AQA A Level Chemistry Paper 3 Practical Revision

• 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.  

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. 

• 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

• 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. 

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. 

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.

• 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. 

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

Alkene 

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

Carboxylic acid: 

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

• 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. 

• 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. 

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

So substances can be fully oxidised. 

• 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

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

• 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. 

• 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

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.

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.

• 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.

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.

See table to learn

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.

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.