Edexcel A Level Chemistry: Core Practicals

Question 1

CP 1: measure molar volume of a gas

Answer 1

1. place excess ethanoic acid (weak acid ∴ slow reaction => less gas is lost) in a boiling tube
2. place CaCO3 in a test tube => has been weighed by difference
3. tip CaCo3 into ethanoic acid and quickly replace the bung
4. collect CO2 gas in a gas string/inverted measuring cylinder filled with water at regular time intervals
5. once reaction is completed, measure vol of CO2 produced at regular time intervals
6. repeat each time ↑ mass of CaCO3 by 0.05g

CaCO3 + 2CH3COOH → Ca(CH3COOH)2 + CO2 + H2O

Errors:
- CO2 is slightly soluble in H2O ∴ exact vol ≠ measured
- bung may not be airtight
- loss of gas when replacing the bung
- if solid is lost when heating, more CO2 is given off ∴ molar mass will be less
- using a gas syringe also collects water + HCl produced not just your gas

An ​alternative method => use a tube containing acid inside the vessel containing CaCO3 ∴ tip to mix the reagent => doesn’t involve replacing the bung

Question 2

CP 2: prepare a standard solution from a solid acid and use it to find the concentration of a solution of NaOH

standard solution = solution of known concentration

Answer 2

Making a standard solution:

1. weigh accurately, approximately the required mass of a solid and transfer to a beaker
2. dissolve in 100cm3 of distilled water and using a funnel, transfer into a 250cm3 volumetric flask => rinse beaker and transfer washings to volumetric flask
3. make up to the mark with distilled water
4. stopper and invert => mix ∴ uniform conc.
5. calc actual conc. of your solution
6. prepare apparatus for titration
   - unknown conc. => conical flask
   - known conc. => burette
7. pour 25cm3 NaOH into conical flask and add 4 drops of methyl orange
8. titrate until you have concordant results

Errors:
- some solid is left in beaker when transferring to the conical flask
- going over the mark in volumetric flask => fewer moles NaOH in volumetric flask ∴ smaller titre => removing excess solution w pipette will remove some of the dissolved NaOH
- forgetting the funnel => smaller titre
- washing burette with water dilutes NaOH ∴ larger titre
- washing the conical flask with water has no effect on titre => n(NaOH) is unaffected
- if titre for structure w 2 carboxylic acid groups = 10.20cm3, the tire for structure w 1 carboxylic acid group = 20.40cm3
- tip of burette must be filled with solution

Question 3

CP 3: find the concentration of a solution of HCL

Answer 3

1. wash 250cm3 volumetric flask with distilled water
2. transfer 25cm3 HCl using a volumetric pipette into volumetric flask
3. make up to the mark with distilled water
4. prepare apparatus for titration
   - known conc. NaOH in burette
   - unknown conc. HCl in conical flask with 4 drops of phenolphthalein
5. titrate until you have concordant results then find mean titre
6. end-point of titration => colourless

Errors:
- swirl conical flask so acid and base fully react
- use a white tile to observe colour change
- Phenolphthalein used in this titration may turn colourless at the end point if you leave the solution to stand => mixture absorbs CO2 ∴ mixture becomes more acidic
- larger titre => smaller % error

Question 4

CP 4: investigation of the rates of hydrolysis of some halogenoalkanes

Answer 4

1. set up 3 test tubes, each with 1 cm3​ ​ of ethanol (dissolves the halogenoalkane => insoluble in H2O) and 2 drops of a halogenoalkane [iodo/bromo/chloro]
2. place the test tubes in a water bath (60°C) with a test tube of 0.1moldm-3 AgNO3 and leave all to reach a constant temp
3. quickly add 1 cm3​ of a solution of silver nitrate to each test tube containing a halogenoalkane and start a stopwatch
4. measure and record the time taken for the precipitates to form in each of the test tubes

Results:
- yellow ppt of AgI forms fastest => weaker C-I bond ∴ hydrolysed fastest => fastest rate of reaction
- cream ppt of AgBr forms next
- white ppt of AgCl forms slowest => strongest C-Cl bond ∴ hydrolysed slowest => slowest rate of reaction

Errors:
- water is used as the nucleophile instead of OH- => AgOH ppt would form instead

3° halogenoalkanes react fastest

Question 5

CP 5: oxidation of ethanol

Answer 5

1. place acidified potassium dichromate (VI) in a pear-shaped flask, cool using an ice bath => cooling as the reaction with H2SO4 is very exothermic
2. add anti-bumping granules => provides SA for bubbles to form => violent boiling
3. add a few drops of ethanol and stir
4. set up reflux apparatus and place flask in water bath => reflux prevents VAPOUR escaping
5. heat with bunsen burner and boil for 20 mins
6. collect product via distillation => ethanoic acid

To collect ethanal set up distillation apparatus

To collect pure, dry sample
1. add anhydrous NaSO4 (drying agent) => clear solution
2. filter and redistill product
3. collect 1 degree either side of know BT of product

Observations:
- colour change orange dichromate (VI) → green chromium (III)

Errors:
- water in should be at the bottom, water out should be at the top => keeps condenser full of water => ↑ efficiency of condensing
- seal apparatus w thermometer/stopper => prevents gas escaping

Question 6

CP 6: chlorination of 2-methylpropan-2-ol using conc. HCl

Answer 6

1. add conc. HCl and 3° alcohol to conical flask, swirl
2. place bung and swirl flask gently, open the bung to release the pressure from fumes regularly and repeat for 20 mins
3. add anhydrous CaCl2​ and shake => ensures unreacted alcohol is in the lower (aq) layer => there should be 2 distinct layers
   [upper (organic) layer contains the desired product, lower layer is the aqueous layer]
4. transfer mixture to separating funnel
5. allow the layers to separate and discard the lower (aqueous) layer => retain upper organic layer in separating funnel
6. add solution of NaHCO3​ => removes unreacted HCl
7. swirl gently and remove the bung to release pressure of CO2 frequently
8. remove the bung, then run off and discard the aqueous layer
9. run the organic layer into conical flask, add anhydrous Na2​S​O4​ ​=> drying agent => removes water
10. swirl and leave flask => colour change cloudy → clear
11. decant organic liquid into pear-shaped flask
12. distill to purify the product

To lower pressure => invert funnel AND open tap

Question 7

CP 7 and 15: analysis of some inorganic and organic unknowns

Answer 7

Test for group 1 and 2 metal cations
- flame test
- red colour masks lilac colour
- nichrome wire = inert/produces no colour
- HCl cleans wire and allows metal ion salt to stick to it
- hold nichrome wire IN blue flame

Test for carbonate/hydrogen carbonate ions
- add acid
- effervescence of CO2
- Co2 is collected and bubbled through limewater (Ca(OH)2) => turns cloudy => CaCO3 ppt forms

Test for sulfate ions
- add acidified BaCl2 in presence of HCl => removes/reacts with carbonate/sulfate ions
- white ppt of BaSo4 forms

Test for ammonium ions
- add NaOH (aq) => forms NH3 (g)
- NH3 turns red litmus paper blue

Test for halides
- AgNO3
- chloride ions => white ppt => soluble in dil. NH3
- bromide ions => cream ppt => soluble in conc. NH3
- iodide ions => yellow ppt => insoluble in NH3

Test for alkenes
- bromine water => orange-brown → colourless
- acidified potassium manganate (V) oxidises alkene to diol => purple → colourless

Test for alcohols
- add PCl5 => misty fumes of HCL
- react with Na metal => effervescence

Test for COOH
- add NaHCO3 => effervescence of CO2

Test for aldehydes and ketones
- 2,4-DNP
- Fehling’s/Benedict’s
- Tollen’s

Test for CH3C=O
- iodine in presence of NaOH
- forms yellow triodomethane ppt

Question 8

CP 8: to determine the enthalpy change of a reaction using Hess’s Law

Answer 8

1. weigh accurately, approximately required mass of K2CO3 into a test tube
2. using a burette, dispense 30cm3 of HCl into a polystyrene cup
3. measure temp of acid every 30s for 2.5 mins
4. add K2CO3 into polystyrene cup with HCl at 3 mins and stir
5. measure temp every 30s for 5 mins
6. plot graph of temp against time on a graph
7. extrapolate lines of time of mixing to to find max ⃤T

Errors:
- we assume the specific heat capacity of the solution to be the same as water
- polystyrene is more insulating than glass

• stirring => uniform heating
• heated slowly => easier to identify temp @ which sample boils

Question 9

CP 9: finding the Ka value for a weak acid

Answer 9

1. titrate 25 cm3​ ​ of the acid (conical flask) being tested against NaOH (burette) using phenolphthalein
2. after titration, add another 25 cm3​ ​of ethanoic acid to the same flask
3. use pH meter to find pH => will equal pKa => exactly half of the acid has
   been neutralised ∴ this is the half-equivalence point [A−​ ]​ = [HA] ∴ Ka = [H+​ ]​ ∴ pKa = pH
4. pKa to Ka = 10−​pKa

Errors:
- burette reading are subjective to end-point of titration
- test pH meter with known conc. of buffer solution => see if it’s working

Question 10

CP 10: investigating some electrochemical cells

Answer 10

1. for each half cell, clean the strips of either metal or platinum with sandpaper => so there’s SA exposed for the reactions to occur
2. soak a strip of filter paper in KNO3 to make salt bridge => allows movement of ions ∴ balances the charges
3. use a high resistance voltmeter to find the value of the Ecell

Errors:
- electrodes must be made from inert substance ∴ won’t react with water
- differences in experimental and theoretical values usually stems from condition not being standard => 100kPa, 298K, 1moldm-3 solutions

Question 11

CP 11: redox titration

Answer 11

Preparing iron (II) solution

1. use mortar and pestle => crush tablets into powder
2. add H2SO4 and stir => dissolve tablets
3. filter the solution into a volumetric flask => removes insoluble impurities onto filter paper
4. wash beaker and filter paper with distilled water to get remaining solution into volumetric flask
5. dilute the solution in the volumetric flask by adding H2SO4 and making up to the mark => H2SO4 ensures sufficient H+ to reduce OA (MnO4-)

Titration

1. prepare the titration apparatus
2. add KMnO4 solution (OA) of known conc. ​to burette
3. add known volume of iron (II) solution (RA) to the conical flask
4. titrate the solution
   [no indicator required => KMnO4​​ is self indicating => end point is pink colour.]
5. repeat until you get concordant titres
6. calculate a mean titre

Errors:
- ensure as much iron salt is dissolved

Question 12

CP 12: the preparation of a transition metal complex

Answer 12

1. accurately weigh out the mass of CuSO4 and dissolve in water
2. in a fume cupboard, add conc. NH3
3. stir mixture and pour into ethanol, then cool mixture in ice bath until crystals of product form
4. filter mixture by suction filtration and collect the crystals in the funnel, then wash the crystals with cold ethanol (ensures no soluble impurities form on product as It dries)
5. leave crystals on the funnel/press crystals between 2 sheets of filter paper to dry
6. record mass and calc the % yield

Errors:
- losses => incomplete reaction, not all crystals crystallise
- gains => mass of impure/wet crystals

Question 13

CP 13: Rates of reaction

A: following the rate of the iodine-propanone reaction by a titrimetric method
B: investigating a ‘clock reaction’ (Harcourt-Esson, iodine clock)

Answer 13

A:
- H2SO4 acts as catalyst
- large excess of propanone and H2SO4 is used ∴ their conc. remain constant => measure influence of iodine on the reaction

1. using a pipette add a sample of mixture (iodine, propanone and H2SO4) to excess NaHCO3 immediately after removing it from the flask => quench the reaction
2. start a stopwatch
3. withdraw samples every three minutes and repeat this process
4. titrate these samples with sodium thiosulphate, adding starch indicator
   near the end => will turn from blue-black → colourless as iodine reacts with thiosulphate
5. repeat for all samples to find their conc.
6. use these results to plot a graph of conc. against time => graph should be a straight line => 0 order w.r.t iodine ∴ not involved in the r.d.s of the reaction ∴ has no effect on the rate

B:
- in the experiment you’ll vary the conc. of iodine whilst keeping the conc. of the persulfate constant, and vice versa ∴ you’ll measure the order w.r.t. each reactant

1. add persulphate to test tube containing potassium iodide, sodium thiosulfate and starch
2. start stopwatch
3. when the solution goes blue-black stop ​the clock

Errors:
- adding starch ↑ vol => affects the conc. of reactants ∴ the amt they change over time

Question 14

CP 14: finding the activation energy of a reaction

Answer 14

plotting​ ln(​​t​​)​​ against 1/T should produce a straight graph, with gradient Ea​/​R => allows the ​activation energy​​ to be found

Question 15

CP 16: the preparation of aspirin

Answer 15

1. weigh out 2-hydroxybenzoic acid and transfer to a pear shaped flask
2. add acetic anhydride and 8 drops of conc. H3​P​O4​
3. warm mixture under reflux for 5 mins
4. add cold deionised water to the solution and cool flask in an iced beaker until precipitation is complete
5. filter the mixture by suction filtration
6. dissolve impure product in minimum vol of warm ethanol
7. add warm water
8. place the boiling tube into ice water for 15 mins
9. filter the purified derivative by suction filtration
10. wash with small amt of solvent and dry the purified product using filter paper/oven
11. collect sample in a dry, pre-weighed sample vial and calc mass of product ∴ calc the % yield of the product
12. measure the melting point of the product