Chemistry RPs

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RP 1a: making a standard solution

• weigh a weighing boat on a mass balance

• add the acid to mass balance and record mass

• tip acid into beaker record mass

• subtract initial from final

• add 100cm3 distilled water to beaker mix until acid dissolves

• transfer to volumetric flask with washings

• add distilled water up to 250cm3 mark

• invert multiple times to mix

RP 1b: acid-base titrations

• fill burette with known concentration of acid/alkali with known volume

• add fixed volume of acid/alkali (unknown conc.) into conical flask with volumetric pipette

• add 3 drops of indicator and swirl white tile under

• slowly add volume in burette, swirling between additions

• add dropwise at first sight of colour change

• stop at permanent colour change

• record volume and repeat until titres are concordant calculate mean

RP 2: measuring an enthalpy change

• measure acid using burette and pour into polystyrene cup

• record solution temp. for at least 4 mins before addition with thermometer

• weigh powdered carbonate on watch glass

• reweigh watch glass afterward and calculate difference

• add carbonate stir with thermometer and close lid

• record temp. at 1 min intervals until trend is observed

• plot graph temp (y)/time (x), extrapolate to point of addition

• determine temp change at addition point

RP 3: effect of temp. on rate of reaction

• place X underneath beaker in water bath

• use water bath to keep temp. consistent at 25 degrees

• add fixed volume of acid to beaker, then thiosulfate

• start stopwatch immediately - record time taken for cross to disappear

• monitor water bath temp. using a thermometer regularly

• repeat at 30, 35, 40, 45, 50 degrees

• calculate rate = 1/t

RP 5a: distillation as a separation technique

• add 25cm3 reaction mixture to round bottom flask

• add anti bumping granules (smaller bubbles)

• use electric heater, clamp flask to Liebig condenser, end of condenser should drop into beaker in ice cold water bath

• turn on tap, allow water to fill condenser

• heat gently at boiling point of desired product tracked using thermometer (down at T junction)

• keep temp constant at this point - compound will evaporate and condense in water bath

RP 6: organic tests

• (gently heat wb) acidified K₂Cr₂O₇ - 1^o = orange → green aldehyde 2^o = orange → green (ketone) 3^o no change

• aldehyde + tollen’s (heat gently wb) - colourless → silver mirror

• aldehyde + fehling’s (heat gently wb) - blue → brick red ppt.

• chloroalkane + acidified AgNO3 (heat gently wb) - white AgCl ppt forms slowly

• acyl chloride + acidified AgNO3 - white steamy HCl fumes, immediate white AgCl ppt

RP 7a: initial rate of reaction

→ iodide ions + H₂O₂ → I₂ reacts with S₂O₃^2- the excess I2 reacts with starch - blue black

• measure required volume of KI, Na₂S₂O₃, starch and water into conical flask

• measure H2O2 separately, immediately start timer after addition

• stop when solution is blue-black

• repeat with diff. concs/ of KI

→ can measure the amount of time taken for a fixed amount of X to be produced

RP 7b: continuous monitoring of reaction

• measure 50cm3 HCl in conical flask using burette

• attach gas syringe to conical flask with delivery tube

• weigh 0.2g Mg and add to conical flask

• place bung firmly and start stopwatch

• record vol of H2 gas every 30 secs for 3 mins

→ can also measure change in mass, remove small samples (quench reaction) then titrate to find conc., colorimetry

RP 8: EMF of a cell

• clean pieces of copper and zinc using emery paper

• degrease them using cotton wool + propanone

• place copper into beaker with 50com3 CuSO4 solution

• place zinc into beaker with 50cm3 ZnSO4 solution

• soak filter paper in KNO3 for salt bridge & place between beakers each end in solution

• use crocodile clips connect each half cell to voltmeter, record voltage

• repeat with different metals

→ if metal is non-conductive us Pt electrode

→ salt bridge should be unreactive unlike KCl

→ wire not use for bridge because it would set up its own electrode system

RP 9: pH changes

• calibrate pH probe using known buffers & plot calibration curve

• transfer 25cm3 acid to conical flask with burette

• measure initial pH using pH probe record results

• add 2cm3 alkali, stir, measure pH and record it

• repeat and when approaching endpoint add smaller volumes of alkali

• add alkali until in excess

• use pH calibration curve to adjust pH results from experiment

• plot corrected pH values against alkali volume

RP 10a: making aspirin

• 20cm3 benzoic acid + 4cm3 ethanoic anhydride

• add 5 drops H2SO4 & mix

• heat beaker in water bath over Bunsen burner, leave to cool to room temp

• mix with 10cm3 distilled water

• pour mixture through Buchner funnel & filter - crystals will form

RP 10b: obtaining pure crystals

• dissolve impure compound in min vol of hot solvent (ethanol) 70^oC

• hot simple filtration

• leave to cool in ice bath

• filter under reduced pressure with Buchner flask

• pour a small volume of cold water through crystals to wash away soluble impurities

• dry crystals with pieces of filter paper

→ loss of yield: crystals lost whilst washing, some product stayed in solution after recrystallisation

RP 10c: preparing and purifying an ester

• 50cm3 propanol + 50cm3 ethanoic acid round bottom flask with anti bumping granules

• add 10 drops H2SO4 & fit reflux condenser onto flask and heat under reflux for 20 mins

• cool mixture and transfer to a separating funnel

• add 20cm³ of aqueous Na2CO3 and shake

• shake gently then open cork, close and repeat to remove CO2 gas

• allow to separate into aqueous & organic layer & add 5g anhydrous MgSO4

• distil organic layer with anti bumping granules & heat with water bath just below b.p. of the ester

• collect ester in conical flask and weigh ester

→ anti bumping granules prevent large bubbles explode

→ Na2CO3 reacts with any unreacted acid

→ H2SO4 is a catalyst

RP 12: TLC

• draw pencil line 1cm above bottom of TLC plate

• use capillary tube to add tiny drop of each solution on pencil line - allow dry

• pour solver in beaker, place TLC plate with pencil line just above solvent

• place beaker in fume cupboard with lid

• remove plate 1cm before solvent reaches top, mark solvent front

• leave to dry in fume cupboard

• calculate Rf value from centre of spot