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A-Level Chemistry > Practical skills > Flashcards

Practical skills Flashcards

1
Q

Describe a method for a titration

A
  • Accurately pipette 25cm^3 of alkali into conical flask
  • Touch the surface of the alkali with pipette
  • Add a few drops of the appropriate indicator
  • Fill a burette with a standard solution of acid using a funnel at eye level
  • add the acid into the alkali whilst swirling the mixture
  • add the acid dropwise at the end point
  • note the reading on the burette
  • repeat the titration to get 2-3 concordant results
  • calculate an average from the concordant results
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2
Q

Describe a method for making a solution

A
  • Weigh a beaker using a top pan balance
  • Add the solid into the beaker and weigh them
  • Record the difference in mass
  • Add 100 cm^3 of distilled water into the beaker
  • Stir with a glass rod until the solid has dissolved
  • Transfer the solution using a funnel into a volumetric flask
  • Rinse the beaker and the funnel and add washings from the beaker and glass rod into the volumetric flask
  • Add distilled water up to the graduation mark
  • make sure that the bottom of the meniscus touches the graduation mark
  • stopper the flask and shake and invert it several times to ensure a uniform solution
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3
Q

Suggest a reason for removing the funnel from the burette after it has been used

A

A droplet from the funnel could enter the burette affecting the volume/readings/titre

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4
Q

Suggest one other source of error in using the burette

A

Air bubbles

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5
Q

Why is it important to fill the space below the tap in the burette before starting the titration?

A

The space would fill during the titration

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6
Q

Why is a 250cm^3 conical flask preferred to a 250cm^3 beaker for a titration?

A

It is easier to swirl the contents in a conical flask without spilling the contents

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7
Q

Suggest a reason why repeating a titration makes the value for the average titre more reliable

A
  • Anomalies can be eliminated
  • Concordant results can be collected
  • An average can be calculated
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8
Q

What effect would an air bubble have on the value of the first titre?

A

The value would be higher than the true value

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9
Q

What effect would an air bubble have on the average titre?

A
  • No effect

- The titre value can be discarded

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10
Q

How can the percentage uncertainty be calculated?

A

Percentage uncertainty = error/measurement * 100

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11
Q

How can the percentage uncertainty of an apparatus be reduced?

A
  • Using a larger mass so that you can have a larger titre/reading
  • Using a more concentrated solution of the alkali so that a larger volume of the acid can be used
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12
Q

What could be done to obtain an accurate average titre for a titration?

A
  • conduct more experiments

- obtain concordant results

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13
Q

Suggest ideas on how the overall technique can be improved when making a solution

A
  • weigh by difference
  • wash the stirring rod/beaker after use
  • shake the final solution
  • make the sure the bottom of the meniscus touches the mark
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14
Q

What affects the volume of a gas?

A

pressure

temperature

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15
Q

What are the potential errors in using a gas syringe?

A

gas escapes before bung is inserted
syringe sticks
some gases like carbon dioxide and sodium dioxide are soluble in water

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16
Q

why are only a few drops of indicator added to solutions?

A

Indicators are weak acids and too much of them can affect the titration result

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17
Q

What does it mean when the results are concordant?

A

the results are accurate and reproducible

the technique is good/consistent

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18
Q

Why is it necessary to test several samples when testing batches?

A

The amount/concentration of the chemical being tested may vary between samples

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19
Q

How does the lid on a crucible improve the accuracy of the experiment?

A

It prevents loss of the solid

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20
Q

When are equal volumes of gas with an equal number of molecules measured?

A

When the gases are measured under the same conditions

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21
Q

Why doesn’t the Mr need to be exact to turn an empirical formula into the molecular formula?

A

The molecular formula will be a whole number multiple of the empirical formula

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22
Q

Describe a method for removing the water of crystallisation from calcium sulphate crystals

A

Weigh an empty, clean, dry crucible and lid
Add 2g of hydrated calcium sulphate to the crucible and weigh them
Heat strongly with a bunsen burner for a couple of minutes
Allow cooling
Weigh the crucible and contents again
Heat the crucible again and reweigh until you reach a constant mass

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23
Q

Why are large amounts of hydrated calcium sulphate not used in the experiment?

A

The decomposition is likely to be incomplete

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24
Q

Why are small amounts of hydrated calcium sulphate not used in the experiment?

A

Percentage uncertainties in weighing will be too high

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25
Q

Describe a method of using a gas syringe to calculate the Mr of propanone

A

Extract 0.20cm^3 of propanone into a hypodermic syringe and then measure the mass of the syringe
Using hand protection, remove a gas syringe from the oven and note the volume of air already in the barrel
inject the propanone though the self-seal cap into the barrel. the plunger will move straight away.
Put the gas syringe back into the oven
Measure the mass of the empty hypodermic syringe immediately
After a few minutes measure the volume of the gas in the gas syringe, record the temperature of the oven shelf and the pressure of the room

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26
Q

Describe a method to dilute a solution

A

Pipette 25cm^3 of the original solution into a 250cm^3 volumetric flask
Makeup to the mark with distilled water using a pipette for the last few drops
Invert the flask several times to ensure a uniform solution

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27
Q

What is the colour change of phenolphthalein?

A

pink in alkali

colourless in acid

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28
Q

What is the colour change of methyl orange?

A

red in acid

yellow in alkali

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29
Q

Why is it important for the burette to be rinsed out before a titration?

A

The acid or alkali added may be diluted by residual water or may react with substances left from a previous titration. The concentration of the substance would be lowered and a larger titre would be used.

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30
Q

Why is distilled water added to the conical flask during a titration to wash the sides of the flask?

A

the water does not react with the reagents or change the number of moles of the acid/alkali added

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31
Q

Outline the equation for the manganate redox titration with Fe2+ ions

A

MnO4- + 8H+ + 5Fe2+ → Mn2+ + 4H2O + 5Fe3+

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32
Q

Why is an indicator not used in the manganate redox titration?

A

the titration is self indicating because of the significant colour change from reactant to product

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33
Q

Describe a procedure for finding out how much iron is in iron tablets

A

Weigh accurately two ferrous sulphate tablets
Grind up the tablets with a little 1M sulphuric acid, using a pestle and mortar
Through a funnel, transfer the resulting pastel into a 100cm^3 volumetric flask. Use further small volumes of 1M of sulphuric acid to rinse the ground up tablets into the flask
Then add sufficient 1M sulphuric acid to make up the solution to exactly 100cm^3. Stopper the flask and shake it to make sure that all the contents are thoroughly mixed
Titrate 10cm^3 portions of the solution with 0.0050M potassium manganate

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34
Q

What would be the result of adding insufficient sulphuric acid?

A

the solution would not be acidic enough
MnO2 will be produced
MnO4- 4H+ + 3e- → MnO2 + 2H2O
the brown MnO2 will mask the colour change and lead to a greater volume of manganate being used in the titration

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35
Q

Why can conc. HCl not be used in the manganate redox titration?

A

the Cl- ions will be oxidised to Cl2 by the manganate ions

a greater volume of manganate will be used and poisonous Cl2 will be produced

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36
Q

Why can nitric acid not be used in the manganate redox titration?

A

It is an oxidising agent. The Fe2+ ions will be oxidised to Fe3+ ions
a smaller volume of manganate will be used

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37
Q

What is the overall equation for the manganate titration with hydrogen peroxide?

A

2MnO4(-) + 6H+ + 5H2O2 → 5O2 + 2Mn(2+) + 8H2O

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38
Q

What is the overall equation for the manganate titration with ethanedioate?

A

2MnO4(-) + 16H+ + 5C2O4(2-) + 10CO2 + 2Mn(2+) + 8H2O

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39
Q

Why is the reaction between the manganate and ethanedioate ions slow to begin with?

A

the reaction is between two negative ions

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40
Q

What is the conclusion made if the %uncertainty due to the apparatus is < %difference between the actual and calculated value?

A

there is a discrepancy in the result due to other errors

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41
Q

What is the conclusion made if the %uncertainty due to the apparatus is > %difference between the actual and calculated value?

A

there is no discrepancy and all errors in the results can be explained by the sensitivity of the equipment

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42
Q

Outline a procedure to find the enthalpy change of neutralisation of a solution

A

Add a known volume of acid to an insulated container and measure the temperature
Then add a known volume of alkali and record the temperature of the mixture at regular intervals over a period of time
Find the temperature change for the experiment. Use it to calculate the enthalpy change of the reaction.

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43
Q

How is an accurate temperature obtained?

A

Record the temperature at regular intervals before the start of the reaction
Plot the temperature against the time
Extrapolate the graph to when the reactants are added together

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44
Q

Identify the errors in calorimetry

A
  • energy transfer from surroundings
  • the method assumes that all solutions have the same specific heat capacity of water
  • the specific heat capacity of the calorimetry is neglected
  • incomplete reaction
  • density of the solution is taken to be the same as water
45
Q

Outline a method to measure the enthalpy change of solution of anhydrous copper(II) sulphate

A

-Weigh out between 3.90g and 4.10g of anhydrous copper(II) sulphate in a dry weighing bottle. Use weighing by difference to record an accurate mass
-Using a volumetric pipette, place 25cm^3 of deionised water into a polystyrene cup and record its temperature at the beginning, start the timer and then record the temperature again every minute,stirring the liquid continuously.
-At the forth minute, add the powdered anhydrous copper(II) sulphate rapidly to the polystyrene cup and stir.
Record the temperature every minute up to 15 minutes continuously stirring the mixture
Plot a graph of temperature against time
Use the graph to find the temperature change
use q=mc(change in)T
chang in H = q/moles

46
Q

Why can the enthalpy change of forming a hydrated salt not be done experimentally?

A

It is impossible to add the exact amount of water without the solid dissolving and it is not easy to measure the temperature change of a solid

47
Q

Identify the errors in measuring enthalpies of combustion using flame calorimetry

A
  • energy losses from calorimetry
  • incomplete combustion of the fuel
  • incomplete transfer of energy
  • evaporation of fuel after weighing
  • heat capacity of calorimeter not accounted for
  • measurements not carried out under standard conditions
48
Q

Outline a method for the reaction between sodium thiosulphate and hydrochloric acid

A

Measure out fixed volumes of sodium thiosulphate hydrochloric acid using a measuring cylinder
Use a water bath to gently heat both solutions to the desired temperature before you mix them
Mix the solutions in a conical flask. Place the flask over a black cross which can be seen through the solution. Watch the black cross disappear through the cloudy sulfur and time how long it takes to disappear.
Repeat the experiment for different temperatures keeping the volume and concentrations the same

49
Q

What is the equation for the reaction between sodium thiosulphate and hydrochloric acid?

A

Na2S2O3 + 2HCl → 2NaCl + SO2 + S + H2O

50
Q

How would you test for group 2 ions?

A

Add 10 drops of 0.1 moldm^3 of the metal ion solution in a test tube.
Add about 10 drops of 0.6moldm^3 of sodium hydroxide into the test tube.
Add sodium hydroxide until excess
OR
Add 10 drops of 0.1 moldm^3 of the metal ion solution in a test tube.
Add about 10 drops of 1.0moldm^3 of sulphuric acid into the test tube.
Add sulphuric acid until excess

51
Q

What observations would be made with group 2 hydroxides?

A

the solubility of the hydroxides increases down group 2

magnesium and calcium solutions would produce white precipitates

52
Q

What observations would be made with group 2 hydroxides?

A

the solubility of the sulphates decreases down the group

strontium and barium solutions produce white precipitates

53
Q

How would you test for ammonium ions?

A
  • Add 10 drops of 0.1 moldm^3 of ammonium chloride in a test tube.
  • Add about 10 drops of 0.4 moldm^3 of sodium hydroxide solution into the test tube and shake the mixture.
  • Warm the mixture in the test tube gently using a water bath
  • Test the fumes released from the mixture by holding a piece of damp red litmus paper in the mouth of the test tube.
  • Alkaline ammonium gas turns the red litmus paper blue
54
Q

How would you test for sulphate ions?

A

BaCl2 solution acidified with HCl

white precipitate of BaSO4 formed

55
Q

Why is HCl added when testing for sulphate ions?

A

The HCl reacts with carbonate impurities that may give a false positive result

56
Q

How would you test for halide ions?

A

acidified silver nitrate solution added dropwise into a container containing halide ions

57
Q

Which observations would be made when testing for halide ions?

A

Flourides - no precipitate
Chlorides - white precipitate
Bromides - cream precipitates
Iodides - yellow precipitates

58
Q

How can silver halide precipitates be differentiated?

A

Silver chloride dissolves in dilute ammonia
Silver bromide dissolves in concentrated ammonia
Silver iodide is insoluble

59
Q

How would you test for the presence of carbonate ions?

A

Add any dilute acid and observe effervescence

Bubble the gas through limewater to test for carbon dioxide- the limewater will turn cloudy

60
Q

How would you test for hydroxide ions?

A

Alkaline hydroxide ions will turn red litmus paper blue

61
Q

Give the equations and observations for the reactions between NaF and NaCl with conc. sulphuric acid

A

NaF + H2SO → NaHSO4(s) + HF(g)
Observations: White steamy fumes of HF
NaCl(s) + H2SO4(l) → NaHSO4(s) + HCl(g)
Observations: White steamy fumes of HCl

62
Q

Give the equations and observations for the reactions between NaBr and conc sulphuric acid

A 
NaBr + H2SO4 → NaHSO4 + HBr
2HBr + H2SO4 → Br2+ SO2+ 2H2O
Observations:
 white steamy fumes of HBr
red fumes of Bromine
colourless, acidic gas of sulphur dioxide
63
Q

Give the equations and observations for the reactions between NaI and conc sulphuric acid

A 
NaI + H2SO4→ NaHSO4 + HI
2HI + H2SO4 → I2 + SO2 + 2H2O
6HI + H2SO4 → 3 I2 + S + 4 H2O
8HI + H2SO4 → 4I2 + H2S + 4H2O
Observations:
white steamy fumes of HI
black sold and purple fumes of iodine
colourless, acidic gas of SO2
yellow solid of sulphur
H2S, a gas with a rotten egg smell
64
Q

Give the reagent than can be used for the partial oxidation of a primary alcohol and the observation

A

acidified potassium dichromate

orange to green

65
Q

Describe a method for the partial oxidation of propan-1-ol

A

-Place about 10 cm3 of dilute sulfuric acid in a flask and add about 3g of potassium dichromate(VI) and 2 or 3 anti- bumping granules. Shake the contents of the flask until solution is complete.
-Add 1.5 cm3 of propan-1-ol in drops from a dropping pipette, shaking the flask so as to mix the contents
-Use a distillation apparatus to carry out the experiment
-Gently heat and slowly distil 2 cm3 of liquid into a test tube, taking care that none of the reaction mixture
splashes over

66
Q

Why are bunsen burners not used to heat the mixture during a distillation?

A

organic chemicals are normally highly flammable and could be set on fire with a naked flame

67
Q

Give the reagent and observation for the full oxidation of primary alcohols

A

acidified potassium dichromate
orange to green
heat under reflux

68
Q

Give a method for the full oxidation for the propan-1-ol

A

-Measure 5 cm3 of water into a boiling tube. Add 6 g of sodium dichromate(VI), shake and set aside to dissolve.
-Put about 1.5 cm3 propan-1-ol into a 50 cm3 round bottomed flask and add about 5 cm3 of water and two or
three anti-bumping granules. Put a condenser on the flask for reflux.
-Add 2 cm3 of concentrated sulfuric acid down the condenser in drops from a dropping pipette. While the
mixture is still warm, start to add your sodium dichromate(VI) solution down the condenser in drops from a
dropping pipette. The energy released from the reaction should make the mixture boil. -Add the solution a drop
at a time so that the mixture continues to boil without any external heating.
-When all the sodium dichromate(VI) solution has been added, use a low Bunsen burner flame to keep the
mixture boiling for 10 minutes, not allowing any vapour to escape. -At the end of that time remove the Bunsen burner and arrange the apparatus for distillation. Gently distil 2-3
cm3 of liquid into a test tube.

69
Q

During a reflux experiment, why must the end of the condenser not be sealed?

A

as gas builds up, the pressure could cause the apparatus to explode

70
Q

Why are anti-bumping granules added to the apparatus?

A

To prevent vigorous, uneven boiling by making smaller bubbles

71
Q

Outline a method for fractional distillation in the lab

A

Heat the flask, with a Bunsen burner causing the vapours of all the components in the mixture
to be produced. Vapours pass up the fractionating column. The vapour of the substance with the lower boiling point
reaches the top of the fractionating column first.
The thermometer should be at or below the boiling point
of the most volatile substance. • The vapours with higher boiling points condense back
into the flask.
Only the most volatile vapour passes into the condenser. The condenser cools the vapours and condenses to a
liquid and is collected

72
Q

How would you test for an alkene?

A

Bromine water

observation: orange to colourless

73
Q

How would you test for an aldehydes and ketones?

A

Aldehyde
Tollen’s reagent - silver mirror
Fehling’s solution - brick red precipitate

Ketones
no visible change for both tests

74
Q

How would you test for a carboxylic acid?

A

Add sodium carbonate to a solution

Effervescence would be observed if a carboxylic acid was present as carbon dioxide is produced

75
Q

What is the initial rate of a reaction?

A

It is the rate right at the start of the reaction. This can be found by calculating the gradient of the tangent at t=0.

76
Q

How does the initial rates method work?

A

Repeat an experiment several times using different initial concentrations of the reactions, changing of the concentrations at a time, keeping the rest constant.
Calculate the initial rate of each experiment
Use the initial rates and concentrations to work out the orders of each reactant to work out the rate equation.

77
Q

What are the equations for the iodine clock reaction?

A

H2O2(aq) + 2H+(aq) + 2I–(aq) → I2(aq) + 2H2O(l)

2S2O3(2-)(aq) + I2(aq) → 2I(–)(aq) + S4O6(2–)(aq)

78
Q

What are the different techniques that can be used to investigate the rates of reactions?

A

measure:

  • the change in volume of a gas
  • the change of mass
  • colorimetry
  • the change in electrical conductivity
  • optical activity
79
Q

Outline a method for measuring the EMF of an electrochemical cell with zinc and copper sulphate solutions

A

-Clean the zinc and copper foils with emery before use.
-Degrease the metal using some cotton wool and
propanone.
-Place the copper strip into a 100 cm3 beaker with
about 50 cm3 of 1 mol dm–3 CuSO4 solution.
-Place the zinc strip into a 100 cm3 beaker with about
50 cm3 of 1 mol dm–3 ZnSO4 solution.
-Use a strip of filter paper soaked in saturated
potassium nitrate solution for the salt bridge
-Connect the Cu(s)|Cu2+
(aq) and Zn(s)|Zn2+
(aq) half- cells by connecting the metals using the crocodile clips
and leads provided to the voltmeter

80
Q

What is the salt bridge usually made of?

A

A piece of filter paper soaked in potassium nitrate

81
Q

What is the purpose of the salt bridge?

A

allows for the transfer of ions to balance the charge

82
Q

Outline a method for constructing a pH curve

A
  1. Transfer 25cm3 of acid to a conical flask with a volumetric
    pipette
  2. Measure initial pH of the acid with a pH meter
  3. Add alkali in small amounts (2cm3
    ) noting the volume
    added
  4. Stir mixture to equalise the pH
  5. Measure and record the pH to 1 d.p.
  6. Repeat steps 3-5 but when approaching endpoint add in
    smaller volumes of alkali
  7. Add until alkali in excess
83
Q

Describe how to carry out recrystallisation to purify an organic solid

A

-Dissolve the impure solid in a minimum volume of a hot solvent- this should give a saturated solution of the impure solid
Filter the hot solution through a heated funnel to remove any insoluble impurities
Leave the solution to cool down slowly. Crystals of the product will form as as it cools
Remove the liquid containing the soluble impurities from the crystals by filtering the mixture under reduced pressure.
Pour the mixture into a filter paper lined Buchner funnel, that’s sitting in a side arm flask attached to a vacuum line.
Finally, wash the crystals with ice-cold solvent to remove any soluble impurities from their surface. Leave the purified crystals to dry.

84
Q

How can impurities affect the melting and boiling points of pure substances?

A

Pure substances have specific melting and boiling points.
Impurities lowers the melting point and the boiling point is raised.
The melting and boiling points occur across a wide range of temperatures.

85
Q

Outline a method that can be used to determine the purity of a substance

A
  • Pack a small sample of the solid into a glass capillary tube and place it inside the heating element of the melting point apparatus.
  • Increase the temperature until the sample turns from solid to liquid
  • Measure the melting range from where the solid begins to melt to where it completely melts.
  • Compare the measured melting point to the data values
  • Impurities lower the melting point and broaden the melting range
86
Q

Describe a method for the preparation of aspirin

A

Add to a 50 cm3 pear-shaped flask 2.0 g of 2-hydroxybenzoic acid and 4 cm3 of ethanoic anhydride.
To this mixture add 5 drops of 85% phosphoric(v) acid and swirl to mix.
Fit the flask with a reflux condenser and heat the mixture on a boiling water bath for about 5 minutes.
Without cooling the mixture, carefully add 2cm3 of water in one portion down the condenser.
When the vigorous reaction has ended, pour the mixture into 40 cm3 of cold water in a 100 cm3 beaker, stir and rub the sides of the beaker with a stirring rod necessary to induce crystallisation and, finally, allow the mixture to stand in an ice bath to complete crystallisation.
Collect the product by suction filtration and wash it with a little water.

87
Q

Describe a method for the purification of aspirin

A

Using a measuring cylinder, measure out 15 cm3 of ethanol into a boiling tube.
Prepare a beaker half-filled with hot water from a kettle at a temperature of approximately 75 °C.
Use a spatula to add the crude aspirin to the boiling tube with ethanol and place the tube in the beaker of hot water.
Stir the contents of the boiling tube until all of the aspirin dissolves into the ethanol.
Pour the hot solution containing dissolved aspirin through a warmed filter funnel and fluted filter paper to hot filter
Then pour filtrate into 40 cm3 of water in a conical flask.
Allow the conical flask to cool slowly and white needles of aspirin should separate.
Cool the whole mixture in an ice bath.
Filter off the purified solid under reduced pressure and allow it to dry on filter paper. Record the mass of the dry purified solid

88
Q

Describe a separation technique that removes water soluble impurities from a product

A

Pour the mixture into a separating funnel and add water
Shake the funnel and then allow it to settle
The organic layer and aqueous layer are immiscible and so separate out into two distinct layers
then open the tap and run each layer off into a separate container

89
Q

Outline solvent extraction

A

Take an organic solvent in which the product is more soluble than it is in water.
Add it to the impure product solution and shake well
The product will dissolve into the organic solvent, leaving the impurities dissolved in the water. The solvent containing the product can then be run off using a separating funnel

90
Q

How would you purify a product by drying it?

A

Add an anhydrous salt such as magnesium sulphate. The salt is used as a drying agent-it binds to any water present to become hydrated.
first, add the salt to the organic layer and it will clamp together. Keep adding the drying agent until disperses evenly when you swirl the flask.
Filter the mixture to remove the solid drying agent- place a piece of filter paper into a funnel that feeds into a flask and pour the mixture into the filter paper

91
Q

Describe a method for preparing and purifying cyclohexene from cyclohexanol

A

a) Pour about 20 cm3 of cyclohexanol into a weighed 50 cm3 pear-shaped flask. Reweigh the flask and record
the mass of cyclohexanol.
b) Using a plastic graduated dropping pipette, carefully and with frequent shaking, add to the flask
approximately 8 cm3 of concentrated phosphoric acid.
c ) Add a few anti-bumping granules to the flask and assemble the distillation apparatus, so that the contents
of the flask may be distilled. Heat the flask gently, distilling over any liquid which boils below 100 °C.
d) Pour the distillate into a separating funnel and add 50 cm3 of saturated sodium chloride solution. Shake
the mixture and allow the two layers to separate.
e) run off the lower layer into a beaker and then transfer the upper layer, which contains the crude cyclohexene, into a small conical flask.
f) Add a few lumps of anhydrous calcium chloride or anhydrous sodium sulfate(VI) or anhydrous magnesium
sulfate to the crude cyclohexene to remove water. Stopper the flask, shake the contents and allow this to
stand until the liquid becomes clear.
g) Decant the liquid into a clean, dry, weighed sample container.
h) Reweigh the container, calculate the mass of dry cyclohexene produced

92
Q

Describe a method for the preparation and purification of an ester e.g. propyl ethanoate

A
  1. Propan-1-ol (50 cm3) and ethanoic acid (50 cm3) are mixed thoroughly in a 250 cm3 round-bottomed flask.
  2. Concentrated sulfuric acid (10 cm3) is added drop by drop to the mixture, keeping the contents of the flask well-shaken and cooled in an ice-water bath.
  3. When the acid has all been added, a reflux condenser is fitted to the flask and the mixture gently boiled over an electric heating mantle for about 30 minutes.
  4. The mixture is cooled, and the apparatus rearranged for distillation. The crude ester (about 60 cm3) is distilled off.
  5. The distillate is placed in a separating funnel and shaken with about half its volume of 30% sodium carbonate solution, with the pressure being released at intervals. The lower aqueous layer is then discarded.
  6. The crude ester is shaken in a separating funnel with about half its volume of 50% calcium chloride solution, which removes unreacted alcohol. The lower layer is discarded.
  7. The ester is run into a clean, dry flask containing some
    anhydrous calcium chloride and swirled.
  8. The ester is filtered into a clean, dry flask, with a few anti-bumping granules, and distilled. The fraction boiling between 100°C and 103°C is collected.
93
Q

Outline a method for carrying out thin-layer chromatography(TLC)

A

a) Wearing gloves, draw a pencil line 1 cm above the bottom of a TLC plate and mark spots for each sample, equally spaced along line.
b) Use a capillary tube to add a tiny drop of each solution to a different spot and allow the plate to air dry.
c) Add solvent to a chamber or large beaker with a lid so that is no more than 1cm in depth
d) Place the TLC plate into the chamber, making sure that the level of the solvent is below the pencil line. Replace the lid to get a tight seal.
e) When the level of the solvent reaches about 1 cm from the top of the plate, remove the plate and mark the solvent level with a pencil. Allow the plate to dry in the fume cupboard.
f) Place the plate under a UV lamp in order to see the spots. Draw around them lightly in pencil.
g) Calculate the Rf values of the observed spots.

94
Q

How are the Rf values calculated?

A

Rf value= distance moved by amino acid/ distance moved by solvent front

95
Q

Why is it necessary to wear gloves during a TLC experiment?

A

to prevent contamination from the hands to the plates

96
Q

Why is a pencil used instead of a pen in TLC?

A

it will not dissolve in the solvent

97
Q

Why is it necessary to use a small drop in TLC?

A

too big a drop will cause different spots to merge

98
Q

Why is a lid used in TLC?

A

to prevent the evaporation of the toxic solvent

99
Q

Why is the depth of the solvent important?

A

if the solvent is too deep, it will dissolve the sample spots from the plate

100
Q

What precautions should be taken when doing a TLC?

A

work in a fume cupboard if the solvent is toxic or flammable fumes from escaping the room

101
Q

How can colourless chemicals be revealed?

A

UV light

iodine- spots appear purple

102
Q

How can chromatography separate chemicals?

A

Separation by chromatography depends on the balance between the solutility in the mobile phase and the retention in the stationary phase

103
Q

How can spectrometry be used to determine the concentration of coloured ions

A

Add an appropriate ligand to intensify the colour
Make up solutions of known concentrations
Measure absorption or transmission
Plot graph of absorption against concentration
Measure the absorption of an unknown solution and determine its concentration from the calibration curve

104
Q

Outline a method to identify metal ions using test tube reactions

A

Measure out samples of the unknown metal ion solution into three separate test tubes
in the first test tube, add sodium hydroxide solution dropwise using a pipette and record the observations. Then add until in excess and record the observations
in the second test tube, add ammonia solution dropwise and record any changes.
Then add the ammonia until in excess and record any changes
In the third test tube, add sodium carbonate solution dropwise and record the observations

105
Q

Give the observations when [Cu(H2O)6]2+ reacts with OH- or NH3, excess OH-, excess NH3 and Na2CO3

A

OH- or NH3 - blue precipitate Cu(OH)2(H2O)4
excess OH- - no change
excess NH3 - deep blue solution [Cu(NH3)4(OH)2]2+
Na2CO3 - green-blue precipitate CuCO3

106
Q

Give the observations when [Fe(H2O)6]2+ reacts with OH- or NH3, excess OH-, excess NH3 and Na2CO3

A

OH- or NH3 - green precipitate Fe(OH)2(H2O)4
excess OH- - no change
excess NH3 - no change
Na2CO3 - green precipitate FeCO3

107
Q

Give the observations when [Al(H2O)6]3+ reacts with OH- or NH3, excess OH-, excess NH3 and Na2CO3

A

OH- or NH3 - white precipitate Al(OH)3(H2O)3
excess OH- -colourless solution [Al(OH)4(H2O)2]-
excess NH3 - no change
Na2CO3 - white precipitate Al(OH)3(H2O)3

108
Q

Give the observations when [Fe(H2O)6]3+ reacts with OH- or NH3, excess OH-, excess NH3 and Na2CO3

A

OH- or NH3 - brown precipitate Fe(OH)3(H2O)3
excess OH- - no change
excess NH3 - no change
Na2CO3 - brown precipitate Fe(OH)3(H2O)3

A-Level Chemistry (30 decks)

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