Methods

Question 1

Calorimetry - 8

Answer 1

• Make up standard solutions of different known concentrations
• Choose a suitable filter with the complimentary colour
• Zero colorimeter with water
• Measure absorbance of standard solutions
• Plot calibration graph of absorbance vs concentration
• Measure unknown
• Read off concentration from calibration curve

Question 2

Calorimetry: order/rate of reaction

Answer 2

• Use a suitable filter with the complementary colour
• Zero with water
• Put a sample of the reaction mixture into the colorimeter
• Take absorbance readings at set time intervals
• Convert absorbance readings to concentrations using the calibration curve
• Plot graph of concentration v time
• Find half-lives from graph
• Constant half-life = first order

Question 3

Carbonates reacting with HCl - 9

Answer 3

• The mass of a weighing bottle containing the carbonate is measured
• Transfer 50 cm3 of 2 mol HCl [an excess] from a measuring cylinder to a conical flask
• Add Na2CO3 to acid in flask to saturate acid with CO2
• So no gas given off when carbonate reacts with acid dissolves in acid
• Cotton wool plug placed in neck of flask and flask and acid weighed
• Weighed carbonate is added to acid in flask and cotton wool plug quickly replaced in neck of flask to prevent loss of acid spray
• Mass of empty weighing bottle is measured
• Reaction in flask stopped, flask left for 10 minutes to allow CO2 to diffuse from flask
• Mass of flask and contents after reaction are measured

Question 4

Recrystillisation - 10

Answer 4

• Heat the impure sample with hot solvent
• With a minimum amount until solid just dissolves
• Filter – vaccum filtration used so solution remains hot
• Leave solution to cool
• Crystals formed when solution cools
• Vacuum filter off crystals
• Wash with cold solvent - minimises further loss of product but ensures any soluble impurities left washed through
• Air dry/oven in temperature lower than solid melting point
• First filtration of hot solution removes insoluble impurities
• After crystallisation soluble impurities stay in solution

Question 5

Mass spec

Answer 5

• Sample vapourised and ionised
• Ions are accelerated
• To the same kinetic energy
• Move into drift region
• Heavier ions move across to detector more slowly
• Different masses take different times to reach detector

Question 6

Rates of iodine reaction: methods 3

Answer 6

• Concentration of iodine by titration with thiosulfate
• Colour / absorbance of iodine by colorimetry
• Acidity – concentration of H+ by pH meter

Question 7

Potassium manganate titration - 10

Answer 7

• Fill burette with KMnO4 / MnO4– solution
• Use graduated pipette for sodium ethanedioate
• Place known amount of solution in flask
• Add excess sulfuric acid
• Warm
• Titrate until pink colour persists
• Add manganate dropwise near end
• Repeat for concordance
• No need for indicator as MnO4 2- is only coloured reagent
• Colour change takes place during reaction from colourless to pale pink

Question 8

TLC 11

Answer 8

• Draw pencil-line near bottom of plate
• Place drops of mixture on the line
• Place plate in solvent, line above solvent level and add lid
• Allow solvent to rise through spots
• When solvent nears top of plate, remove plate
• Dry plate
• Transfer to fume cupboard to evaporate solvent
• Locate spots by adding fluorescent dye then viewing under UV light/iodine gas
• Circle spots
• Compare heights/position of spots from mixture with the 3 standard compounds OR calculate Rf values of spots and compare with those of the standards
• Repeat process after further recrystallisation

Question 9

Strength of haloalkane C-X bond - 12

Answer 9

• Dissolve haloalkane in ethanol in test tubes
• Stand in beaker of hot water
• Add solution of silver nitrate
• Start timing
• Ethanol solvent as haloalkanes don’t dissolve in water
• Hot water needed reaction too slow in cold
• Equal amounts/volumes of reactant for ‘fair test’
• precipitate forms/tubes go cloudy
• Note time on first appearance
• Precipitates caused by halide ions reacting with silver ions
• Shorter time means faster rate
• Rate of hydrolysis in order iodo(fastest), bromo and chloro
• Bond strengths increase in above order

Question 10

Thermal stability of carbonates - 11

Answer 10

• Crush lumps of calcium carbonate to a powder using a pestle and mortar
• Add calcium hydroxide to distilled water and filter
• Glass test tube/boiling tube to contain the solid fitted with a bung carrying a delivery tube which dips into a solution of calcium hydroxide in a test tube/boiling tube.
• Tube is heated and is approximately horizontal
• Gas is passed through limewater
• Measure time for cloudiness to first appear
• Measure time taken to obscure a cross on paper
• Longer time has greater thermal stability
• Takes longer to go cloudy down the group
• Equal mass of MgCO3 and CaCO3 and same volume of limewater
• Same volume and concentration of calcium hydroxide solution

Question 11

Enthalpy of solution - 3

Answer 11

• Place known volume/mass of water in a polystyrene beaker
• Weigh out a known mass of the salt and add to water with stirring
• Measure temperature of water before and after

Question 12

Group 2 reaction with water - 7

Answer 12

• Measuring vol of hydrogen per unit time
• Fair testing, same no of moles of metal, same surface area, same temperature
• Gas bubbles produced
• Metal decreases in size/disappears
• Faster rate of reaction for Ba
• Ca cloudier than Ba
• Both metals react to produce hydroxides and hydrogen gas
• Hydroxides more soluble down the group

Question 13

Flame test - 2

Answer 13

• Nichrome wire dipped in sample and concentrated HCl

- Placed in blue/roaring Bunsen flame

Question 14

Making a soluble salt with metal/insoluble base - 5

Answer 14

• Add together acid for anion and metal/base for cation until fizzing stops/reaction complete
• Filter
• Partially evaporate
• Fully evaporate to produce crystals
• Filter and dry

Question 15

Rate of reaction: titration 3

Answer 15

• Withdraw sample using pipette at given times
• Run into conical flask
• Titrate quickly with known concentration of NaOH in burette

Question 16

Heating to constant mass - 2

Answer 16

• Heat, cool and weigh

- Repeat until mass constant

Question 17

Difference between paper and thin layer chromatography

Answer 17

Stationary phase is: silica/alumina in TLC not paper

Question 18

Diluting conc solution to form standard solution

Answer 18

• Vol to use: final conc/initial conc x volume required
• Transfer vol to use to volumetric flask
• Use a dropping pipette to fill solution to required volume with distilled water in volumetric flask

Question 19

Standard solution -11

Answer 19

• Calculate mass of solute needed
• Weigh mass of solute into weighing bottle [zero bottle first]
• Transfer into beaker, carefully washing out weighing bottle with jet of distilled water and allowing washing to drop into beaker
• Add a small amount of distilled water and stir with glass stirring rod until all solute has dissolved
• Transfer solution into 1dm3 volumetric flask containing 500cm3 of distilled water using glass funnel
• Use distilled water to rinse inside of beaker and glass stirring rod at least twice and pour washings into volumetric flask
• Shake flask gently
• Fill up with distilled water until close to actual mark
• Add more distilled water using a pipette drop by drop after removing funnel and rinsing it properly
• Until lowest level of meniscus is on mark when at eye level
• Stopper flask and invert several times

Question 20

How can filters be tested to see which is the appropriate complementary colour? 2

Answer 20

• Try different filters with the solution

- Select filter showing maximum absorption

Question 21

Seperation of substances insoluble in water with impurities that dissolve in water

Answer 21

• Pour mixture into separating funnel
• Add water
• Shake funnel
• Run off bottom aqueous less dense layer
• Swirl top layer with anhydrous sodium sulfate for 20 mins
• Filter

Question 22

Filtration - 3

Answer 22

• Pour mixture into Buchner funnel with filter paper at bottom
• Below funnel have sealed side arm flask attached to a vaccum line
• Reduced pressure in flask will force liquid through funnel

Question 23

Compare smells of different esters - 8

Answer 23

• Add 1 drop of conc sulfuric acid to each test tube
• Add 10 drops of RCOOH to sulfuric acid in one test tube
• Add 10 drops of alcohol to mixture
• Carefully lower tube into beaker of hot water
• After 1 minute remove tube
• When cool, pour mixture into boiling tube half full of sodium carbonate solution
• Smell product by wafting odour towards nose with hand
• Repeat with different combinations of COOH and alcohol

Question 24

Reflux - 4

Answer 24

• Boil a liquid in a flask attached to a vertical / upright condenser for a prolonged time
• Mixture evaporates into vapour
• Vapour then condensed and returned to mixture as liquid drops back into the flask
• Allows any substance initially evaporating to be fully oxidised

Question 25

Distillation - 7

Answer 25

- Heat in flask - With a still head containing thermometer - If it evaporates at room temp then have a cooled collecting vessel - The condenser is horizontal and attached at the side - Keep condenser cool with running water in a tube at either side to minimise loss of substance through evaporation - Mixture are evaporated and condensed and collected in vessel at boiling point - Mixture is separated

Question 26

How can Rf values be calculated for amino acids? 4

Answer 26

- Spray ninhydrin solution [developing agent] onto paper to turn spots purple - Circle spots with a pencil - Measure distance travelled by solvent from baseline to solvent front - Measure distance travelled by amino acid by measuring from baseline to spots - Calculate Rf: distance travelled by spot/distance travelled by solvent

Question 27

Acid base titration - 9

Answer 27

- Use a graduated pipette to add 25 cm3 of alkali to a clean conical flask - Add a few drops of indicator - Put the conical flask on a white tile to see colour change more easily - Fill the burette with acid and note the starting volume. - Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix - Stop adding the acid when the end-point is reached - Note the final volume reading: this is the rough titre - Now do accurate titration: run acid into within 2cm3 of end point, then add the acid dropwise - Repeat for 3 concordant titres

Question 28

Making insoluble salts - 4

Answer 28

- Pick 2 solutions with ions needed and mix - Salt precipitates out - Filter from solution - Wash and dry on filter paper

Question 29

Making soluble salts with alkali - 6

Answer 29

- Titrate acid using indicator with alkali [soluble bases] to know amount of alkali needed to neutralise acid - Repeat titration to combine these volumes without indicator - Filter - Partially evaporate - Fully evaporate to produce crystals - Filter and dry