AQA A Level Chemistry Paper 3 Practical Revision

Required Practical 1: Acid-Base Titrations

  • Substance of unknown conc. goes in conical flask.
  • Use a conical flask because it is easier to swirl without spilling.
  • Use 25cm^3 pipette/pipette filler to accurately measure amount to put in conical flask. 
  • Standard solution is known mass of substance of unknown conc. being made up to 250cm^3 using distilled water. Measure in volumetric flask. 
  • Ensure meniscus is on line. 
  • Rinse equipment with distilled water, then substance that will be put in it. 
  • Readings to 2 d.p, with last digit a 0 or 5. 
  • Ensure air bubbles are removed. 
  • Phenolphthalein is pink in alkali and colourless in acid. 
  • Methyl orange is yellow in alkali and red in acid. 
  • Do a rough run, then repeats until 2 concordant titres found (within 0.1 of eachother). 
  • Use average titre in calculations. 
  • Distilled water can be used to wash edges of conical flask to ensure all acid and base react. This will not affect amount reacting. 
  • Corrosive reactants: Wear gloves and goggles. Wash spilt substances immediately.
  • Unknown substances: treat as toxic.  
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Additional points

Errors: 

  • Each piece of apparatus has a sensitivity error. 
  • Error of equipment is equal to sensitivity error/measurement made x100. 

  • Remember to times sensitivity error by two if two measurements made. For example, in titrations. 
  • Overall error is all the equipment errors added up.
  • When reducing errors, think about things that lower sensitivity errors, like making bigger measurements, or making bigger measurements. Use the equation to help.
  • Sensitivity errors: cause each reading to be different from the true value by the same amount - equipment error.
  • Random errors: readings different to true value in an unpredictable way - human error.

Converting mol dm^-3 to g dm^-3

  • Multiply concentration by Mr. 
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Required Practical 2: Enthalpy change

  • Could be measurement of enthalpy of combustion using calorimeter method - burning fuel.
  • This can be inaccurate because of heat loss to surroundings, and loss of volatile, flammable liquids by evaporation.
  • Could also be measurement of enthalpy change of solution. Using a polysterene cup and creating a graph with two lines of best fit to get accurate temperature change. 
  • Energy change = -q/n
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Required Practical 3: Rate of reaction & temp

  • Eg. 1. Timing how long a precipitate takes to form (however this is subjective so not reliable).
  • Eg. 2. Measuring decrease in mass as an indication of rate. 
  • Eg. 3. Measuring gas given off using a gas syringe. This is an indication of rate. Very accurate measurements.
  • Eg. 4. Cross method using sodium thiosulphate and hydrochloric acid. Time how long it takes for precipitate to form so cross underneath is no longer seen. 
  • Carry these out at different temperatures to see how rate is affected. 
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Required Practical 4: Tests for cations and anions

Halide ions: 

  • Acidify with dilute nitric acid.
  • Test with silver nitrate, dilute ammonia and concentrated ammonia (use fume cupboard). 

Hydroxide ions: 

  • Add ammonium ions. Ammonia gas will be produced. 
  • Test for ammonia gas; distinct smell, red litmus goes blue, dip hydrochloric acid, using a glass rod, into test tube and a white smoke is formed. 

Carbonate ions: 

  • Dilute hydrochloric acid
  • Pipette gas formed into test tube of lime water and bubble it through. 
  • White precipitate formed. 
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Required Practical 4: Tests for cations and anions

Sulphate ions: 

  • Add dilute hydrochloric acid. 
  • Add barium chloride. 
  • White precipitate formed. 

Notes: acids often added to stop other ions interfering in the test. Sulphate ion test links to solubility of the group 2 metals. 

Ammonium ions: 

  • Add sodium hydroxide.
  • Warm.
  • Ammonia produced.
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Required Practical 5: Distillation

  • Eg. Oxidation of ethanol to ethanal. Product must be collected before it is oxidised to ethanoic acid.
  • Use anti bumping granules.
  • Oxidising agent: acidified potassium dichromate. 
  • Solution colour change from orange to green. 
  • Fruity smell. 
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Required Practical 6: More tests...

Alcohol 

  • Sodium 
  • Positive test: gentle fizzing
  • Negative test: no fizzing or violent fizzing
  • Safety - ensure no water or acid in sample. 

Distinguis primary and secondary alcohols from tertiary alcohols

  • Acidified potassium dichromate. 
  • Warm. 
  • Primary/secondary alcohols: will be oxidised so colour change of orange to green. 
  • Tertiary alcohols: will not be oxidised so no colour change. 

Aldehydes 

  • Tollen's reagent: silver mirror
  • Fehling's solution: blue to brick red colour change
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Required Practical 6: More tests... (2)

Alkene 

  • Bromine water
  • Positive test: pale yellow to clear solution 

Carboxylic acid: 

  • Solid sodium hydrogencarbonate 
  • Positive result: fizzing and CO2 produced. 
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Quenching reactions

  • Stopping a reaction from continuing any further.
  • Useful if measure rate of reaction at a set time.
  • One way can be to remove the reagent. For example if it is an acid, react it with a base, or vice versa. 
  • Quickly cooling the mixture to stop reaction. 
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Data types

  • Discrete: certain values on a scale. 
  • Continuous: any value on a scale. 
  • Categoric: values can be sorted into categories. 
  • Ordered: when categories can be put in order. 
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Filtration

Seperates solids from liquids. 

Method 1: funnel and filter paper 

Method 2: filtration under reduced pressure 

  • uses Buchner apparatus

Method 3: decanting 

  • allows solid and liquid or two immiscible liquids to be seperated
  • allows heavier substance to sink to the bottom and lighter to be drained off
  • Centrifuge can speed up process 

Method 4: evaporation 

  • evaporating substances off
  • eg. water
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Heating under reflux

So substances can be fully oxidised. 

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Back Titrations

  • You can react an unknown solid in excess acid 
  • React 25cm^3 of acid with base to work out number of moles used.
  • Multiply up to get moles that react with solid
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Experimental errors

of your experimental measurements by finding the percentage error ... (http://dev.physicslab.org/img/91a4adf8-b12a-4649-a6f8-ac85b3e3d1e0.gif)

If experimental error is greater than the total apparatus error, then the results are inaccurate. 

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Testing the purity of a product

  • See what temperature the product boils or melts at. 
  • Compare to data book value. 
  • Closer to correct value, and the smaller the range the product melts/boils at. the more pure the product. 
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Safety when using potassium cyanide

  • Irritant 
  • Extremely dangerous if ingested or inhaled
  • It can react with moisture to produce toxic HCN gas
  • Wear gloves, goggles and a lab coat
  • Use a fume cupboard
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Purifying organic compounds

Removing water 

  • Add an anhydrous salt 
  • Eg. Calcium chloride, Magnesium sulphate 

Remove impurities by washing 

  • Product contaminated with left over reagents or side products. 
  • Eg. use aqueous sodium hydrogencarbonate to remove acid 

Recrystallisation

  • Dissolve impure solid in just enough hot solvent so that solution is saturated
  • Filter through heated filter to remove insoluble impurities (so it doesn't crystalise too early)
  • Leave to cool. Crystals of product form.
  • Filter under reduced pressure to remove soluble impurities in liquid. 
  • Wash with ice cold solvent to remove any soluble impurities on surface (cold so crystals don't dissolve again). 
  • Leave to dry. Can use paper towel to dry quicker.
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Required Practical 7: Rate of reaction

Initial Rate 

  • Meausure change in conc. over a period of time early on in the reaction 
  • Plot graph and measure gradient of tangent to get close approximation to initial rate
  • Repeat this with different conditions that you are investigating (eg. temp)
  • Can use this to work out order of reaction/rate equation

Continuous Monitoring

  • Measuring conc. of product/reactant at different time intervals 
  • Make sure to quench reaction
  • Carry out a titration so conc. can be calcultated
  • Can also track a reaction where a colour change occurs with a colorimeter
  • Concentration is proportional to light absorbed.
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Required Practical 8: Electrochemical cell EMF

  • Standard electrode potentials measured in: 100kPa, 298K, zero current and concentrations are 1.0 mol dm^-3 for all ions
  • Voltmeter with a very high resistance
  • Salt bridge to allow ions to flow and complete circuit: 
    • Soak chromatography paper in saturated solution of potassium nitrate
    • Pick different solution if potassium nitrate would react with anything in cells
  • Ecell = Eright - Eleft 
  • Ecell = Ereduced - Eoxidised
  • When you read the voltmeter, if the value is positive the cells are in standard arrangement. 
  • If voltage value is negative, cell is not in standard arrangement.
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Required Practical 9: pH changes in titrations

Investigating how pH changes when a weak acid reacts with a strong base and when a strong acid reacts with a weak base

  • Remember the graphs learnt in theory lessons.
  • Using a pH meter to measure pH at varying volumes, using rough run as a guide. 
  • Carry on beyond the end point until the volume at the end point is approximately doubled.
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Required Practical 11: Transition metal ions

See table to learn

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Required Practical 12: Thin Layer Chromatography

Remember...

  • Rf values
  • Stationary phase: thin layer of silica 
  • Mobile phase: solvent 
  • Rf values are specific to conditions so ensure data book values you are comparing to were obtained under the same conditions your Rf value was obtained under
  • Watch glass prevents evaporation of solvent 
  • Dry the chromatography paper in the fume cupboard
  • Can use UV light to look at spots if they are hard to see.
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Required Practical 10: Preparations

Preparation of a pure organic solid and test of its purity (eg. aspirin)

  • React substances to produce the product and filter off using Buchner funnel under reduced pressure
  • Recrystallisation technique to purify solid 
  • Test its purity by...
    • Melting substance and seeing how close it is to the data book value for the melting point. Also noting if it occurs over a narrow or wide range of temperatures.
    • Infrared spectrum would show impurities in the fingerprint region compared to known sample of aspirin. 

Preparation of a pure organic liquid (eg. ethyl ethanoate)

  • Know difference between reflux and distillation apparatus. 
  • For ethyl ethanoate - heat under reflux.
  • Safety: using heating mantle instead of bunsen as ethanol is flammable. 
  • Seperating funnel technique for two immiscible liquids.
  • Distill product. 
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