Chemistry Practicals

Question 1

Heating in a crucible, to obtain a sample of clean, dry calcium sulphate crystals.

Answer 1

Heating in a crucible
This method could be used for measuring mass loss in various thermal decomposition reactions and also for mass gain when reacting magnesium in oxygen.
The water of crystallisation in calcium sulfate crystals can be removed as water vapour by heating as shown in the following equation.
CaSO4.xH2O(s) → CaSO4(s) + xH2O(g)
Method.
*Weigh an empty clean dry crucible and lid .
*Add 2g of hydrated calcium sulfate 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).
The lid improves the accuracy of the experiment as it prevents loss of solid from the crucible but should be loose fitting to allow gas to escape.
Large amounts of hydrated calcium sulfate, such as 50g, should not be used in this experiment as the decomposition is likely to be incomplete.
Small amounts of the solid , such as 0.100 g, should not be used in this experiment as the percentage uncertainties in weighing will be too high.
The crucible needs to be dry otherwise a wet crucible would give an inaccurate result. It would cause mass loss to be too large as the water would be lost when heating.
Example 1. 3.51 g of hydrated zinc sulfate were heated and 1.97 g of anhydrous zinc sulfate were obtained. Use these data to calculate the value of the integer x in ZnSO4.xH2O
Calculate the mass of H2O = 3.51 – 1.97 = 1.54g
Calculate moles of = 1.97 ZnSO4 161.5
= 0.0122
Calculate ratio of mole of = 0.0122 ZnSO4 to H2O 0.0122
=1
Calculate moles of H2O
=7 X=7
= 1.54 18
=0.085
= 0.085 0.0122

Question 2

If something is (…) what risk does it pose what should you do?
-Dilute acid
-Strong acid
-Flammable
-Toxic
-Oxidising

Answer 2

Dilute acids and dilute alkalis are irritants and you should ear goggles.

Strong Acid and Strong Alkalis are corrosive and you should wear googles.

Flammable- Keep away from naked flames

-Toxic-Wear gloves and avoid skin contact, wash hands after use

-Oxidising -Keep away from flammable or easily oxidised materials.

Question 3

What is better to use and why, a volumetric pipette or measuring cylinder?

Answer 3

A volumetric pipette is better because it has a smaller uncertainty.

Question 4

In a titration what should you pour the solution with unknown concentration into?

Answer 4

A Conical flask is better to use than a beaker because as you are swirling the solution it is less like to spill because of the narrower neck.

Question 5

What should before you do a titration with the burette and equipment in general?

What must you also do before beginning the titration?

Answer 5

You should rinse out the burette because it may have water in it which will dilute the acid or alkali being added into the burette.
In order to rinse it out you should use the substance which you are going to put in the burette.

You must take the funnel which you added to solution to the burette with out of the burette. During the titration it could drip more solution into the burette and lead to an inaccurate( lower) titre volume.

Question 6

What is the danger if you add too much indicator?

Answer 6

The indicators are usually weak acids so adding too much will affect the titration result.

Question 7

What are the colour changes for methyl orange and phenolphthalein?

Answer 7

Phenolphthalein:
Acid to Alkali
Colourless to pink

Methyl Orange:
Acid to Alkali
Red to Orange to Yellow

Question 8

What must you make use that there are none of in the burette solution?

Answer 8

There are no air bubbles in the burette.

Question 9

How should you rinse the conical flask in a titration?

Answer 9

YOU should rinse it with distilled water to wash the sides of the flask so all the acid or alkali is in the solution. Adding water will not affect the moles of a cid or alkali in the conical flask.

Question 10

When taking average?

Answer 10

Make sure the volumes are within 0.1 cm^3 of each other, they are concordant.

Question 11

What is the problem with titrating mixtures?

Answer 11

We do not know or must make sure that the other substances in the mixture do not have acid or alkali properties.

Question 12

Two possible manganate titration reaction rates?

Answer 12

With Fe 2+ the MnO4- ions are attracted to due to the opposite charges.

With C2O4 2- and MnO4- they are both negatively charged so the reaction begins slow but as Mn 2+ and Mn 3+ is produced they act as catalysts and speed up the rate of reaction. Therefore the reaction is an autocatalytic reaction. The catalyst for the reaction is produced in the reaction.

Question 13

What acid should you use to provide the H+ ions in order for a redox manganate titration to occur?

Answer 13

You should use Dilute Sulphuric acid.

Should not use weak acids as only partially dissociates so cannot supply the full number of H+ ions necessary.

Question 14

Practical method for determining how much of something is in a tablet?

Answer 14

Detailed Procedure : how much iron is in iron tablets
* Weigh accurately two ‘ferrous sulphate’ tablets.
* Grind up the tablets with a little 1 mol dm-3 sulphuric acid, using a pestle and mortar.
* Through a funnel, transfer the resulting paste into a 100cm3 volumetric flask. Use further small volumes of 1 M
sulphuric acid to rinse the ground-up tablets into the flask.
* Then add sufficient 1 mol dm-3 sulphuric acid to make up the solution to exactly 100cm3. Stopper the flask and
shake it to make sure that all the contents are thoroughly mixed. They will not all be in solution although the Fe2+
ions which were present in the tablets will be dissolved.
* Titrate 10.0 cm3 portions of the solution with 0.0050 M potassium manganate(VII). The end-point is marked by the
first permanent purple colour.

Question 15

Calorimetry, what possible combinations could they ask you to perform calorimetry on?

What must you do with one of these types of calorimetry.

Answer 15

They could ask you to do calorimetry on a solution and a solid sample or two solutions. ( solid dissolving in a solution)

If you have a solution and sample then you should measure the temperature of the solution before the addition of the sample. ( two solution reacting together)

If two solutions then you should measure the temperature of both solutions and an average temperature taken to be use in the calculation q=mc delta theta.

Question 16

Where in the calorimetry method could an error occur?

Answer 16

Energy could be transferred to the surroundings

The specific heat capacity. This method assumes that all solution have the same specific heat capacity as water.

We ignore any thermal energy absorbed by the thermometer and the apparatus.

Density of the solution is taken to be the same as water.

The dissolving may be slow or incomplete.

Question 17

What is the uncertainty in each of these types of apparatus?

Answer 17

Mass balance: smaller division(resolution) if digital is the uncertainty.

Volumetric flask:
0.1cm^3 uncertainty

25cm^3 pipette:
0.1 cm^3

Burette( you take 3 reading, at the start of the initial amount, at the end and at the end point of the reaction) so 3x smallest uncertainty.

Question 18

How can you reduce percentage uncertainty in a burette?

Answer 18

Use a lower concentration of the solution in the burette so a greater volume is required for titration.(same number o moles required to react with solution in conical flask)

Increase the Volume or the Concentration of the solution in the conical flask. (to require more moles of the solution in the burette.)

Question 19

What must you remember when using the enthalpy change= Q/ moles equation?

Answer 19

The moles are the moles of the limiting reagent. So if a sample is being added to a solution then it is the moles of the sample added.

Also the mass in the q=mc delta theta equation is the mass of the solution if a solid sample is being added to a solution.

If two solutions, then the mass is the combined mass of both solutions.

Even if 1:1 ratio and same moles of both is added, use the moles of only one of the reactants.

Also the q in the enthalpy change equation is in kJ so must be converted from joules to kilojoules and also the moles are the moles of the solid sample of the moles of the limiting reagent (if there is no limiting reagent and both are exact amounts then just use moles of one/

Question 20

In flame calorimetry what are the possible errors in the experiment?

Answer 20

Idea is to determine the enthalpy of combustion of a substance. Substance is burnt and used to heat up water in a sample.

There could be incomplete combustion of the fuel.

Energy could be lost to the surrounding from the calorimeter.

The heat capacity (the thermal energy absorbed by the thermometer is not included or taken into account.)

Question 21

In the disappearing cross experiment what two substances do we react together?

In the disappearing cross experiment what equation are we using and why do we need the time taken for the cross to disappear.

Answer 21

WE react sodium thiosulfate and hydrochloric acid together to produce sulphur which covers the cross.

WE are using the Arrhenius equation so k=Ae^-Ea/RT
and we talk natural logs and since k ( the rate constant) is proportional to the rate then we can plot ln rate against 1/T. Rate of reaction is change in conc/time however we assume the the change in conc in the reactions the same because the same amount of reactants are being added each time we repeat the experiment at different temperatures and the same amount of sulphur (the substance which covers the cross) is being produced. SO therefore same charge in conc.

Question 22

What should you add when testing for sulphate ions?

Answer 22

You should add HCL to the BaCl2 to remove any carbonate impurities which would form a white barium carbonate precipitate.

The colour of barium sulfate is white ppt.

Question 23

What should you add when testing for the halve ions?

Answer 23

Silver nitrate solution with HNO3, Nitric acid to remove any carbonate impurities (Ag2 CO3)which would give a false result.

Also AgCl does dissolve in concentrated ammonia also.

Question 24

HOW do you test for carbonate ions?

Answer 24

Add any dilute acid to the solution. You will observe effervescence of CO2 if carbonate ions are present. You can bubble this gas through limewater and if it is CO2 it will turn limewater cloudy.

Question 25

How do you test for hydroxide ions?

Answer 25

Hydroxide ions will turn red litmus paper blue.

Question 26

How do you test for Group 2 metal ions?

Answer 26

You place 10 drops of 0.1 moldm^-3 metal ion solution in a Test tube. Add 10 drops of 0.6 moldm-3 sodium hydroxide solution( to add hydroxide ions to the solution). Continue to add sodium hydroxide until in excess. If metal ion is Mg2+ Mg(OH)s is sparingly soluble so will form a white precipitate. Ca(OH)2 also white ppt because partially soluble in water. Sr and Ba will not form precipitates due to their high solubility. Same process with sulphates. Add metal ions and sulphuric acid. Ba and Sr will form white precipitates. Mg and Ca will no form a precipitate.

Question 27

How do you test for Ammonium ions?

Answer 27

Add sodium hydroxide solution to the solution. Warm gently and shake. Test fumes from mixture with red litmus paper. I fit turn blue then ammonia gas is released and ammonium ions are present.

Question 28

What are the observations with halide salts and sulphuric acid?

Answer 28

Br2 and orange gas and SO2 gas a colourless, acidic gas are produced in the redox step of Bromine. Fluorine and Chlorine are not strong enough reducing agents to reduce the S in H2SO4 so only undergo acid-base reaction. White steamy fumes of HF, HCL, HBR and HI are evolved. Black solid and purple fumes of iodine are produced. Yellow solid of Sulfur. H2S Hydrogen sulfide, a gas with a bad egg smell.

Question 29

What can beakers be used for?

Answer 29

Transporting solutions, water baths or reactions can be conducted in them.

Question 30

Why do we use measuring cylinders?

Answer 30

We use measuring cylinders to measure volumes. Always read from the bottom of the meniscus.

Question 31

What are conical flasks used for?

Answer 31

Usually fr reactions to occur in them. The measurements are imprecise o should not be use when accuracy is necessary. Usually clamp the top of the conical flask unless you want to shake the flask.

Question 32

What is a teat/Volumetric pipette and when do we use it? What is a graduated pipette? Difference between the two?

Answer 32

Teat pipettes are the ones we tend to use in the practicals to add small volumes of solutions. A Graduated pipette is a pipette which has much more accurate measurements on its side which allows for more accurate volumes to be measured when transferring solution. VOlumetric pipettes are used to transfer highly accurate volumes of a solution. They have a fixed graduation mark and should be used when you want to transfer a set, precise volume of solution to a beaker or something. It Is calibrated for a specific volume. Graduated pipette are generally less accurate than Volumetric pipettes and can be used when you want to deliver different volumes of solutions within a given range. It has markings on the Side which allow for different volumes of solution to be measured.

Question 33

Volumetric Flask?

Answer 33

A volumetric flask is used when making a standard solution. Has a line to show the mark which you should fill it up to.

Question 34

How do you separate two immiscible liquids?

Answer 34

Using a separating funnel.

Question 35

What apparatus do you use to crush up a solid into a powder?

Answer 35

Use a Mortar and Pestle.

Question 36

What apparatus do we use when doing Column Chromatography?

Answer 36

We use a Column, a very long column with a solvent reservoir on the middle and a porous material at the bottom.

Question 37

Why do we use Distillation?

Answer 37

To purify an Organic product. The temperature which you heat the round bottomed flask to should no be higher than the boiling point of the reactant in the round bottomed flask otherwise it will evaporate and will condense into the conical flask which is meant for the purified organic product. "In general to separate an. organic product from it's reacting mixture." Use a water bath to heat the round bottomed flask because the reactant in the flask might be flammable. If we want to heat something s o that it reacts and then collect the product by condensing it into a separate flask then we use distillation.

Question 38

Why do we use reflux?

Answer 38

We use reflux to continuously heat the reactants. If we want something to react and we don't want to collect the intermediate product by condensing and collection git separately then we can use reflect as the intermediate will condense again and react further. We use anti-bumping granules also to ensure small bubbles are produced and that it is not dangerous. "To heat organic reaction mixtures for long periods of time" Always make sure to keep the top os the column open to ensure there is no pressure build up.

Question 39

Where must the thermometer be in distillation and what other problems do you have to consider?

Answer 39

The thermometer must be at the junction of the column and the condenser. See sheets. Heat the reaction mixture in the pear-shaped flask using a water bath and not with naked flame as most organic reactant are flammable so this could set on fire. You could place the conical collection flask in a beaker of ice in order to improve condensing of the distillate.

Question 40

To maximise yield of the distillate in distillation where should you heat the reaction mixture up to?

Answer 40

You should heat it to the approximate boiling point of the distillate and not higher as this may mean some of the original reaction mixture may evaporate and condense in the distillate also.

Question 41

What must you do when handling potassium dichromate and sulphuric acid and why?

Answer 41

Potassium Dichromate- You must wear gloves as it is highly toxic and a carcinogen. Sulphuric acid- You must wear gloves because it is corrosive.

Question 42

What should be added to the round-bottomed flask in reflux?

Answer 42

Anti-Bumping Granules should be added to reduce the size of bubbles which could damage the apparatus. Must leave the top of the column open so there is no pressure build up.

Question 43

What is the the method for preparing Tollens' Reagent? Fehling's?

Answer 43

Place Silver Nitrate into two boiling tubes and add sodium hydroxide to form silver oxide. Add Ammonia solution drowse until a clear, colourless solution is formed. Add Fehling's A into two test tubes and then add Fehling's until blue precipitate redissolves. Add a few drops of the unknown and leave in the water bath for a few minutes.

Question 44

When is fractional distillation used and what reaches the top first?

Answer 44

It is used to separate liquids with similar boiling points. The vapour of the substance with the lower boiling point reaches the top of the column first. Heat should be at or below the boiling boiling point of the most volatile substance. (Highest B.P.)