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A-level Chemistry > Practical Techniques > Flashcards

Practical Techniques Flashcards

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1
Q

Draw out a diagram for reflux and give the reaction conditions.

A 
Reaction conditions:
Heat
Antibumping granules
Alcohol
Acidified potassium dichromate (oxidising agent)
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2
Q

What are the uses of anti bumping granules?

A

Used to prevent the formation of large bubbles and reduced boiling vigorously. Also prevents splash and therefore loss of mixture.

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3
Q

What happens when heating under reflux?

A

Mixture is heated to boiling point for a prolonged time.
Vapour is formed which escapes the evaporation mixture. Then this condensed back into a liquid and returns to the mixture.
Any original solution or part completion must be oxidised to ensure completion.

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4
Q

Draw the apparatus of distillation.

Explain the importance of the thermometer and the ice bath.

A

Thermometer- identifies the chemicals separated based on their boiling points.

Ice bath- ensures that the mixture once condensed doesn’t evaporate off.

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5
Q

Why is distillation used?

A

To separate an organic product from a reaction mixture.

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6
Q

Why do we use melting point analysis?

A

All pure chemicals have a distinct melting and boiling point.

Pure samples have a precise melting point.
Impure samples will have an average range.

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7
Q

Draw the apparatus for melting point analysis

A

Thiele tube

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8
Q

What are the steps taken in melting pint analysis?

A
  1. Place a few grains of organic product in a melting point tube ( melted capillary tube)
  2. Attach the capillary tube to a thermometer using a rubber band:
  3. Submerge the capillary tube and thermometer in the oil or a thiele tube (not the rubber band)
  4. Using a micro Bunsen burn r heat the side arm of the thiele tube. Heat slowly when approaching melting point.
  5. Note the temperature when the substance starts and stops melting.
  6. Compare melting point to data book value to determine purity.
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9
Q

What observations would you make when doing melting point analysis?

What is the main error that may occur?

A

Impure sample:
Have a wide range for melting point.
Melting point is below/before the true melting point.

Pure sample:
Melt at lower temperatures due to inaccuracies with the equipment.
( eg. thermometer reading a different temperature to the reaction mixture)

Temperature on the thermometer may not be reading the correct temperature for the actual sample in the melting point tube.

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10
Q

When do we carry out filtration under reduced pressure?

A

When we carry out synthesis of a chemical, the organic product will often crystallise out of the mixture.

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11
Q

Draw the apparatus for filtration under reduced pressure.

A

Büchner funnel
Filter paper
Side arm flask

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12
Q

What are the steps in filtration under reduced pressure?

A
  1. Dissolved crystals are placed in the minimum hot solvent
  2. Allow to cool/crystallise (scrape side of beaker to encourage crystallisation)
  3. Pour into funnel and rinse the beaker with distilled water. Allow vacuum to suck through the liquid.
  4. Rinse through their cold solvent to remove soluble impurities (repeat)
  5. Leave to suck through for a while to remove moisture.
  6. Put in a warm place or desiccator.
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13
Q

What is the purpose of doing recrystallisation?

A

To purify products after synthesis

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14
Q

What are the steps in recrystallisation?

A
  1. Dissolve the sample in the LEAST amount of HOT SOLVENT possible.
  2. Carry out gravity filtration as soon as possible on the hot solvent using filter paper.
  3. Allow to cool and crystallise, can use scratch beakers to encourage crystallisation./ place in an ice bath.
  4. Collect the product (purified) using vacuum filtration washing the crystals with cold solvent to remove and excess filtrate and impurities.
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15
Q

Explain the purpose of using:

LEAST amount of HOT SOLVENT

Filtered hot

Cooled in and ice bath

Washed with cold solvent/water

A

LEAST amount of HOT SOLVENT- do only the desired product is dissolved and no other impurities are ( to increase yield) and using hot solvent increases the solubility of the product.

Filtered hot- to remove any insoluble impurities

Cooked in an ice bath- increased number of crystals forming (encourage crystallisation)

Washing with cold solvent/water- remove any soluble impurities

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16
Q

How do you make up a solution?

A
  1. Weigh the mass of the solid to be tested on a 2 d.p balance.
  2. Transfer the solid to a beaker and ensure all contents of the solid are transferred by rinsing the weighing boat with distilled water.
  3. add 100cm^3 of distilled water to the beaker and use a glass rod to stir and dissolve the solid.
  4. Rinse of the sides of the beaker and the stiring rod to ensure all the solid is dissolved into the water.
  5. Pour the solution into a 250cm^3 graduated flask using a funnel. Rinse the beaker, rod and funnel with distilled water into the volumetric flask ensuring all solution is present.
    Make the solution up to the mark lining up the bottom of the meniscus with the line using distilled water.
    Place a stopper in the top of the flask and invert the flask several times to ensure there is a uniform solution.
17
Q

How can you dilute a solution?

A
  1. Pipette 25cm^3 of original solution into a 250cm^3 volumetric flask.
  2. make up to the mark with distilled water using a dropping pipette for the last few drops.
  3. Invert the flask several time to ensure a uniform solution
18
Q

What are the potential errors when using a gas syringe?

A
  • Gas escapes before the bung is inserted
  • Syringe sticks
  • Some gases like CO2 or Sulphur dioxide are soluble in water so the true amount of gas is not measured.
19
Q

What is the method for carrying out a titration?

A
  1. Rinse equipment with its relative solutions (eg. burette rinse with acid)
  2. Pipette 25cm^3 of alkali into a conical flask
  3. After rinsing the burette with acid, fill the burette up to the 0 mark and ensure that the tap is closed.
  4. Add a few drops of indicator to the conical flask which is suitable for the pH of the titration being carried out. Place the conical flask on a white tile to make the observation of the colour change easier.
  5. Slowly add the acid to the conical flask whilst constantly swirling the contents of the flask around to ensure a uniform solution.
  6. When you are close to the end point, add the acid in drop by drop to calculate the exact point at which endpoint is established,
  7. Calculate the volume of the titre added to the solution in order to calculate the concentration of the alkali.
  8. Repeat steps until at least 2 concordant results are obtained with readings within 0.1 of each other.
20
Q

How do you calculate percentage error?

A

error/Measurement of apparatus X 100

To calculate total percentage error, calculate error for each equipment and add up the percentages.

21
Q

What are the possible errors with the titration methods, and how can they be resolved?

A

Apparatus such as beakers have high % errors. To solve use apparatus with lower % errors such as pipettes and burettes.

Reduce error reading in burette, make a titre with a larger volume, by either increasing the volume and concentration of the substance in the conical flask or by decreasing the concentration of the substance in the burette.

22
Q

How can you reduce errors with measuring mass?

A

Use a more accurate balance or a larger mass when weighing a solid.
Weigh the sample both before and after addition and then calculate difference ensuring a more accurate measurement of the mass added.

23
Q

What is the method for measuring Enthalpy change?

A
  1. Wash the equipment with solutions to be used
  2. Dry the polystyrene cup after washing, and place this into a beaker for insulation and support
  3. Weigh the solid reagent and add the known mass to the cup using the before and after weighing method and using a 2dp balance.
  4. Measure out desired volume of solutions (eg 50cm^3 of water) and place this into the polystyrene cup.
  5. Measure the initial temperature of the solution every 30 seconds for 3 minutes.
  6. After 3 minutes add the second reagent to the cup
  7. Record the temperature for the next 5 minutes every 30 seconds at 4 minutes on wards.
  8. record the final temperature and measure the mass of the solid reagent after time is up.
24
Q

Explain how you would use the results from the calorimetry method to calculate the enthalpy change

A
  1. Draw a graph of Temperature against time.
  2. Extrapolate lines to when the solid was added to find the initial and final temperatures.
  3. Calculate the change in temperature (T final - T initial)
  4. Using Q=MCT work out energy transferred.
  5. Calculate amount of mass used and the moles Mass o/Mr
  6. Find Enthalpy change by (+or-) q/mol
25
Q

What are the errors with the Calorimetry method?

A

Heat Transferred to the surroundings
approximation of specific heat capacity
Reaction or dissolving may be incomplete or slow
Density of solution is taken to be the same as water

26
Q

What is the Continuous monitoring method?

A

• Measure 50 cm3 of the 1.0 mol dm–3 hydrochloric acid
and add to conical flask.
• Set up the gas syringe in the stand
• Weigh 0.20 g of magnesium.
• Add the magnesium ribbon to the conical flask, place the
bung firmly into the top of the flask and start the timer.
• Record the volume of hydrogen gas collected every 15
seconds for 3 minutes.

27
Q

How do you construct a pH curve for an acid-base titration?

A
  1. Measure initial pH of the acid
  2. Add alkali in small amounts noting the volume added
  3. stir the mixture to equalise the pH
  4. Measure and record the pH to 1 Dp
  5. When approaching end point add in smaller volumes of alkali
  6. Add until alkali is in excess

Ensure pH meter is calibrated first and that this experiment is at a constant temperature to improve the accuracy.

28
Q

What is the method for Thin layer chromatography?

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. or spray with Ninhydrin and put in an oven.
Draw around them lightly in pencil.
g) Calculate the Rf values of the observed spots

29
Q

What is done during the method of chromatography to avoid inaccuracies?

A

Wearing plastic gloves - prevents contamination from hands to the plate

pencil line - will not dissolve in the solvent

tiny drops - too big drops will cause different spots to merge

depth of solvent - if the solvent is too deep it will dissolve the sample spots from the plate

lid - prevent evaporation of toxic solvent

dry in fume cupboard - solvent is toxic

U V lamp - used if the spots are colourless and not visible.

30
Q

Why may the result of chromatography be different to the expected results?

A

Some substances may not separate dur to having similar rf values.

some spots may contain more than one compound.

31
Q

What is the method for measuring hydrated salts

A

•Weigh an empty clean dry crucible and lid .
•Add 2g of hydrated compound to the crucible and
weigh again
•Heat strongly with a Bunsen for a couple of minutes
•Allow to cool
•Weigh the crucible and contents again
•Heat crucible again and reweigh until you reach a constant
mass ( do this to ensure reaction is complete).

32
Q

How can you test the the concentrations of transition metals using their coloured ions?

A

Using colorimetry:

  • Add an appropriate ligand to intensify colour
  • Make up solutions of known concentration
  • Measure absorption or transmission
  • Plot graph of absorption vs concentration
  • Measure absorption of unknown and compare
33
Q

How can you calculate the initial rate of reactions?

A

Used to clock reactions to measure rates of reaction

Measure the time taken for a certain amount of product to form as you vary the concentration of the reactants.

Often an observable end point (colour change)

plot graph of concentration against time

Used to find initial rate when t=0

34
Q

What is the method for setting up an electrochemical cell?

eg Copper and zinc cell

A

a) Clean a piece of copper and a piece of zinc using emery paper or fine grade sandpaper.
b) Degrease the metal using some cotton wool and propanone.
c) Place the copper into a 100 cm3 beaker with about 50 cm3 of 1 mol dm–3 CuSO4 solution.
d) Place the zinc into a 100 cm3 beaker with about 50 cm3 of 1 mol dm–3 ZnSO4 solution.
e) Lightly plug one end of the plastic tube with cotton wool and fill the tube with the solution of
2 mol dm–3 NaCl provided.
f) Plug the free end of the tube with cotton wool which has been soaked in sodium chloride. Join
the two beakers with the inverted U-tube so that the plugged ends are in the separate beakers.
g) Connect the Cu(s)|Cu2+(aq) and Zn(s)|Zn2+(aq) half-cells by connecting the metals (using the
crocodile clips and leads provided) provided to the voltmeter and read off the voltage.

A-level Chemistry (33 decks)

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