurately weigh solid and weighing bottle and record

4. Empty solid into glassware where it will be used

5. Accurately reweigh empty weighing bottle

6. Subtract recorded mass for empty weighing bottle from mass recorded for solid and weighing bottle

Measuring a volume of a liquid using a pipette

Pipettes used for accurately dispensing a fixed volume of a liquid, usually 10 or 25cm³

1. Ensure pipette is completely clean- rinse with water and then a small volume of the solution that will be pipetted

2. Dip pipette into solution and, using a pipette filler, draw enough liquid into the pipette until it is exactly the right level (read using bottom of meniscus at eye level)

3. Run liquid out of pipette into glassware until it stops. Touch the pipette on the side and remove (will be drop left in)

Measuring a volume of a liquid using a burette

1. Clean burette by rinsing out with water and then small volume of solution to be used

2. Make sure burette tap is closed. Pour solution into burette using small funnel (fill above zero line)

3. Use clamp to hold burette in place and allow some solution to run into beaker until there are no air bubbles (record reading to the nearest 0.05 cm³

4. Carry out the titration to the end point

5. Record the reading on the burette to the nearest 0.05 cm³. Subtract the reading taken at the beginning from this to find the titre.

Measuring volumes of gases

The volume of gas produced in a reaction can be measured using either a gas syringe or an inverted burette/measuring cylinder.

In order for as much as possible to be collected, the system must be 'gas tight.'

The volume of gas collected in an inverted burette is the initial volume minus the final volume

Synthesis- reflux

Heating under reflux is used for reactions involving volatile liquids- ensures reactants and/or products do not escape while the reaction is in progress

It also provides a safe way of heating flammable liquids such as alcohols.

1. Put reactants into pear-shaped or round-bottomed flask and add a few anti-bumping granules (used to burst bubbles in boiling mixture and reduce chance of boiling over)

2. Do not stopper the flask as this would cause pressure to build up and glassware could crack or stopper could fly out (could cause accidents)

3. Attach condenser vertically to the flask so water flows into condenser at the bottom and out of the condenser at the top (ensures condenser always full of cold water)

4.Heat so reaction mixture boils gently, using Bunsen burner or heating mantle

5. When refluxing correctly, any vapours should reach no more than half way up the condenser before condensing back into liquid. The liquid should drip back into the reaction flask steadily

Purifying an organic liquid product

Once organic liquid products have been synthesised they must then be purified to remove any solvents or impurities present

1. When the organic product is mixed with another immiscible liquid (e.g. an aqueous liquid) the two layers can be separated using a separating funnel- the layers separate, with the denser liquid forming the lower layer

2. Allow the layers to settle and then run off and dispose of the aqueous layer. Run the organic layer into a clean conical flask

3. If acidic impurities are present, add sodium hydrogen carbonate solution and shake well to remove them. If the crude product is alkaline and needs neutralising, then add a dilute acid until the mixture is neutral.

4. Dry the crude product by adding anhydrous sodium sulphate and swirling the mixture. It is possible to use other anhydrous salts such as calcium chloride.

5. The pure product can then be separated by distillation

How can you make sure you run off the correct layer?

Add more water into the separating funnel and leave the layers to settle again- the layer that is bigger will be the aqueous layer

How can water-soluble inorganic salts be made?

Either by reacting an acid and a soluble base or reacting an acid with an insoluble base

Reacting an acid with a soluble base

1. Carry out an acid-base titration to find out how much acid is needed to neutralised 25 cm³ of the alkaline solution

2. Transfer 25 cm³ of the alkaline solution to a clean conical flask

3. Using a burette, add the correct amount of acid to neutralise the alkali (do not add any indicator)

4. Transfer the neutralised solution to a clean evaporating basin and heat over a Bunsen flame to evaporate the water- make sure not to heat too strongly to avoid spitting

5. Stop heating once crystals of solid start to appear

6. Leave mixture to cool in evaporating basin

7. Filter mixture

8. Wash solid residue with cold distilled water

9. Transfer residue to watch glass and heat in oven to dry solid- make sure set at temperature below melting point of salt

10. At regular intervals, remove from oven, cool in desiccator, and weigh

11. Once solid has dried to a constant mass, stop heating and leave to cool in a desiccator

Reacting an acid and an insoluble base

1. In a beaker, warm excess insoluble base in dilute acid

2. Continue to warm (but not boil) until solution is neutral (use universal indicator paper for this step), adding more solid base if needed

3. Leave to cool

4. Filter off excess base and transfer filtrate to clean, dry evaporating basin

5. Heat evaporating basin until crystals begin to appear

6. Cool the basin and its contents

7. Filter mixture and discard the filtrate

8. Wash the solid with cold distilled water

9. Transfer residue to 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.

10. Heat to constant mass

Making water-insoluble inorganic salts

Insoluble salts can be prepared by the reaction of two soluble salts in solution.

1. Add equal volumes of desired salt solutions in a beaker to form a precipitate of the insoluble salt

2. Filter the precipitate

3. Wash precipitate several times with cold distilled water

4. Transfer filtered, washed precipitate into a clean watch glass and place in drying oven (temperature set below melting point of salt)

5. Heat to constant mass

What is distillation used for?

Used to separate a mixture of miscible liquids with unique boiling points.

By heating mixture, each pure component is vaporised, condensed, and collected.

Components will evaporate in order of boiling point- one with lowest bp will evaporate first

Quickfit glassware commonly used- ground glass joints that can be sealed using grease to prevent loss of reagents

Purification using distillation

1. Put mixture into pear-shaped flask and add a few anti-bumping granules.

2. Set up distillation apparatus (thermometer at top of flask, water in at bottom and water out at top of condenser, beaker at bottom of tube to collect products)

3. Heat mixture until it boils gently, using a Bunsen flame or a heating mantle (heating mantles easier when using flammable liquids)

4. When vapour temperature is approx 2°C below bp of liquid that is about to be collected, put the collecting beaker in place

5. Collect the distilled liquid until the temperature of the vapour rises above the bp. Stop heating.

6. If another compound is required of a higher bp, repeat step 5 using a clean collecting beaker.

What is chromatography used for?

Used to separate small quantities of organic compounds, purify organic substances, and follow reaction progress over time.

How does chromatography work?

Different organic compounds have different affinities for a particular solvent, and so will be carried through the medium at different rates.

What is the difference between paper chromatography and TLC?

They have different stationary phases- the former uses paper whereas the latter uses a silica plate.

Method for chromatography

1. Spot the test mixture and reference samples on a pencil line 1 cm from the base of the paper/plate. Pencil is used as it will not run into the solvent.

2. Suspend the plate in a beaker containing the solvent (below baseline) and cover the beaker with a watch glass to prevent the solvent evaporating.

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

4. Locate any spots using iodine, ninhydrin, or under a UV lamp

5. Match Rf values with those of known compounds using the same chromatography solvent

How is Rf calculated?

Rf= distance travelled by spot/ distance travelled by solvent

Why is recrystallisation used?

Used to purify solid crude organic products with small amounts of impurities

What properties does a recrystallisation solvent need?

Dissolves the product when hot but not when cold.

Recrystallisation method

1. Select appropriate solvent

2. Dissolve mixture in minimum amount of hot solvent- smaller amount gives better yield

3. Filter to remove any insoluble impurities and retain filtrate- best to preheat filter funnel and conical flask to prevent recrystallisation during this stage

4. Leave filtrate to cool until crystals forms

5. Collect crystals by vacuum filtration

6. Dry crystals in oven or by leaving them in the open covered by an inverted filter funnel

Vacuum filtration

Used to separate a solid from a filtrate rapidly

1. Connect conical flask to vacuum pump via the side arm (do not switch on pump yet)

2. Dampen piece of filter paper and place flat in Buchner funnel

3. Switch vacuum pump on and then carefully pour in mixture to be filtered- pump creates partial vacuum so filtrate gets pulled through quickly

4. Disconnect flask from pump before turning pump off to avoid 'suck back'

Melting point analysis

Used to determine melting point of organic solids- can be used as evidence of identity/purity

1. Seal end of glass melting point tube by heating to melting in a Bunsen flame

2. Tap open end of tube into solid so small amount goes into tube- tap tube so solid falls to bottom of sealed end

3. Fix tube in apparatus and heat surrounding liquid gently, stirring to ensure even heating throughout (temp will rise very slowly)

4. Note temperature at which solid starts and finishes melting (difference between two is the melting range)

5. Compare experimental value to published value for melting point. Wider melting range means more impure. A pure compound will melt within 0.5°C of the true melting point.

What is a standard solution?

A solution of accurately known concentration

What can standard solutions be used for?

Used to determine concentration of another unknown solution.

Can also be used to find purity of a solid.

Making a standard solution from a solid

1. Calculate mass of solute required.

2. In a weighing bottle, weigh out amount accurately (use 2dp electric balance.) Make a note of mass to the nearest 0.01g.

3. Pour 100 cm³ of distilled water into a 250 cm³ beaker. Carefully transfer the weighed solute into the water from the weighing bottle.

4. Reweigh the weighing bottle- the difference between the mass of the bottle and solute and the mass of just the bottle will give the mass of solute transferred.

5. Stir mixture in beaker to ensure complete dissolving of solute.

6. Transfer solution to clean 250 cm³ volumetric flask. Rinse beaker and stirring rod well with distilled water, making sure all washings go into volumetric flask.

7. Add distilled water, swirling at intervals to mix the contents until the level is within about 1 cm of the 250 cm³ mark on the neck of the flask.

8. Using a dropping pipette, add distilled water so the bottom of the meniscus is level with the mark on the neck at eye level.

9. Insert stopper in flask and invert it, shaking thoroughly to ensure complete mixing

Making a standard solution by dilution

If the concentration of an existing solution is too high, it can be lowered using dilution. This existing solution is known as the stock solution. For example, concentration may be diluted by a factor of 10 to produce 250 cm³ of a 0.1M from a standard solution of 1M.

1. Rinse clean dry beaker with stock solution and then half fill it.

2. Use pipette filler to rinse clean 25 cm³ pipette with some of stock solution.

3. Fill pipette to 25 cm³ mark- with bottom of meniscus level with mark at eye level

4. Run solution into 250 cm³ volumetric flask

5. Add distilled water to solution, swirling at intervals to mix contents until level within about 1 cm of mark on neck of flask.

6. Use dropping pipette to add distilled water so bottom of meniscus is level with mark on neck at eye level.

7. Insert stopper and invert several times to thoroughly mix

How to calculate volumes needed

C₁V₁ = C₂V₂

Acid-base titration

Used to determine concentration of acid/alkali accurately.

1. Rinse burette with some of acid solution then fill with acid. Run a little through burette into waste beaker to fill tip. Record initial reading to nearest 0.05 cm³.

2. Fill clean 25.0 cm³ pipette with some of alkaline solution

3. Run alkaline solution into clean 250 cm³ conical flask

4. Add 2 or 3 drops of suitable indicator and swirl to mix.

5. Run acid from burette into flask. Swirl flask continually and watch for first hint of colour change (can use white tile to see easier.) First titration used as trial run to give rough indication of how much acid required. Record titre.

6. Refill and record initial reading. Repeat steps 2-5 but run acid in to 1 cm³ before rough titre then add drowse until colour change.

7. Repeat until there are three concordant results (within 0.10 cm³ of each other) and use these to find the average titre

How do iodine-thiosulfate titrations work?

Involve redox reactions.

Used to find concentration of oxidising agent that is strong enough to oxidise iodide ions too iodine.

This liberated iodine is titrated with thiosulfate ions, with starch as an indicator.

Once the amount of thiosulfate ions are calculated, the amount of chemical being analysed can be calculated.

Step 1 of iodine-thiosulfate titrations (in this case using chlorate(I) ions)

1. Pour some chlorate(I) solution into clean dry beaker.

2. Rinse 25 cm³ volumetric pipette with water and then the chlorate(I) solution- ensures pipette is clean and solution is not diluted.

3. Transfer carefully measured 25 cm³ aliquot of the chlorate(I) solution to a conical flask using a volumetric pipette and filler- equipment ensures low uncertainty. Make sure bottom of meniscus level with mark at eye level.

4. Before emptying solution into flask, make sure to dry outside with paper towel and after emptying, touch the tip of the pipette to the inside of the flask- ensures 25 cm³ are measured as accurately as possible.

5. Add excess iodide ions using a measuring cylinder to transfer 15 cm³ of 0.5M potassium iodide to the flask- since the amount of iodine liberated is determined by the amount of chlorate(I) used, the volume of KI does not need to be precise.

6. Add excess hydrogen ions using a measuring cylinder to transfer 20 cm³ of 1M sulfuric acid.

7. The contents of the flask will be brown due to the iodine produced.

Step 2 of iodine-thiosulfate titrations

1. Wash burette with water and standard solution of 0.100M sodium thiosulfate- ensures burette is clean and solution not diluted.

2. Fill burette with thiosulfate solution, making sure jet is full- titre will be inaccurate otherwise

3. Put conical flask on white tile to make end point easier to see.

4. Record initial burette reading to nearest 0.05 cm³- will be easier to read if a white tile is placed behind to see bottom of meniscus.

5. Start rough titration- gives rough idea of how much needed.

6. Near end point, a pale straw colour is seen. When this is seen, a few drops of starch solution are added, turning the contents blue-black- makes it easier to see colourless end point.

7. Record end reading and record rough titre from this.

8. Repeat titration until concordant results seen- be more accurate by adding solution dropwise so end point is not overshot.

Why is an indicator usually not needed for a redox titration?

Redox reactions are usually self-indicating as one of the reactants is usually coloured.

e.g. when manganate(VII) ions are reduced there is a colour change from purple to colourless. If the ions are being added to a colourless solution, the end point is seen when the first pale pink colour is seen.

What is colorimetry used for?

To determine the concentration of a coloured solution which absorb certain wavelengths of light. The amount of light absorbed/transmitted can be measured to give the concentration (proportional relationship.)

Can be used to follow the progress of a reaction involving a colour change.

Colorimetry method

1. Select filter with complementary colour to solution being tested, e.g. yellow filter for purple solution. This allows only those wavelengths absorbed most strongly to pass through the sample.

2. Make up range of standard solutions of test solution- should be solutions above and below concentration of unknown.

3. Zero colorimeter using cuvette of pure solvent (water in most cases)

4. Measure absorbance of each of the standard solutions and plot calibration curve to determine concentration of unknown.

5. Measure absorbance of the unknown sample and use the curve to determine the concentration against the absorbance.

Measuring the energy transferred when a fuel burns (calorimetry)

1. Using a measuring cylinder, pour known volume of water into copper calorimeter. Record its temperature.

2. Weigh a spirit burner- keep the cap on to reduce loss from evaporation.

3. Support calorimeter over spirit burner containing fuel to be tested- surround with draught excluder to reduce energy losses as heat to surroundings

4. Remove cap from spirit burner and light wick

5. Use thermometer to stir water the whole time it is being heated- carry on heating until temperature has risen by around 15-20°C

6. Extinguish spirit burner and put cap back in place. Keep stirring water and make note of highest temperature reached.

7. Weigh burner again.

How to process results of calorimetry experiment

Q= mc∆T

Then divide Q by 1000 to convert J to kJ

Use mass of fuel burned (mass at start vs mass at end of spirit burner) and Mr of fuel to work out moles of fuel burned.

Divide energy (Q) in kJ by the moles of fuel burned to give kJ/mol (∆cH)

Measuring energy transferred when reactions occur in solution

Allows calculation of ∆rH through measurement of changes in temperature when known quantities of reactants react together.

1. Using measuring cylinder, add known volume of known concentration of acid to insulated vessel and record temperature.

2. Using measuring cycling, add known volume of known concentration of alkali. Stir well to mix reactants.

3. Top vessel with lid with a hole.

4. Place thermometer through hole in lid and record changes in temperature every 30 seconds until there is no further temperature change/

5. Calculate the maximum increase in temperature.

Process results using same method as calorimetry but using moles of acid/alkali rather than fuel.

How is error minimised when measuring enthalpy change of neutralisation?

Insulated vessels minimise transfer of thermal energy to surroundings.

Measuring energy transferred when solids and solutions react

1. Use measuring cylinder to add known volume of known concentration of reactant solution to insulated vessel. Record temperature.

2. Add known mass of solid reactant (in excess)

3. Top vessel with lid with hole in.

4. Place thermometer through hole and record temperature changes every 30 seconds until no more change.

5. Plot graph of temperature against time to find maximum temperature change- once a line of best fit has been drawn, the temperature change immediately after the reactants were added can be estimated.

The theoretical maximum temperature change cannot be achieved experimentally as the reaction does not occur instantaneously and some heat is lost the surroundings. Therefore, extrapolation must be used.

Electrolysis of aqueous solutions

An electrical current is passed through the electrolyte. An electrical circuit must be set up with a power pack/battery and electrodes (usually graphite as inert and inexpensive)

Gaseous products may be collected using inverted test tubes- filled with water at start of reaction and gaseous products displace the water so they are collected in the tubes.

Electrolysis can be done to purify an impure metal but in this case the anode must be made of the impure metal and the cathode should be made of the pure metal.

How is a pH meter calibrated?

pH is temperature-dependent so must be calculated for a specific temperature.

1. Wash electrode with distilled water.

2. Transfer electrode into buffer solution of pH 7.00- check bulb of electrode is full immersed in the solution and wait for the reading to stabilise (ensure the meter reads 7.00)

3. To measure acidic solutions, calibrate using an acidic buffer solution with a pH of around 4.00. To measure alkaline, use an alkaline buffer solution of around pH 10.00.

4. To measure both acidic and alkaline solutions with a wide range of values, calibrate with both acidic and alkaline buffers, as well as a pH 7.00 buffer.

Cracking a hydrocarbon vapour over a heated catalyst

1. Set up apparatus (inverted test tube/cylinder in water, boiling tube clamped over bunsen flame containing porcelain chips and mineral wool soaked in liquid paraffin, tubing from boiling tube into water underneath inverted cylinder)

2. Ensure there is space above the catalyst (Al₂O₃) to allow gases to pass freely over it.

3. Place several test tubes in water in collection trough.

4. Heat catalyst strongly- ensures that when alkene vapour passes over it the temperature is high enough to allow cracking to take place

5. Heat alkane gently, collecting any gases that pass into collection tubes, changing and corking full tubes. Continue to heat whilst changing collection tubes to prevent 'suck-back'

6. Discard first tube of gas- will just contain displaced air rather than product

7. Continue heating catalyst and alkane mixture to be cracked until several tubes of gas collected or until no more gas produced.

8. Remove delivery tube from collection trough before stopping and heating catalyst and alkane mixture- prevents suck back

9. Leave to cool then dismantle apparatus.

10. Test any liquid products from middle collection tube with bromine water- should remain yellow/brown

11. Test gas collected in tubes by shaking with bromine water- should decolourise

Testing for unsaturation

1. Add a few drops of bromine water to a few drops of unknown liquid/gas and shake well

2. If unknown is an alkene, it will decolourise (colour change from brown to colourless.) If it is not unsaturated, there will be no colour change.

How to test the degree of unsaturation

1. Use equal volumes of compounds being tested

2. Add bromine water dropwise from a burette

3. Bromine water will decolourise

4. The compound that decolourised the greatest titre is more unsaturated.

Accuracy

Measure of closeness of agreement between an individual test result and the accepted reference value. Closer= more accurate.

Precision

Closeness of agreement between independent measurements obtained under the same conditions.

Depends only on distribution of random errors, i.e. the spread of measurements, and does not relate to the true value.

Error

The difference between an individual measurement and the true value of the quantity being measured.

Uncertainty

An estimate attached to a measurement which characterises the range of values within which the true value is asserted to lie.

Usually expressed as range such as 1 ±0.05

Reliability

Opposite of uncertainty- if uncertainty is greater, measurement is less reliable

How to measure uncertainty

Half a division on either side of the smallest unit on scale being used.

Depends on quality of apparatus.

% uncertainty = uncertainty/measured quantity x 100

If a measurement occurs twice, the uncertainty must be doubled, e.g. measuring titre involves using a start and end value.

Overall uncertainty= all uncertainties added together