Required practicals

Question 1

Making a standard solution

Answer 1

Add mass of solution you are trying to make, e.g. sodium carbonate, to a weighing boat. Weigh mass of weighing boat and sodium carbonate added. Fill a 250 cm3 beaker with 50cm3 water. Add sodium carbonate from the weighing boat to the beaker. DISSOLVE in water in beaker.Weigh weighing boat again and do mass of weighing boat and sodium carbonate before transfer- mass of weighing boat after transfer to see the mass of sodium carbonate added to the solution. Or wash in with distilled water.
Stir the solution and add more water if necessary.
Transfer solution to a volumetric flask, RINSE beaker with water to ensure all is transferred to volumetric flask.
Make up the volumetric flask to 1cm3 below the mark with distilled water. Insert stopper and shake to mix the contents. Use dropping pipette to bring bottom of meniscus to the mark with distilled water. Insert stopper and SHAKE thoroughly several times, inverting it too.
Do mass/mr for moles of sodium carbonate added.
Then do mol/vol for concentration of solution.
We now have a solution we know the concentration of to use in the titration.

Question 2

Enthalpy changes- burning alcohols

Answer 2

Record initial mass of spirit burner with alcohol in.
Clamp a copper calorimeter 7cm3 above spirit burner with 100cm3 water in.
Light wick and heat water until temp increases by 30 degrees. Measure with thermometer.
Record mass of spirit burner after.
Do initial mass of spirit burner- mass of spirit burner after to find mass of alcohol used.
Do mass/Mr to find mol of alcohol used.
Do q=mc delta t and divide by 1000 to get KJ not J and then delta H= q/mol of alcohol used to find enthalpy change. Repeat for each alcohol.

Question 3

Disappearing cross practical- how rate of reaction varies with temperature

Answer 3

Set up a water bath with 2 holes in, put test tube in each hole one with a cross under it and one without.
Add 10cm3 1moldm-3 HCl to test tube without cross under it. Add 10cm3 0.05 mol dm-3 sodium thiosulfate solution to test tube with cross under it. Add hot water using a kettle at about 55 degrees. Leave for 3-5 mins. Add 1cm3 acid to sodium thiosulfate solution and measure time taken for cross to disappear from view. Record temp of water. Repeat for different temperatures as water cools down.
Plot graph of 1/time against temp. 1/time is measure of the rate.

Question 4

Why can 1/time be used as a measure of the rate in the disappearing cross practical?

Answer 4

Initial rate of reaction= amount of sulfur produced/time but amount of sulfur produced is fixed as when cross disappears from view that is a certain amount of sulfur produced to make the water cloudy.

Question 4

Preparation of cyclohexene by the dehydration and distillation of cyclohexanol

Answer 4

Add 20cm3 cyclohexanol into a pear shaped flask that has already been weighed.
Weigh again to determine mass of cyclohexanol added.
Add 8cm3 phosphoric acid and a few anti bumping granules.
Distil anything that boils below 100 degrees. Collect distillate in a cooled collection vessel to reduce evaporation of the product.
Transfer to a separating funnel and add 50cm3 NaCl solution. Shake and leave to allow the two layers to separate. Decant lower layer to dispose of and then transfer upper layer to small conical flask and add anhydrous CaCl2 to remove water. Add stopper, shake and leave to stand until liquid becomes clear.
Decant into a clean dry sample container that has already been weighed to calculate the mass of cyclohexene produced. Determine % yield and test with Br2 to see if it’s an alkene.

Question 4

Distillation things to remember

Answer 4

If flammable substances present, use water bath or electric heater to heat mixture.
Use anti bumping granules to prevent bumping- large bubbles forming so prevents these boiling over, contaminating the product.
Condenser tilted down so any liquid can run into the collection flask.
Cooled collection vessel to reduce evap of product.
Bulb of thermometer should be at the t junction, connecting to condenser to measure correct BP.
Water must enter at lowest point and leave at the highest point to go against gravity as this ensures water fills the condenser(preventing backflow) maximising heat transfer for condensation.
Apparatus should not be sealed near collection vessel to allow air to escape as if sealed, as air is heated it expands so apparatus may crack.

Question 5

Organic tests

Answer 5

Aldehyde- Fehling’s or Tollen’s.
Alkene- Br water.
Alcohol: Add K2Cr2O7/H2SO4.
Halogenoalkane: Add NaOH, nitric acid, then silver nitrate.
Add dilute and conc ammonia solution.

Carboxylic acid:
add sodium hydrogen carbonate, and bubble gas produced through limewater

Question 6

Fehling’s test for aldehyde

Answer 6

Add 1cm3 Fehling’s A. Add Fehling’s B until blue ppt redissolves to form deep blue solution.
Add Fehling’s solution, anti bumping granules and aldehyde and heat. Brick red ppt if aldehyde present.

Question 6

Tollen’s test for aldehyde

Answer 6

Tollen’s: 1cm3 solver nitrate in test tube. Add one drop of NaOH to form a ppt of silver oxide.
Add dilute ammonia drop by drop until brown ppt redissolves(this is now Tollen’s reagent).
Add a few drops of the organic compound, heat in a hot water bath for a few mins, will see silver mirror if aldehyde present.

Question 7

Test for halogenoalkane

Answer 7

Halogenoalkane: Add NaOH (OH- replace Br to get Br- ions).
Warm and add nitric acid (to remove carbonates) then silver nitrate.
White ppt if Cl, cream ppt if Br, yellow ppt if I.
Add dilute and conc ammonia solution.
Cl dissolves in dilute, Br dissolves in conc, I doesnt dissolve.

Question 7

Test for carboxylic acid

Answer 7

add sodium hydrogen carbonate, will see effervesence. Collect gas produced and bubble through limewater using delivery tube. Cloudy if acid present due to CO2 produced.

Question 8

test for alkene

Answer 8

Few drops Br water turns colourless from orange if alkene (few drops)present.

Question 9

Test for alcohol

Answer 9

Add K2Cr2O7/H2SO4 and heat in water bath, orange to green.

Question 10

Acid base titration

Answer 10

Rinse burette with acid, pipette with alkali and conical flask with distilled water.
Transfer 25cm3 alkali (from standard solution) to conical flask using the pipette. Touch surface of alkali with pipette to ensure all is transferred.
Add few drops indicator and put white tile under to observe colour change easier.
Add acid to burette below eye level and with filter. Ensure jet space is filled. Record initial reading on burette. Add acid to alkali and stop when colour change seen. Measure reading on burette and do final-initial vol to get vol of acid added to alkali. This is the rough titre. Repeat until concordant results obtained (within 0.1cm3). Next ones, add acid slower near vol of what we recorded on rough titre, maybe dropwise.

Question 11

Preparation of ethanal by the oxidation and distillation of ethanol

Answer 11

Add a few anti bumping granules, 15cm3 acidified potassium dichromate to a pear shaped flask. Add 5cm3 ethanol using a teat pipette, shaking gently to mix contents.
Heat gently in distillation apparatus to distil off approdximately 5cm3 distillate in a cooled reaction vessel (immersed in cold water). Find mass of ethanal made and test with tollens.

Question 12

Iodine clock- initial rates method

Answer 12

Fill beaker with potassium iodide, sodium thiosulfate, starch and water. Then add H2O2 and record time taken to turn blue-black.
Rinse with distilled water and repeat with different concentrations of potassium iodide.
Do 1/time to get initial rate and plot graph of initial rate against concentration to determine order.

Question 13

Why iodine clock produces a blue-black colouration

Answer 13

Hydrogen peroxide reacts with iodide ions in KI to form iodine.
Sodium thiosulfate reacts with iodine. Amounts chosen so iodine is in excess so as soon as iodine is in excess the blue black colouration of iodine in starch is seen.

Question 14

Why 1/time can be used for iodine clock to measure rate of reaction

Answer 14

Rate usually amount of substance produced/time. Here, amount of iodine produced to give a blue black colouration for every conc of KI is constant so 1/time can be used. 1 represents amount of substance produced and this is the same for every concentration.
For clock reaction, if slow rate, it will take a long time to observe the colour change and if quick reaction, it will take a short amount of time for colour change. This is an inverse relationship.
We can conclude that the initial reaction rate is inversely proportional to time or initial reaction rate is directly proportional to the inverse of time (1/t).

Question 15

Why 1/time can be used to measure INITIAL rate of reaction for iodine clock

Answer 15

Mainly, we assume there is no significant change in rate of reaction between the start of the reaction and the time the measurement is taken.

Question 16

continuous monitoring method of rate- mg HCl method

Answer 16

Add HCl to conical flask and Mg ribbon with bung placed firmly on top.
Set up gas syringe or upside down measuring cylinder in water connected to beaker via delivery tube.
Record vol of H2 gas collected every 15 seconds for a set amount of time.
Plot vol gas collected vs time graph.
Find rate using tangent at several points on the graph. Now plot rate time graph to get how rate varies over time.
Can take sample out of the reaction mixture at same time intervals and stop the reaction of it by quenching (adding a lot of water to dilute it). May be possible to measure conc of a reactant/product at that time (e.g. by titration)
Can then plot a rate-conc graph to find order of reaction.

Question 17

Continuous monitoring alternative method- colorimeter

Answer 17

React substances together. Measure absorbance using cuvette at regular intervals. Plot absorbance vs time graph.
Use known concentrations and measure absorbance at each conc to plot a calibration curve of conc vs absorbance.
Use calibration curve to determine conc at each time interval from absorbance.
Plot conc-time graph
Measure rate at each conc by doing tangent at conc time graph.
Plot rate-conc graph to determine order.

Question 18

Preparation of a pure organic solid (aspirin) and test of its purity. (recrystallisation and measurement of MP on a different flashcard)

Answer 18

2g 2-hydroxybenzene carboxylic acid, 4cm3 ethanoic anhydride and 5 drops H3PO4 in fume cupboard into pear shapes flask and swirl.
Set up reflex condenser and heat to boiling in a water bath, swirling at regular intervals.
Add 2cm3 water down the condenser to react with any excess ethanoic anhydride in a vigorous reaction..
Pour the mixture into 40cm3 cold water in 100cm3 beaker.
Put in a cold water bath and scratch sides of beaker to induce crystallisation.
Collect products by suction filtration using a Buchner funnel and wash with a little water. Then recrystallisation of aspirin (on another flashcard)

Question 19

Recrystallisation of aspirin

Answer 19

Put crystals into boiling tube and add a bit of water. Put in water bath and dissolve crystals in the minimum vol of hot water. Filter hot to remove any insoluble impurities (not buchner). Cool until crystals reappear.
Filter off the crystals under suction using a Buchner funnel and wash with a small amount of cold water!!.
Pat dry between filter paper and then place in warm over to dry thoroughly.
Measure MP(on another flashcard) and yield of aspirin.
Markscheme:
Dissolve crude product in hot solvent/water

of minimum volume

Filter (hot to remove insoluble impurities)

Cool to recrystallise

Filter under reduced pressure / with Buchner/Hirsch apparatus

wash (with cold solvent) and dry

Question 20

Measuring MP

Answer 20

Put aspirin into electrical MP machine (in capillary tube usually). Use apparatus and change heat until it melts. See temp it melted at and compare to data book value. Impurities tend to lower MP and give a wider range

Question 21

Preparation of a pure organic liquid

Answer 21

Ethanol, ethanoic acid, conc H2SO4 and anti bumping granules to pear shaped flask. Put in boiling water bath and reflux.
Add saturated sodium carbonate solution to allow ethyl ethanoate to separate into an oily layer and react with any unreacted acids)
Transfer to separating funnel and stopper added. Invert and shake 15-20 times, opening stopcock each time to release the pressure, to allow layers to separate. Ethyl ethanoate floats on top of the aq solution. Open stopcock so aq layer drains away. Drain ethyl ethanoate into a clean beaker.
Distil ethyl ethanoate as lost H bonds that were present in the alcohol and carboxylic acid, so lower BP than the two reactants.
Distillate collected and temp it comes off is noted. BP can be used to establish its identity

Question 22

Loss of yield in recrystallisation

Answer 22

Crystals lost when filtering or washing
Some product stays in solution after recrystallisation
Other side reactions occuring.

Question 23

Describe how you would carry out an experiment to determine the enthalpy of solution of anhydrous MgCl2. (Dissolves in/absorbs water)
Use 0.8g MgCl2.
Explain how your results could be used to calculate the enthalpy of solution.

Answer 23

Stage 1 Method

(1a) Measures water with named appropriate apparatus
(1b) Suitable volume/mass / volume/mass in range 10 –
200 cm3/g
(1c) Into insulated container / polystyrene cup (NOT just
‘lid’)
(1d) Add known mass of MgCl2(s)
(1e) Use of ‘before and after’ weighing method. NOT
‘added with washings’

Stage 2 Measurements (could mark from diagram)

(2a) Record initial temperature (min 2 measurements)
(2b) Record T at regular timed intervals for 5+ mins / until
trend seen
(2c) Plot T vs time

Stage 3 Use of Results (3a and 3b could come from
diagram)

(3a) Extrapolate lines to when solid added (to find initial
and final T)
(3b) Tfinal – Tinitial = ∆T / idea of finding ∆T from graph at
point of addition
(3c) q = mc∆T
(3d) amount = mass/Mr (0.80/95.3 = 8.39 x 10–3 mol)(3c) q = mc∆T
(3e) ∆Hsoln = q/8.39 x 10–3 or in words

Question 24

Need to put ion testing in this set

Question 25

Recrystallisation, why could percentage yield be more than 100%?

Answer 25

Product not dried or impurities present in the product

Question 26

Difference between continuous rates and initial rates

Answer 26

Continuous rates you measure the rate at several points during one reaction

Initial rates you measure initial rates of several reactions with different concentrations

Question 27

making a standard solution markscheme answer

Answer 27

weigh solid and transfer using a method that allows exact mass
to be known (there should be two weighings, one of which could
be zeroing, and method could be by difference or with washings
or directly weighed into container)
M2 dissolve in water in suitable container (NOT in 250 cm3 of
water)
M3 transfer with washings into 250 cm3 volumetric/graduated flask
M4 make up to mark / 250 cm3 AND THEN shake / invert / mix

Question 28

Why invert the flask several times when making a standard solution?

Answer 28

to ensure the solution is homeogenous. Ensure the solution is fully mixed and evenly distributed.