Practicals

Question 1

Finding the Molar Volume of a Gas

Answer 1

1. Place 30 cm3 (excess) of ethanoic acid in test tube, add pre-weighed marble chips(CaCO3) and quickly place the bung on.
2. Measure the volume of a gas produced with gas syringe.
3. Make a few measurements.
4. Repeat, increasing the mass of the marble chips by around 0.05g each time.

Question 2

Preparation of a Standard Solution
and Titration

Answer 2

1. Weigh out the mass of required acid salt and transfer to a clean beaker.
2. Add 50 cm3 of distilled water to the weighed out solid and stir with a glass rod to dissolve.
3. Using a funnel, transfer to a graduated volumetric flask by pouring it down the glass
   rod. Wash the rod into the graduated volumetric flask and fill with distilled water to the 250 cm3 mark.
4. Stopper the flask, then mix thoroughly by inverting and shaking vigorously

Question 3

Titration to find the concentration of
an acidic or alkaline solution.

Answer 3

1. First carry out a quick trial titration to find the approximate end-point. Note down the
   value of this titre.
   [Phenolphthalein is pink in the basic solution and colourless in acidic and neutral solutions. Methyl orange is yellow in basic solution, orange in neutral solution, and red in acidic solution.]
2. Carry out a more accurate titration, adding the titrant drop by drop when
   approaching the end-point.
   [When titrating, wash down the walls of a conical flask with a bit of distilled water from time to time. This will ensure that all the titrant ends up reacting with the analyte.]
3. Repeat accurate titrations until you have at least two concordant (within 0.10 cm3) titres.
4. Find a mean titre using these concordant values.

Question 4

Rates of hydrolysis of halogenoalkanes.

Answer 4

1. Set up 3 test tubes, each with 1 cm3
   of ethanol and two drops of a haloalkane.
   [iodo/bromo/chloro alkanes]
2. Place the test tubes in a water bath (60 °C), along with a test tube of 0.1 mol dm−3 silver nitrate and leave all to reach a constant temperature.
3. Quickly add 1 cm3 of a solution of silver nitrate to each test tube containing a haloalkane, and start a stopwatch.
4. Measure and record the time taken for the precipitates to form in each of the test
   tubes (this is a measure of the rate of reaction).

Question 5

Oxidation

Answer 5

1. Place the acidified potassium dichromate solution in a pear-shaped flask. Cool down
   the flask using an ice bath.
2. Add a few anti-bumping granules. These will prevent the formation of large gas
   bubbles that cause violent boiling.
3. Add ethanol dropwise to the pear-shaped flask. Stir to ensure complete mixing.
4. Warm up the flask to room temperature.
5. Set up the reflux apparatus as shown below, placing the flask in a water bath.
6. Heat using the Bunsen burner for 5-10 minutes.
7. Allow some time for the apparatus to cool down. Afterwards, collect the product via
   distillation using the equipment shown in the diagram below.
   [Use the thermometer to prevent the temperature from rising too high. Keep the bulb in vapours, never in solution.]
8. To preparing an aldehyde, conduct the same reaction under distillation conditions
   without the reflux process.
   [For both reactions, the colour change occurs from orange (Cr2O72−) to green (Cr3+).]

Question 6

Chlorination of 2-methlypropan-2-ol.

Answer 6

1. Add concentrated HCl and the tertiary alcohol to a conical flask. Swirl gently.
2. Put the rubber bung in and swirl the flask gently. Open the bung to release the
   pressure from fumes from time to time. Repeat this regularly over 20 minutes.
3. Add some anhydrous CaCl 2 and shake. At this point, there should be two distinct layers.
   [In this case, the upper (organic) layer contains the desired product. The lower layer is the aqueous layer.]
4. Transfer the contents of the flask to separating funnel.
5. Allow the layers to separate and discard the lower (aqueous) layer. Close the tap,
   keep the organic layer in the separatory funnel.
6. Add a solution of NaHCO3
   to remove the unreacted HCl. Swirl gently. Stopper the
   separating funnel and shake it. Invert the separatory funnel and open the tap to release the pressure due to CO2 formed. Repeat twice.
7. Remove the stopper and run off the aqueous layer. Then, run the organic layer into a
   clear conical flask. Add some anhydrous Na2SO4
   which acts as a drying agent.
8. Swirl the contents and leave the flask to stand for a bit. Then, either decant the liquid,
   or filter it.
9. Distill to purify the product.

Question 7

Flame tests for Group I and Group II metal cations

Answer 7

● Dip a nichrome metal wire in the solution of HCl and then into the tested metal ion
solution.
● Carefully place the wire at the top of the flame coming from Bunsen burner.
○ Li+ = red
○ Na+ = orange/yellow
○ K+ = lilac
○ Rb+ = red
○ Cs+ = blue
○ Mg2+ = no colour
○ Ca2+ = brick-red
○ Sr2+ = crimson red
○ Ba2+ = green

Question 8

Ion tests

Answer 8

CO32- and HCO3- ions
● Add aqueous acid.
● Bubbles of CO2gas will be observed.
● Use a delivery tube to pass the CO2
through limewater.
● The solution will turn cloudy due to formation of CaCO3 precipitate.
NH4+ ions
● Add aqueous NaOH and gently warm the mixture.
● The ammonia gas will turn moist pH indicator paper blue.
● Pungent smell given off.
SO42− ions
● Add acidified barium chloride solution.
● White ppt forms (BaSO4).

Question 9

Determination of enthalpy change using Hess’s Law.

Answer 9

1. Place one of the reactants into a polystyrene cup and place a thermometer with it. Start a stopwatch and record the temperature of the liquid every minute.
2. At 4 minutes, add the second reactant and dont record a temperature change for this minute.
3. At 5 minutes continue taking temperature readings each minute for a further ten minutes.
4. Plot temperatures of a graph and extrapolate to find ∆T.
5. Repeat for the second reaction.

Question 10

Finding Ka of weak acid

Answer 10

1. Titrate 25 cm3 of the acid being tested against NaOH, use phenolphthalein indicator.
2. After titration, add another 25 cm3 of ethanoic acid to the same flask.
3. Use the pH meter to find pH. pH will equal pKa because exactly half of the acid has been neutralised so this is the half-equivalence point.
   [i.e. [A−] = [HA], therefore Ka = [H+], and so pKa = pH.]
4. To convert from pKa to Ka, calculate 10−pKa

Question 11

Redox titrations

Answer 11

1. Prepare the titration equipment.
2. Add the standard solution KMnO4
   to burette.
3. Add known volume solution of iron (II) ions being tested to the conical flask.
4. Titrate the solution.
   [No indicator is required as KMnO4 is self indicating; the end point is when you get the first permanent pale pink colour.]
5. Repeat until you get concordant titres.
6. Calculate a mean titre from these concordant values.

Question 12

Preparing a transition metal complex

Answer 12

1. Weigh out the mass of copper sulphate accurately and dissolve in water.
2. In a fume cupboard, add concentrated ammonia.
3. Stir the mixture and pour into ethanol. Then, cool the mixture in ice bath. Crystals of product will form.
4. Set up the vacuum filtration apparatus with a Buchner funnel. Collect the crystals in the funnel. Wash the tube with cold ethanol and filter again, then wash the crystals
   with cold ethanol.
5. Leave crystals on the funnel for a bit to dry. Then, use two filter papers to dry the
   crystals even more.
6. Record the mass and calculate the percentage yield (relative to the hydrated copper
   sulfate).

Question 13

Iodine Reactions.

Answer 13

1. Using a pipette, add a sample of mixture to excess sodium hydrogencarbonate immediately after removing it from the flask to quench the reaction.
2. Start a stopwatch.
3. Withdraw samples every three minutes and repeat this process.
4. Titrate these samples with sodium thiosulphate (Na2S2O3) adding starch indicator near the end. [Will turn from blue-black to colourless as iodine reacts with thiosulphate.]
5. Repeat for all samples to find their concentrations.
6. Use these results to plot a graph of concentration against time.
   [Graph should be a straight line. This means 0 order w.r.t. iodine; it is not involved in the rate-determining step of the reaction and therefore has no effect on the rate.]

Question 14

Activation energy of bromide and bromate(v)

Answer 14

1. Mix bromates, bromides and some sulfuric acid to produce bromide.
   [BrO3– + 5 Br– + 6 H+ → 3 Br2 + 3 H2O]
2. Add phenol:
   [C6H5OH + 3 Br2 → C6H3Br3OH + 3 HBr]
3. As soon as the produced bromine reacts with phenol, it reacts with the indicator. The
   decolourisation of the indicator is the end of the reaction.
4. Record time values for this point at different temperatures.

Question 15

Aspirin Synthesis

Answer 15

1. Weigh out 2-hydroxybenzoic acid and transfer to a pear shaped flask.
2. Add acetic anhydride and 8 drops of concentrated H3PO4
   solution.
3. Warm the mixture under reflux for 5 minutes.
4. Add cold deionised water to the solution and stand the flask in a bath of iced water until precipitation is complete.
5. Filter the mixture under reduced pressure.
6. Dissolve the impure product in the minimum amount of warm ethanol.
7. Add warm water.
8. Place the boiling tube into ice water for 15 minutes.
9. Filter the purified derivative under reduced pressure.
10. Dry the purified product using filter paper.
11. Collect your sample in a dry, pre-weighed sample vial and calculate the mass of the product. Use this information to calculate the percentage yield of the product.
12. Measure the melting point of the product.

Question 16

Errors with Molar Gas Volume

Answer 16

● If using syringe, plunger may not be free moving. It may need a ​lubricant​​.
● CO2​ ​ is ​slightly soluble​​ in water, so the exact volume is not measured.
● Some ​gas escapes​​ between addition of marble chips and sealing the test tube.
● Bung may not be ​airtight​​.
● Transferring the solid. It is important to weigh the tube containing marble chips
before the addition and reweigh after the addition. This method is ‘​weighing by difference​​’ and ensures the amount of CaCO3​ that ends up in the reaction mixture is known..

Question 17

Errors with Preparing a Standard Solution

Answer 17

● Be careful not to ​lose any solid​​ when transferring from the weighing bottle to the beaker.
● When weighing out the solid, ​weigh by difference and then calculate the mass of a solid in a beaker:
mass of (weighing bottle + beaker) - (mass of the bottle after emptying solid)
● General titration procedure is described in ​CP3​​. Find the concentration of NaOH by titrating it with a solution of the acid prepared in this experiment.

Question 18

Errors with Titration

Answer 18

● Allow the titrant some time to drain down walls of burette before reading the burette.
● Swirl the conical flask so it mixes properly with the analyte.
● Use a white tile to make the colour change more noticeable.
● Diluting your solutions may produce a reading with smaller percentage error, (i.e. 10
cm3​ ​ titre has a smaller percentage error than a 30 cm3​ ​ titre).
● Phenolphthalein used in this titration may turn colourless at the end point if you
leave the solution to stand. This because NaOH reacts with CO2​ from the air to form Na2​C​O3​.

Question 19

Errors with rates of halogenoalkane hydrolysis

Answer 19

● Use ​water bath​ to control the temperature.
● Use lower temperatures to reduce the rate of reaction. This will make the time
differences will be more pronounced producing a ​lower uncertainty​.

Question 20

Errors with chlorination of 2-methypropan-2-ol

Answer 20

Some product is lost when ​transferring​​ liquids between the vessels.

Question 21

Errors with Enthalpy Change

Answer 21

● We assume the ​specific heat capacity​​ of the solution to be ​that of water​​.
● Polystyrene is more ​insulating​​ than glass, so less heat is lost

Question 22

Errors with finding Ka of a weak acid

Answer 22

● Burette readings, ​subjective​​ end point of titration

Question 23

Safety with constructing electrochemical cells

Answer 23

● Somesolutionsare​toodangeroustouseat1moldm-​3(e.g.silvernitratewhichis highly oxidising).
● Zinc sulphate and iron (II) sulphate are ​harmful to the environment therefore have to be disposed of safely.
● Electrodes must be made from an ​inert substance and cannot be made from a metal that reacts with water (e.g. Mg).
● Differences between experimental values and theoretical values usually stem from the ​conditions​​ not being standard

Question 24

Errors with redox titrations

Answer 24

● Make sure as much iron salt as possible is dissolved. Warming may help.
● Wash the containers with water so as to get as much iron as possible.
● Use a white tile to better see the endpoint of your titration

Question 25

Errors with preparing a transition metal.

Answer 25

● Losses could be from the reaction not going to ​completion​​ and product staying in solution (i.e. not crystallising out.)
● Gains could be from ​impure or wet​​ crystals.

Question 26

Errors with Iodine reactions

Answer 26

● Inaccurate timing of the appearance of blue colour. Two students could time simultaneously and use an average value.
● Adding starch slightly increases the volume which affects the concentrations of the reactants and thus the amount they change over time.

Question 27

Key Points and Safety for Aspirin Synthesis

Answer 27

Key points
● When washing the product with ice cold water​​, don’t add too much as to minimise product loss).
Safety
● Ethanoic anhydride and conc. acid are ​corrosive​​, so use gloves.