All Experiments & Tests, Module 1-4

Question 1

How would you carry out an experiment to determine the water of crystallisation

Answer 1

• weigh an empty crucible
• add the hydrated salt into the weighted crucible then weight it
• use a pipe-clay triangle supported on a tripod. Then place the crucible on top.
• Heat the crucible and contents gently for about a minute. Then heat it strongly for a further three miniutes.
• leave the crucible to cool. Then weigh the crucible and anhydrous salt.

Question 2

What assumption do we make when we conduct the hydrated salt experiment and how can you ensure the experiment is accurate as possible?

Answer 2

Assumption 1 - All of the water has been lost. If the hydrated and anhydrous forms have different colours, then it is safe to assume all the water has been lost. If the hydrated and anhydrous forms a similar colours , then it is best to heat to constant mass.

Assumption 2 - no further decomposition
Many sales second or further when heated; for example, if heated very strongly, copper (II) sulfate decomposes to form black copper (II) oxide. This can be difficult to judge if there is no colour change.

Question 3

How do you make a standard solution?

Answer 3

1) The solid is first weighed acurately using a balance.
2) Transfer solid from weighing boat to beaker. Wash any solid left behind into the beaker using deionised water.
3) Dissolve your solid fully using deionised water. Stir to ensure the solid is dissolved fully.
4) Transfer solution to volumetric flask. Use a funnel to avoid spillage and rinse the beaker, glass rod and funnel into the flask to ensure most of the solution is transferred.
5) Use more deionised water to fill to the graduation line. Be careful not to go above this line, or else it will be too diluted! Use a pipette to fill to the line when you get near. Make sure the base of the meniscus touches the mark
6) Add a stoper. Then invert a flask a few times. This ensure the solution is thoroughly mixed and ready to use.

Question 4

Write down the methods to carry out a titration experiment

Answer 4

1. Using a pipette, measure the volume of a solution
2. Add the solution into a conical flask and add an indicator into it
3. Add the other solution into a burette and record the volume to the nearest 0.05cm^3
4. Slowly add the solution in the burette into the conical flask
5. Swirl the mixture continuously until the endpoint is reached
6. Record the final burette reading to nearest
7. 05cm^3 to obtain titre
   7) run a quick trial
8. Repeat until concordant results are obtained - agreeing to within 0.10cm^3

Question 5

Describe the gas syringe experiment to work out the identity of unknown metal

Answer 5

• Weigh up a sample of the unknown metal and add to flask
• Using a measuring cyclinder weigh an excess of HCL, add 25cm^3 1.0mol dm-3 to the flask and quickly replace the bung connected to a gas syringe
• When the syringe has stopped moving, Measure the maximum volume of gas in the syringe

Results: 0.14g of unknown metal
Volume of H2 = 84cm^3

Assuming RTP, n(H2) = V(cm^3) / 24 000

• This will give you the moles.
• Then you take the moles and use stoichiometry to work out the moles of the unknown metal.
• Using that information u can work out the molar mass

Question 6

What are the uses of titrations?

Answer 6

Finding the concentration of a solution.

Identification of unknown chemicals.

Finding the purity of a substance.

Question 7

what is the colour of methyl orange in an acid, base and at the endpoint?

Answer 7

Acid - red
Base - yellow
End point - orange

Question 8

Explain the effect on the titre of the following error.

1) The pipette has an air bubble inside.
2) The burette readings are taken from the top, rather than the bottom, of the meniscus.

Answer 8

1) Titre would be less as less solution has been added than the correct pipette volume.
2) No effect as volume is measured by difference between two readings.

Question 9

How can we remove any air bubble inside a burette prior to the titration experiment?

Answer 9

Ensure to run an excess solution out through the tap to remove any air bubbles.

Question 10

Describe the test for carbonates

Answer 10

• Add unknown compound into test tube and add nitric acid to test tube
• Bubbling indicates carbonate might be present, so CO2 gas will be released.
• Bubble the CO2 gas in limewater, Ca(OH)2, which will turn form a milky white ppt.
• Confirming presence of carbonate.

Question 11

What can displacement reactions be used to identify?

Answer 11

Which halide is present in solution.

Question 12

Which compound is used to test for halides?

Answer 12

Silver nitrate solution

Question 13

How can the results for a halide test be further tested?

Answer 13

Solubility in ammonia solution

Question 14

What is disproportionation?

Answer 14

A redox reaction in which the same element is both oxidised and reduced.

Question 15

What is a titration?

Answer 15

A technique used to accurately measure the volume of one solution that reacts exactly with another solution.

Question 16

Describe the test for sulfates

Answer 16

Aqueous barium ions are added to the solution. If a white precipitate forms, there is a sulfate.

Question 17

In the halide tests, what happens to chloride with silver nitrate and then ammonia?

Answer 17

Silver nitrate - white precipitate

Ammonia - soluble in dilute ammonia

Question 18

In the halide tests, what happens to bromide with silver nitrate and then ammonia?

Answer 18

Silver nitrate - cream precipitate

Ammonia - soluble in concentrated ammonia

Question 19

In the halide tests, what happens to iodide with silver nitrate and then ammonia

Answer 19

Silver nitrate - yellow precipitate

Ammonia - insoluble in concentrated ammonia

Question 20

What is the correct order for these anion tests?

Answer 20

Carbonate test, sulphate test, halide test

Question 21

Why is this the correct order for anion tests carbonate, sulphate, halide?

Answer 21

-Carbonates also form a white
precipitate in the sulphate test such as barium carbonate but not vice versa. So first must rule out there isn’t any carbonates present

-Carbonates and sulphates form precipitates with silver nitrate. Such as Ag2CO3 and Ag2SO4.

Question 22

Describe the test for ammonium ions

Answer 22

• Add source of hydroxide ions to solution to be tested, such as NaOH
• If Ammonium ion (NH4+) is present, it’ll react with OH-(hydroxide ion) to produce Ammonia gas (NH3) and liquid water (H20)
• Heat test tube to release ammonia gas, as initially, the NH3 gas is soluble in H20.
• When gas released, you may smell it, but it’s easier to test with red litmus paper.
• Where presence of Alkaline ammonia will turn paper from red to blue.

Question 23

If I want to work out which halide is present in a solution what can I do?

Answer 23

Run a displacement reaction

Question 24

A Cl2 is added to a Br- ion. What observation will you see with and without the organic non-polar solvent cyclohexane present?

Answer 24

Orange colour will form as the chlorine would have displaced the bromide ion

With cyclohexane - you will see a orange colour

Question 25

A Cl2 is added to a I- ion. What observation will you see with and without the non-polar cyclohexane?

Answer 25

Brown colour change as the chlorine would have displaced the Iodine ion

In cyclohexane - it is violet colour

Question 26

A Br2 is added to a I- ion. What observation will you see with and without the non-polar,organic solvent, cyclohexane?

Answer 26

Brown colour change as the halogen bromine would have displaced the Iodine ion

In cyclohexane- it would be a violet colour change

Question 27

At RTP describe chlorine, bromine and iodine

Answer 27

Chlorine is a pale green gas
Bromine is liquid, extremely toxic, and vaporises readily
Iodine is a solid with grey-black crystals

Question 28

Explain the tests that would be carried out on a mixture of ions to determine the halide present?

Answer 28

1) Carbonate test:

If you see bubbles, continue adding dilute nitric acid until the effervescence stops

All carbonate ion will then have been removed and there will be none left to react in the next tests. We don’t use sulfuric acid or hydrochloric acid because it contains sulfate and chloride ions , which will show
up in the sulfate and halide test

2) Sulfate test:

Add an excess of Ba(NO3)2
Any sulfate ion present will precipitate out as barium sulfate
Filter the solution to remove the barium sulfate

3) Halide test:

For the solution left, add AgNO3 (aq)
Any carbonate or sulfate ion present have already been removed. Therefore any PPT must involve halide ions.

Add NH3 to confirm which halide you have

Question 29

How could you distinguish between NaCl, NaBr, and NaI by a simple test?

Answer 29

Add AgNO3(aq) to an aqeuous solution of each
- If Cl- ions are present, a white PPT will form which disolves in dilute NH3(aq)

• If Br- ions are present, a cream PPT will form which dissolves in conc NH3(aq)
• If I- ions are present, a yellow PPT will form which is insoluble in conc NH3(aq)

Question 30

Explain why it is important to carry out the carbonate test before carrying out a sulfate test on an unknown chemical?

Answer 30

Dilute nitric acid reacts with carbonate ions and no precipitate of barium carbonate will then form

Question 31

Explain why, if you are testing a mixture, it is important to use dilute nitric acid, rather than sulfuric or hydrochloric acid, for the carbonate test?

Answer 31

• If sulfuric acid is used, sulfate ions are added and will show up in the sulfate test with Ba2+(aq).
• If hydrochloric acid is used, chloride ions are added and will show up in the halide test with Ag+(aq).

Question 32

Determination of an enthalpy change of combustion:

Describe the experiment for spirit burner

Answer 32

1) using a measuring cylinder, measure out 150cm^3 of water. Then pour it into a beaker and record the initial temperature to the nearest 0.5C.
2) add methanol to the spirit burner. Then weigh it.
3) place the spirit burner under the beaker. Then light it. Then stir the water whilst burning with a thermometer.
4) after 3 minutes, extinguish the flame. Immediately, record the maximum temperature reached by the water.
5) re weigh the spirit burner containing the methanol. Assume that the wick has not been burnt

Question 33

Determination of an enthalpy change of reaction, the experiment.

Answer 33

• Determined by using plastic cups made of polystyrene foam (which are cheap,waterproof and light weight), which offer some insulation against heat lost to the surroundings.
• Record temperature change using thermometer
• Heat energy calculated using mcΔT

Question 34

How is an organic product purified?

Answer 34

• Ensure the tap of the separating funnel is closed,
• Pour the mixture of liquids, place a stopper in the top and invert to mix,
• Allow the layers to settle.
• Add some water to see which layer will increase in volume (aqueous layer)
• Place a conical flask underneath, remove the stopper and open the tap until the lower layer has left.
• Place a second flask underneath to collect the organic layer

Question 35

Describe the test for hydrolysis of haloalkanes:

Answer 35

1) Tube 1: Add 1 cm^3 of ethanol and two drops of 1-chlorobutane (will form white PPT)
Tube 2: Add 1 cm^3 of ethanol and two drops of 1-bromobutane (will form cream PPT)
Tube 3: Add 1 cm^3 of ethanol and two drops of 1-iodobutane (will form yellow PPT)

2) Stand the test tubes in a water bath at 60C

3) Place a test tube containing 0.1 mol dm^-3 silver
nitrate in the water bath and allow all tubes to reach a constant temperature.

4) Add 1 cm^3 of the silver nitrate quickly to each of the test tubes. Immediately start a stop-clock.
5) Observe the test tubes for five miniutes and record the time taken for the precipitate to form.

Question 36

How can acid impurities be removed?

Answer 36

• Adding aqueous sodium carbonate and shaking the mixture in the separating funnel.
• Carbon dioxide gas is made and must be released via the tap (funnel upside down).
• Aqueous sodium carbonate layer is then removed and organic layer washed with water.

Question 37

How can water be removed from organic products?

Answer 37

• A drying agent is added (typically an anhydrous inorganic salt) and then mixed,
• Place a stopper on the flask,
• Leave for ten minutes,
• If the solid is stuck together in a limp, there is still some water present. Add more until some solid is dispersed as a fine powder,
• Decant liquid into another flask.

Question 38

Describe the process of purifying organic products

Answer 38

1) Add rxn mixture to separating funnel
2) place a stopper and invert to mix content
3) Add distilled water allow layers to spectate - layer that increases in V is the aqueous later. This happens because the organic layer has higher density than the aqueous layer.
4) place a conical flask under separating funnel, remove the stopper and open tap to allow organic layer to run and collect.
5) place a second funnel under the separating funnel to collect the other layer
6) label the flasks

Question 39

Describe the process of drying the organic product

Answer 39

1) add organic liquid to a conical flask
2) using a spatula, add some of the drying agent to the liquid and gently swirl the contents to mix together
3) place a stopper on the flask to prevent your product from evaporating away. Leave for about ten minutes.
4) if the solid has all stick together in a lump, there is still some water present. Add more drying agent until some of the solid is dispersed in the solution as a fine powder.
5) decant the liquid from the solid into another flask.

Question 40

Reaction may use acid to react, and thus my result in acid impurity. Describe the process of removing the impurity.

Answer 40

Add aqueous sodium carbonate to spreading funnel

Then shake the mixture.

Any acid present will react with the sodium carbonate releasing carbon dioxide gas.

Question 41

How does infrared spectroscopy work?

Answer 41

• Sample placed inside an IR spectrometer,
• A beam of IR radiation in range 200/cm to 4000/cm passed through the sample
• Molecule absorbs some of the IR frequencies and the emerging beam is analysed to find the absorbed frequencies.

Question 42

What is the typical sequence for identification of an unknown substance?

Answer 42

• Elemental analysis - determine empirical formula
• Mass spectrometry - Determine molecular mass and identify sections in molecule from fragment ions
• Infrared spectroscopy - identify bonds and functional groups present
• Identity of unknown compound can be worked out now.