Core Practicals

Core practical 1 - Finding molar volume of a gas

• use the gas syringe set up with calcium carbonate and ethanoic acid

• palce 30 cm³ of ethanoic acid in excess in conical flask and calcium carbonate and quickly put bun on.

• measure the volume of gas produced by reading from gas syringe

• repeat with increasing masses of marble chips by around 0.05g each time

  Source of error:

• CO2 is slightly soluble in water/acid, so exact volume is not measured, bubble CO2 in acid beforehand or add a tiny bit of solid beforehand, as acid in excess, this won’t affect the result

• some gas escapes between addition of marble chips and sealing test tube, change acid to a weaker acid to slow reaction would reduce gas loss as less gas produced, or reduce concentration of acid

• wait until no effervescence is observed before weighing flask containing acid and marble chips to minimise gas loss.

CP 2 - Preparation of standard solution

• weigh out mass of required acid salt and transfer to clean beaker

• add 50 cm³ of distilled water to the salt and stir with a glass rod to dissolve

• use a funnel and transfer to volumetric flask by pouring down the glass rod

• wash the rod into volumetric flask and fill with distill water to 250cm³ mark

• stopper the flask and mix thoroughly by inverting and shaking vigorously

  Source of Error

• losing solids when trasferring from weighing bottle to beaker

• sample must be pure

CP 3- Titration

• use pipette to measure 25cm³ of acid, pour in conical flask

• fill in burette completely with alkali e.g. sodium hydroxide to the 0cm³ line and record this initial volume

• add an indicator to conical flask e.g. phenolphalein

• slowly open the tap to allow alkali to add to acid, swirl while adding

• stop/close the tap when observe a colour change e.g. colourless to pale pink

• then record the final volume

• repeat the experiment until getting at least three concordant results

• work out titre used for each repeat and take a mean

  Source of error

• air bubbles in the tip of pipette - need to calibrate

• use a white tile to make colour change more noticeable

• phenolphthalein used may turn colourless at end point if leave solution to stand as NaOH reacts with CO2 from air to form Na2CO3

CP4 Rate of Hydrolysis of halogenoalkanes

• set up 3 test tubes with 1cm³ of ethanol (acts as a good solvent for halogenoalkane, allows water and halogenoalkane to mix) each and two drops of a halogenoalkane

• place test tuebs in a water bath of around 60 degree, along with a test tube of 0.1 mol/dm³ of silver nitrate, leave all to reach a constant temperature

• quickly add 1cm³ of silver nitrate to each test tube containing the halogenoalkane and start the stopwatch

• measure and record the time taken for precipitates to form in each test tube

• the fast the formation of precipitate and the weaker the C-halide bond

• C-I is the weakest bond because of the weaker bond enthalpy of C-I bond (iodine is a bigger atom compare to chlorine, so has a worse orbital overlap with C, there is a weaker electrostatic force of attraction between the shared pair of electrons and nuclei)

• For primary/secondary/tertiary halogenoalkane, because tertiary carbocation is more stable(more alkyl group so stronger inductive effect) so takes the shortest time to react

• Note:

• this is a nucleophilic substitution where water acts as the nucleophile

• although oh- is a better nucleophile, silver hydroxide would form instead, insoluble, so. white precipitates of solids silver hydroxide would form immediately, hence cannot observe difference in rate of reaction of halogenoalkanes

CP5 - oxidation of alcohol

• place acidified potassium/sodium dichromate (in excess for carboxylic acid)in a pear-shaped flask

• add a anti-bumping granules to provide a surface for smaller bubbles to form to prevent violent bubbling

• cool pear-shaped flask in a ice water beaker

• set up reflux apparatus,

• add ethanol dropwise to pear-shaped flask to allow reaction to subside after each addition before adding more, stir to ensure complete mixing

• remove icewater, warm up flask to room temperature

• place flask in a hot water bath

• heat using bunsen burner for 20 minutes

• allow apparatus to cool down and then collect product via distillation into a boiling tube

• to preparing an aldehyde conduct the same reaction until distillation conditions but without the reflux process so aldehydes are immediately removed that they cannot be further oxidised, do not use potassium dichromate in excess, use the right amount

  Source of error

• thermometer should not touch the solution, measure vapour temperature

• reflux apparatus hosuld have an open end, do not put lid to prevent pressure building

• for the condenser, water must in at bottom, out at top because due to gravity, some of the container then may not be called as more water at the bottom, hence some aldehyde may escape and reduce yield

CP6 - Chlorination

• add concentrateed HCl and tertiary alcohol to a conical flask. Swirl gently

• put rubber bung in and swirl flask gently. Open the bung to release the pressure from time to time. Repeat reguarly for 20 minutes

• add anhydrous calcium chloride and shake

• transfer contents to a separating funnel

• allow layers to separate and discard the lower layer(aqueous)

• add NaHCO3 to remove unreated HCl. Swirl gently. Stopper the separating funnel and shake. Invert thefunnel and open tap to release pressure due to CO2, repeat a few times

• remove the stopper and run off aqeous layer. Run the organic layer to a conical flask. Add sodium sulfate - drying agent

• swirl contents and leave flask to stand then filter out product

• distill to purify the product

• Note

• Reaction can be taken place in room temperature because teritary alcohol can form tertiary carbocation, which is very stable, so does not need much energy to react

• sodium hydrogen carbonate used instead of sodium hydroxide to remove H+ because OH- could react with halogenoalkane to form alcohol again or elimination reaction to form alkene. There would be also not be co2 forming, which could indicate an end reaction when there’s no more pressure

CP 7 - Analysis of unknown Compound

• flame test

• test for carbonate ions - add hcl acid observe effervescence, bubble limewater, turns milky white/cloudy

• test for sulfate ions - add hcl and bacl2, forms white precipitates

• test for halogenoalkanes - add ethanol and NaOH or water, water bath, then add aqueous silver nitrate, observe white/cream/yellow precipitates

• test for halides: nitric acid and silver nitrate, then aqueous ammonia - iodide insoluble in any ammonia, bromide soluble in concentrated ammonia, chloride soluble in any ammonia

• test for NH4+ - add NaOH, turns red litmus paper blue

• test for -OH groups - add acidified potassium dichromate orange to green for primary/secondary. add pcl5, steamy fumes and effervescne

• test for carboxylic acid - add carbonate ions observe effervescence, litmus paper turns red

• test for carbonyl compounds, add 2,4-DNPH orange precipitates, tollen’s reagent, for aldehyde silver mirror observe, or iodoform(iodine and sodium hydroxide), antiseptic smell and yellow precipitates

CP8 - Enthalpy change using Hess’s Law

• plac solid carbonate into a test tube, weigh solids and test tube, and mass of empty test tube

• place 30cm³ of HCl use a measuring cylinder into a polystyrene cup, measure HCL and container

• place a thermometer with it, add solids, meaasure the inital temperature and record down

• continue measuring the temperature whilst adding solid carbonate to acid and stirring. Reocird the highest temperature reached

• reweigh the empty container for HCl

• repeat using different solid carbonate but record the lowest tempeature reached

  source of error

• polystyrene is more insulating than glass, so less heat lost

CP9 - Finding Ka

• titrate 25cm³ of acid against NaOH, use phenolphthalein indicator

• then add 25cm³ of ethanoic acid to the same flask after titration

• Use pH meter to find pH

• pH equals to pKa because exactly half of the acid has been neutralised so this is the half equivalence point i.e. [A^-] = [HA], so pKa = pH

• then use pKa = -logKa, to find Ka

CP 10 - Electrochemical Cell

CP 11 - Redox titration

• weigh the bottle provided containing impure crystals

• transfer solid into a 250cm³ beaker and reweigh empty bottle

• add 10cm³ of dilute sulfuric acid (need to be in excess) to the beaker, sit and dissolve

• Make up a standard solution of the solid using volumetric flask (filter solution into volumetric flask and rinse beaker with sulfuric acid, then fill to line with distilled water, stopper, invert and shake)

• fill a burette with potassium manganate solution

• transfer 25cm³ of the standard solution into a conical flask

• carry out a rough titration (colour change from colourless to pink)

• titration must be heated because the two reactants have same charge so there’s a strong force of repulsion

• then carry out at least two accurate titrations with two concordant values with 0.1cm³

• For the manganate reaction, a brown precipitate may be formed during titration because MnO2 is formed when there is not enough sulfuric acid so manganate ions are only partially reduced rather fully to Mn(II)

CP 12 - Transition metal complex preparation

CP13a - Iodine reaction - continous method

• label 1 250cm³ flask and 6 conical flask

• measure 50cm³ of iodine solution using a measuring cylinder and add to flask 1

• using clean measuring cylinders and measure 25cm³ of propanone and sulfuric acid to add to flask 2

• add 10cm³ of sodium hydrogencarbonate to each flask 3-7

• rinse and fill burette with sodium thiosulfate

• use a hot plate stirrer to stir flask 1 at a steady rate

• pour contents of flask 2 to 1 and start timer immediately. Leave this timer running throughout the experiment

• after one minute, take 10cm³ sample of flask 1 using a pipette, put in flask 3 to quench the reaction, recording the time when the pipette is half-empty. Swirl the contents of flask

• take four additional 10 sample at 5 minutes intervals, empty each sample to flask 4-7, recoding time for pipette to half empty

• titrate the iodine present with the sodium thiosulfate solution until a pale-yellow colour is reached. Add a few drops of starch indicator to flask 3-7 and titrate until end-point reached. Calculate titres using initial and final volumes

• plot graph of volume of sodium thiosulfate against time to work out order of reaction

• It is important to record the time for pipette to be half-empty as average time is used.

CP 13b - determine rate equation using a clock reaction

• add 5cm³ of sodium thiosulfate, 25cm³ of hydrogen peroxide, 5cm³ of distilled water to a 250cm³ conical flask

• add 25cm³ of sulfuric acid to the conical flask

• add 10 drops of starch indicator to the conical flask. Use a hot plate stirrer to swirl contents at a steady rate

• add 10cm³ potassium iodide to conical flask and immediately start timer

• record the time taken t=for mixture to turn blue-black

• repeat method for experiments 2-5 changing the volume of hydrogen peroxide and water (keep the total volume used constant, e.g. decrease 5cm³ of hydrogen peroxide but increase 5cm³ of distilled water each time).

• Use the time taken for each experiment to calculate the initial rate 1/t

• measure each volume using a measuring cylinder

CP 14 - Activation Energy

• Label two boiling tueb A and B. Mark a dark spot on the side of a 400cm³ beaker

• Use a measuring cylinder, transfer 10cm³ of sodium thiosulfate to boiling tube A

• Use a measuring cylinder, transfer 10cm³ of HCl to B

• Place both tubes in the beaker of water, allow them to reach thermal equilibrium

• Ass solution from B to A and start timer.

• Mix A by gently stirring with the thermometer. Read temperature to nearest degree and record this

• Observe spot on the side of beaker by looking through solution in A. Record the time taken for the spot can no longer be seen due to the formation of sulfur precipitate formed in A

• Use a kettle and beaker prepare water bath with temperature about 65 degree and repeat the experiment with five sets of results at different temperatures

• One should be at room temperature and four between room temperature and 50 degree

• ln(1/t) = (-Ea/R)T + constant, to work out Ea

CP 15 - Analysis of Unknown Compound

CP16 - Synthesis of Aspirin

State the apparatus that would be suitable for breaking up the lumps of solid into powder

• pestle and mortar

bromoethane is very volatile, suggest what could be done with the small beaker to collect bromoethane to prevent the loss of the bromoethane distillate

cool with ice water