Methods

Question 1

Method of a reaction using colorimeter

Answer 1

1. Samples of diff concentrations
2. Calibrate colorimeter - blank solution
3. Absorbance of each solution of known conc
4. Calibration curve of absorbance against conc
5. Reaction mixture in colorimeter and record absorbance at diff times
6. Use calibration curve to convert absorbance to concentration and plot graph of concentration against time
7. Gradient of tangent at any concentration equals rate

Question 2

Rate using quenching and titrating

Answer 2

1. Mix solutions - remove samples by pipette at various times
2. Quench - stop reaction - rapid cooling/large vol of water/add chemical to remove reactant that is not being monitored
3. Titrate - find conc of reactant/product
4. Graph conc against tine - grad of tangent at any concentration equals rate

Question 3

Measuring rate through pH

Answer 3

1. Start reaction
2. Record pH at diff times - pH meter
3. H ion conc using pH = -log(H+)
4. Graph H+ against time - grad of tangent equals rate

Question 4

Rate using volume of gaseous product over time

Answer 4

1. Attach gas syringe to reaction vessel or collect gas over water in inverted measuring cylinder
2. Start reaction
3. Vol of gas at different time intervals
4. Vol of gas against time

Question 5

Calculate rate by measuring the mass if a gaseous product over time

Answer 5

1. Place a flask containing one reactant on a balance and record mass
2. Add second reactant and start clock
3. Mass at different time intervals
4. Graph of mass against time

Question 6

How do you make a buffer solution

Answer 6

1. Use burette to measure out calculated volumes of solutions containing equimolar amounts of weak acid and its salt and add to beaker
2. Mix using magnetic stirrer
3. Record pH using pH meter

Question 7

Determine the shape of a titration curve by measuring the pH

Answer 7

1. Record pH os solution - pH paper
2. Fill burette - known conc
3. Add in 5cm^3 from burette and mix w magnetic stirrer
4. Record pH after each addition
5. Add in 0.5cm^3 near end point
6. Add two 5cm^3 portions after end point
7. Graph of pH against volume of solution added

Question 8

What is the method for preparing, recrystallising and determining the melting point of 2,4-dinitrophenylhydrazones

Answer 8

See notes

Question 9

Use Fehling’s solution and Tollens’ reagent to distinguish between aldehydes and ketones

Answer 9

1. Add a few drops of unknown compound to 1cm^3 of freshly prepared Tollens’ reagent in clean test tube
2. Warm in hot water bath
3. Freshly prepare Fehling’s solution - mix 1cm^3 of Fehling’s solution A with 1cm^3 of Fehling’s solution B
4. Add few drops of unknown compound - warm in water bath

Question 10

What is the method for preparing carboxylic acid from an alcohol

Answer 10

See notes

Question 11

How do you prepare a liquid Ester from a carboxylic acid and an alcohol

Answer 11

See notes

Question 12

How do you prepare methyl 3-nitrobenzoate

Answer 12

See notes

Question 13

Titrating iodine with sodium thiosulfate solution using starch and hence estimate oxidising agents by their reaction with excess acidified potassium iodide

Answer 13

1. Rinse pipette with sodium iodate(V) solution, using pipette filler, pipette 25cm^3 of standard potassium iodate(V) solution into conical flask
2. Add approximately 1.5g of potassium iodide (excess) and excess sulfuric acid
3. Rinse burette with standard sodium thiosulfate, fill burette w it
4. Add standard sodium thiosulfate from burette until solution is straw coloured, add 3 drops of starch indicator
5. Titration until blue-black to colourless
6. Repeat until results are concordant

Question 14

Titrating acidified potassium manganate(VII) solution with reducing agents

Answer 14

1. Rinse pipette with solution of reducing agent and use pipette to transfer 25cm^3 to conical flask
2. Rinse burette w standard solution of potassium manganate(VII) and then fill burette
3. Add excess sulfuric acid using measuring cylinder to conical flask
4. Add potassium manganate(VII) solution from burette into conical flask and swirl until colourless to pink
5. Repeat until results are concordant

Question 15

Determine purity of a group II metal oxide or carbonate by back titration

Answer 15

1. Weigh out mass of the solid
2. Add excess HCl
3. Add to 250 volumetric flask - make up w deionised water until meniscus on mark
4. Rinse pipette with solution from volumetric flask and pipette 25cm^3 to conical flask
5. 3 drops phenolphthalein
6. Rinse burette with alkali and fill
7. Add alkali until indicator changes colour
8. Repeat until concordant

Question 16

Carrying out paper and thin-layer chromatography and measure the Rf values of the components and interpret the chromatogram

Answer 16

1. Base line in pencil close to bottom
2. Spot samples on line using capillary tube - dry - repeat till spot concentrated
3. Place paper in tank with shallow amount of solvent - cover with lid - allow solvent to run up over spots until solvent almost reaches top - mark solvent front - allow to dry
4. If substances are colourless then spray with locating agent eg ninhydrin
5. Measure dist travelled by the solvent front and each spot - calculate the Rf

Question 17

Two-way chromatography

Answer 17

Chromatography method then

1. Rotate through 90°
2. Run in second solvent
3. Locating agent if colourless
4. Rf

Question 18

Carrying out the reduction of acidified ammonium vanadate(V) with zinc and observing the sequence of colours

Answer 18

1. Dissolve some ammonium vanadate(V) in HCl in conical flask
2. Add two spatulas of zinc as reducing agent
3. Stopper with cotton wool, allow H to escape and slow entry of air
4. Observe sequence of colours

Question 19

Determining the electrode potentials of a series of cells and predicting their values using standard electrode potentials

Answer 19

1. Set up a half cell - a metal dipping in 1M solution of its ions in a beaker
2. Attach via wires, and a voltmeter to a second half-cell using a different metal and a 1M solution of its ions
3. Use salt bridge to connect two beakers
4. Record the voltage

Question 20

Preparing aspirin using salicylic acid and ethanoic anhydride

Answer 20

1. Place 1g of 2-hydroxybenzoic acid in a dry pear shaped flask and add 2cm^3 of ethanoic anhydride
2. Safely add 8 drops of concentrated phosphoric(V) acid
3. Heat under reflux for 30 mins
4. Add water to hydrolyse any unreacted ethanoic anhydride to ethanoic acid
5. Pour the mixture onto 400g of crushed ice in a beaker
6. Remove product by suction filtration
7. Recrystallise from water
8. Dry in low temp oven
9. Determine melting point

Question 21

Using chromatography to compare the purity of laboratory-made aspirin with commercial tablets

Answer 21

1. Base line with pencil close to bottom and draw two pencil crosses on it
2. Place some laboratory-made aspirin solid on watch glass and dissolve in a few drops of solvent such as ethanol - spot solvent on pencil cross using capillary tube, dry and repeat until conc - repeat for commercial solid
3. Place in solvent, cover with lid, run up until near top - mark solvent front and allow to dry
4. Spots visible by placing in beaker containing iodine crystals
5. Rf

Question 22

How do you determine something’s melting point

Answer 22

1. Place solid in capillary tube/melting point tube sealed at one end
2. Heat slowly
3. Record temp at which solid starts and finishes melting
4. Repeat and average the temperatures
5. Compare temp with known values in data book