Edexcel A level Chemistry Core Practicals

core practical 1 - measuring the molar volume of gas

METHOD [7]

METHOD:
1. known volume of ethanoic acid in boiling tube

2. place 0.05g of calcium carbonate in test tube

3. weigh contents

4. tip contents into ethanoic acid and seal with bung

5. measure volume of gas produced, collected in a measuring cylinder over a water bath

6. Reweigh the test tube

7. Repeat experiment increasing the mass of calcium carbonate by 0.05g each time.

do not exceed 0.4g of calcium carbonate

core practical 1 - measuring the molar volume of gas

REACTANTS USED FOR THIS EXPERIMENT [2]

ethanoic acid and calcium carbonate

core practical 1 - measuring the molar volume of gas

MAIN SOURCE OF ERROR OF THIS PROCEDURE AND ADJUSTMENT TO ERADICATE ERROR

main source of error: loss of gas during the time taken to replace the bung

adjustment: put ethanoic acid in test tube in a vessel so the contents can be tipped into an already sealed system.

core practical 1 - measuring the molar volume of gas

why should you not exceed 0.4g of calcium carbonate?

a mass above 0.4g will result in the volume of gas produced exceeding the capacity of the measuring cylinder

core practical 2 - preparing a standard solution from a solid acid and using it to find the concentration of solution of NaOH

METHOD

METHOD:

1. weigh empty test tube. scoop 2.5g of sulfamic acid into test tube and reweigh.

2. dissolve sulfamic acid in ~100cm3 of water in a beaker

3. Transfer solution and washings into volumetric flask and make solution up to mark with distilled water

4. prepare apparatus for titration. Burette contains acid, conical flask contains NaOH.

5. pour an aliquot of NaOH of unknown concentration into conical flask.

6. use methyl orange as indicator

7. Titrate contents until 2 concordant titres have been achieved

core practical 2 - preparing a standard solution from a solid acid and using it to find the concentration of solution of NaOH

INDICATOR USED

methy orange

core practical 2 - preparing a standard solution from a solid acid and using it to find the concentration of solution of NaOH

how to calculate percentage error of a 250cm3 volumetric flask with an accuracy of +- 0.6cm3

accuracy/volume

(0.6/250) x 100

= +-0.24%

core practical 2 - preparing a standard solution from a solid acid and using it to find the concentration of solution of NaOH

TYPE OF EXPERIMENT

titration

core practical 2 - preparing a standard solution from a solid acid and using it to find the concentration of solution of NaOH

Why must the pipette be rinsed with sodium hydroxide after washing it with water?

Water that is left in the pipette will dilute the NaOH solution, changing the number of moles used.

core practical 2 - preparing a standard solution from a solid acid and using it to find the concentration of solution of NaOH

Why is there no need to dry the conical flask after washing between trials?

water will not change the number of moles of NaOH in the conical flask because the volume has already been measured.

core practical 2 - preparing a standard solution from a solid acid and using it to find the concentration of solution of NaOH

name a different indicator that could be used for this experiment and the colour change

phenolphthalein

pink to colourless

core practical 3 - finding the concentration of a solution of HCl (aq)

METHOD

METHOD:

1. wash volumetric flask with distilled water

2. use pipette to transfer 25cm3 of HCl solution into volumetric flask. Make solution up to mark with distilled water

3. prepare apparatus for titration. The burette contains NaOH and flask contains dilute HCl

4. pour 25cm3 aliquot of dilute HCl into conical flask and use indicator, phenolphthalein

5. titrate contents. End point is indicated by the contents in the flask becoming pale pink. Colour must persist for 5 seconds or more.

core practical 2 - preparing a standard solution from a solid acid and using it to find the concentration of solution of NaOH

why is it better to have a larger titre volume?

a larger titre volume will produce a smaller percentage error

core practical 2 - preparing a standard solution from a solid acid and using it to find the concentration of solution of NaOH

when NaOH is stored, what does it react with and how will this change the concentration and volume?

NaOH reacts with carbon dioxide in air.

the concentration will decrease

the volume will not change

core practical 4 - investigating the rates of hydrolysis of halogenoalkanes

METHOD

METHOD:

1. set up a water bath at 50 degrees

2. fill three test tubes each with 5cm3 of ethanol and add four drops of each halogenoalkane

3. loosely bung the test tubes

4. pour 5cm3 silver nitrate solution into three test tubes and place in same water bath

5. when both solutions reach the same temperature as the water bath, add the silver nitrate to the halogenoalkane and replace bung. start the stop clock

6. measure time taken for a precipitate to appear and stop the stop clock

7. repeat for other halogenoalkanes and then for primary, secondary and tertiary halogenoalkanes

core practical 4 - investigating the rates of hydrolysis of halogenoalkanes

SOLUTIONS REQUIRED

ethanol
halogenoalkanes
silver nitrate

core practical 4 - investigating the rates of hydrolysis of halogenoalkanes

WHAT TYPE OF REACTION IS THIS (in terms of mechanism)

nucleophilic substitution

water is the nucleophile

core practical 4 - investigating the rates of hydrolysis of halogenoalkanes

if NaOH was used to hydrolyse the halogenoalkanes what must happen before adding in the silver nitrate?

any excess NaOH must be neutralised by HNO3 before adding the silver nitrate.

core practical 4 - investigating the rates of hydrolysis of halogenoalkanes

equation for reaction of 1-bromobutane with water

C4H9Br + H2O = C4H9OH + HBr

core practical 4 - investigating the rates of hydrolysis of halogenoalkanes

why is ethanol used?

ethanol acts as a common solvent

using ethanol ensures that the organic liquids can dissolve so it can react with water molecules

core practical 4 - investigating the rates of hydrolysis of halogenoalkanes

What makes water a nucleophile and why can't hydroxide ions be used?

water has a lone pair of electrons on its oxygen atom

hydroxide cannot be used as if they were a precipitate of silver hydroxide would form instantly

core practical 5: the oxidation of ethanol

METHOD

METHOD

1. add 20cm3 of acidified sodium dichromate solution to a pear shaped flask. cool the flask in ice water bath.

2. set the flask up for reflux, keeping it in the ice water bath

3. place anti-bumping granules into the pear shaped flask

4. pipette 1cm3 of ethanol slowly down the reflux condenser. Allow reaction to subside after each addition of ethanol before adding more

5. when all ethanol has been added, remove the water bath and allow to warm to room temperature

6. position flask in a hot water bath and maintain boil for 20 minutes

7. Distill product using apparatus to collect product.

core practical 12: preparation of a transition metal complex

what is the product

tetraaminecopper(II) sulfate-1-water

core practical 5: the oxidation of ethanol

EQUIPMENT

reflux condenser apparatus:
- single reflux flask connected to the pear shaped flask

- flask half submerged in ice water

- condensing jacket with water flowing from the bottom to the top

- Bunsen burner under tripod stand

- open top to allow ethanol in

distillation apparatus
- thermometer placed where the gas will condense

- water flowing from bottom to top in condensing jacket

- not sealed apparatus to release pressure

core practical 5: the oxidation of ethanol

why are anti bumping granules used?

allows even and smoother boiling

gives a surface area for gas bubbles to form

prevents nucleation

core practical 5: the oxidation of ethanol

PRODUCT FORMED

carboxylic acid

core practical 5: the oxidation of ethanol

5 ways to analyse the results to test for product

1. measure pH with universal indicator

2. add acidified potassium dichromate

3. add calcium carbonate powder

4. add a length of magnesium ribbon

5. add fehling's solution

core practical 6: chlorination of 2-methylpropan-2-ol with concentrated hydrochloric acid

METHOD

METHOD:

1. pour 10cm3 of 2-methylpropan-2-ol and 35cm3 of conc. HCl into a large conical flask. Swirl contents.

2. place bung on the mouth of flask. swirl, then remove bung to release pressure. continue this for 20 minutes until two layers form - the upper layer is the crude product

3. add 6g of powdered anhydrous calcium chloride to flask and swirl until dissolved.

4. transfer reaction mixture to a separating funnel. Run off and discard lower layer.

5. Add 20cm3 sodium hydrogen carbonate solution to separating funnel. swirl funnel and remove bung to release pressure caused by CO2, at frequent intervals. Run and discard lower aqueous layer.

6. repeat the washing with sodium hydrogen carbonate. Run off and discard lower layer, then run and obtain organic layer in conical flask.

7. add anhydrous sodium sulphate. Place bung and swirl until clear.

8. decant organic liquid into pear shaped flask and set up flask for *distillation.

9. collect fraction boiling between 50 and 52 degrees

core practical 6: chlorination of 2-methylpropan-2-ol with concentrated hydrochloric acid

EQUIPMENT

- separating funnel apparatus

- distillation apparatus

core practical 6: chlorination of 2-methylpropan-2-ol with concentrated hydrochloric acid

NECESSARY REACTANTS etc

- 2-methylpropan-2-ol

- concentrated hydrochloric acid

- anhydrous calcium chloride

- sodium hydrogen carbonate solution

- anhydrous sodium sulphate

core practical 6: chlorination of 2-methylpropan-2-ol with concentrated hydrochloric acid

why is anhydrous calcium chloride used?

to ensure any unreacted alcohol is in the bottom layer of the conical flask before separating.

core practical 6: chlorination of 2-methylpropan-2-ol with concentrated hydrochloric acid

what is removed from the reaction when the crude product is shaken with sodium hydrogencarbonate?

unreacted HCl is removed.

HCl + NaHCO3 = NaCl + CO2 + H2O

core practical 6: chlorination of 2-methylpropan-2-ol with concentrated hydrochloric acid

test formed on the distillate

add 5cm3 of ethanol and 1cm3 of aqueous NaOH to the distillate

warm mixture in water bath

add XS nitric acid followed by few drops of silver nitrate solution

core practical 7: The analysis of inorganic and organic unknowns:

METHOD

PART 1:
4 tests on each liquid:
- bromine water
- acidified potassium dichromate
- fehlings solution
- ethanol, aqueous NaOH, nitric acid and silver nitrate

PART 2:
4 tests on each inorganic solids
- flame test
dissolve solid in 10cm3 of distilled water then and separate into 3 portions
- add dilute nitric acid, silver nitrate and dilute ammonia
- add dilute nitric acid and barium chloride
- chlorine water

core practical 7: The analysis of inorganic and organic unknowns:

why is nitric acid added in the test for halide ions using silver nitrate

to react with any carbonate ions so that a precipitate of silver carbonate DOES NOT form.

core practical 8 - determining the enthalpy change of a reaction using Hess's Law

METHOD

METHOD:
1. place 3g of solid potassium carbonate into test tube and weigh contents.

2. use a burette to dispense 30cm3 of (2 mol dm-3 HCl into a polystyrene cup supported in a beaker

3. measure initial temperature of acid

4. add potassium carbonate to the acid and record the highest temperature reached.

5. reweigh empty test tube

repeat with 3.5g of potassium HYDROGENcarbonate and record the lowest temperature reached.

core practical 8 - determining the enthalpy change of a reaction using Hess's Law

why is it not possible to measure the enthalpy change for decomposition of potassium hydrogencarbonate directly?

heat energy must be supplied

therefore, the temperature change measured is not solely due to the decomposition

core practical 8 - determining the enthalpy change of a reaction using Hess's Law

why are the reactions conducted in a polystyrene cup rather than a glass cup?

polystyrene is a better insulator than glass

less heat energy is lost to/gained by surroundings

so the temperature changes are more accurate

core practical 9: find the Ka for a weak acid

METHOD

METHOD:

1. calibrate the pH probe

2. pipette 25cm3 of 0.1 mol dm-3 ethanoic acid into a conical flask

3. Fill a burette with sodium hydroxide solution

4. add two or three drops of phenolhthalein indicator to conical flask

5. titrate until mixture just turns pink

6. pipette a further 25.0cm3 of 0.1 mol dm-3 ethanoic solution into the conical flask and record this pH.

core practical 9: find the Ka for a weak acid

TYPE OF EXPERIMENT

TITRATION

core practical 9: find the Ka for a weak acid

sources of uncertainty in this experiment and adjustments

inaccuracy in burette readings - read from the bottom of meniscus

difficulty identifying the exact end point - use a white tile

core practical 9: find the Ka for a weak acid

type of indicator used

phenolphthalein

colourless to pink

core practical 9: find the Ka for a weak acid

why is a further 25cm3 of ethanoic acid solution added after the solution turns pink

when solution in conical flask turns pink, the acid is essentially half neutralised.

core practical 10: investigating electrochemical cells

METHOD

METHOD:

1. clean strips of zinc and copper using sandpaper

2. set up a zinc half cell with zinc sulfate solution and the strip of zinc

3. set up a copper half cell with copper sulfate solution and the strip of copper

4. make an electrical connection between the two beakers by joining them with a strip of filter paper that has been dipped into a saturated solution of potassium nitrate - SALT BRIDGE

5. Join metal strips by a voltmeter

6. Record electrode potential and repeat with different combinations of metal/metal ion half cells

core practical 10: investigating electrochemical cells

calculating E cell

E cell RH - E cell Lh

core practical 10: investigating electrochemical cells

why may electrode potential values for the cells be slightly different to theoretical values

the experiment not under standard conditions

core practical 10: investigating electrochemical cells

why is silver nitrate at 1.0 mol dm-3 not used?

- highly oxidising

- expensive

- dangerous at 1.0 mol dm-3

core practical 10: investigating electrochemical cells

magnesium may be used in a half cell. Describe a problem that might be observed with this system?

magnesium reacts slowly with water in the solution

... raising the concentration of magnesium ions

the equilibrium will move to oppose this change and form more magnesium atoms

core practical 11: redox titration

METHOD

METHOD:

1. crush iron tablets using pestle and mortar

2. Transfer crush tablets to a weighing boat and measure combined mass

3. empty crush tablets into a beaker and reweigh weighing boat

4. add 100cm3 1.5 mol dm-3 of sulfamic acid to beaker and stir to dissolve as much as possible

5. filter solution into volumetric flask. rinse beaker and add washings to funnel. Make up to the mark with distilled water

6. pipette 25cm3 of solution into conical flask.

7. titrate the iron (II) solution with potassium manganate (VII) solution until mixture has just turned pink for more than 5 seconds.

core practical 11: redox titration

procedural errors

- stirring may not be sufficient to ensure all iron dissolves - warm solution

- transfer of solution and filtering - ensure beaker and filter paper is rinsed

- solution may not be mixed - invert volumetric flask several times to ensure a uniform solution

- glassware measurements not read accurately - read marks from bottom of meniscus

- end point may not be clear - use white tile

core practical 12: preparation of a transition metal complex

METHOD

METHOD:

1. weigh between 1.4 - 1.6g of copper sulfate. Weigh a test tube, record mass, add copper sulfate and to test tube.

2. add 4cm3 of water to test tube

3. prepare a hot water bath and stand in water bath. stir until dissolved

4. remove test tube and add while stirring 2cm3 of concentrated ammonia solution to the copper sulfate solution

5. pipette 6cm3 of ethanol into a beaker. Pour test tube contents into beaker.

6. cool mixture in an ice bath

7. using a buchner funnel and flask, filter crystals. wash beaker with cold ethanol and add washings to the funnel. rinse crystals with cold ethanol

8. scrape crystals off filter paper onto fresh filter paper and dry

9. record mass of crystals

core practical 12: preparation of a transition metal complex

why may the percentage yield differ from theoretical yield?

losses may be due to reaction not going to completion

some product staying in solution

and not crystallising

gains could be due to crystals not being dry or not being pure

core practical 13a: following the rate of the iodine-propanone reaction by a titrimetric method

METHOD

METHOD:
1. mix 25cm3 of 1mol dm-3 propanone with 25cm3 of 1mol dm-3 sulfuric acid

2. start stop clock as soon as 50cm3 of 0.02mol dm-3 iodine solution is added

3. using a pipette, withdraw 10cm3 samples every 3 minutes of the mixture and transfer it to a conical flask

4. stop reaction by addition of solid hydrogencarbonate. Note time at which reaction stopped.

5. titrate the withdrawn sample with 0.01mol dm-3 sodium thiosulfate (VI) solution, using starch indicator

core practical 13a: following the rate of the iodine-propanone reaction by a titrimetric method

what is the purpose of the sulphuric acid?

sulfuric acts as a catalyst for the reaction between propane and iodine

core practical 13a: following the rate of the iodine-propanone reaction by a titrimetric method

SOLUTIONS USED

propanone

sulfuric acid

iodine solution

solid sodium hydrogen carbonate

sodium thiosulfate

core practical 13b: investigating a 'clock reaction'

METHOD

METHOD:

1. measure 10cm3 of potassium iodide solution into a small beaker

2. add 5cm3 of sodium thiosulfate to the potassium iodide

3. add 10 drops of starch to the mixture

4. measure 10cm3 of the sodium peroxodisulfate and put into mixture and start stop clock

5. stop stop clock when a blue colour appears and note time

repeat steps 1-5 using different volumes of sodium PEROXODISULFATE and POTASSIUM IODIDE solution

core practical 13b: investigating a 'clock reaction'

SOLUTIONS USED

- potassium iodide solution

- sodium thiosulfate

- starch

- sodium peroxodisulfate

core practical 13b: investigating a 'clock reaction'

what volume must stay constant and how is this achieved

the total volume, including the sodium thiosulfate solution, must add up to 25cm3

this can be achieved by adding the correct volume of distilled water

core practical 13b: investigating a 'clock reaction'

analysis of results

- calculate concentration of iodide ions in each 25cm3 solution

- use the times recorded to work out rate when 1/t is proportional to rate

- plot RATE AGAINST CONCENTRATION graph

- do the same for the peroxodisulphate ion

core practical 13b: investigating a 'clock reaction'

main sources of uncertainty and adjustments to minimise the uncertainties

- misjudging the appearance of the blue solution - use a white tile

- addition of starch increases total volume of mixture

- measurement uncertainties occur in measuring small volumes - use a graduated pipette or burette

core practical 14: finding the activation energy of a reaction

METHOD

METHOD:

1. pipette 10cm3 of phenol solution and 10cm3 of bromide/bromate solution in one boiling tube

2. add four drops of methyl red indicator to the mixture

3. pipette 5cm3 of sulfuric acid solution into another boiling tube

4. stand both boiling tubes in a water bath of 75 degrees

5. when contents meet temperature of water bath, mix contents by pouring rapidly from one tube to the next and then back into the initial boiling tube and start stop clock

6. leave boiling tube in water until methyl red indicator disappears

7. repeat entire experiment at 65, 55, 45, 35, 25 and 15 degrees.

core practical 14: finding the activation energy of a reaction

SOLUTIONS USED

phenol

bromide/bromate solution

methyl red indicator

sulfuric acid

core practical 14: finding the activation energy of a reaction

what is the function of methyl red indicator?

when all the phenol has reacted

the bromine continuously produced in first reaction

will then react with the indicator and bleach it.

core practical 15: making aspirin

method

1. reflux a mixture of 2-hydroxybenzoic acid, ethnic anhydride and conc. sulfuric acid - heat until solid dissolves

2. add ice or distilled water to break down unreacted ethanoic anhydride.

3. stand beaker in iced water until precipitate appears

4. filter using Buchner funnel

5. wash crystals with min. volume of iced water

6. recrystallise aspirin in minimum volume of 1:3 parts ethanol:water

7. filter and dry. measure melting temperature

core practical 15: making aspirin

why must a range of temperature be recorded

impurities cause solid to melt over a range of temperatures

the more narrow the range, the more pure the sample.

core practical 15: making aspirin

safety

ethanoic anhydride and concentrated sulfuric acid = corrosive

2-hydroxybenzoic acid = harmful

core practical 15: making aspirin

which functional group of the 2-hydroxybenzoic acid reacts with the ethanoic anhydride?

the hydroxyl group

core practical 15: making aspirin

what would you expect to be the main impurity in the sample

unreacted 2-hydroxybenzoic acid

core practical 15: making aspirin

reactants to product in structural formula

2-hydroxybenzoic acid + ethanoic anhydride = aspirin
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