Chemistry 9701 AS Level Notes - Paper 3

When recording results:

The number of decimal places used should be equal to half of the smallest division of scale of the instrument.

For digital instruments (except digital stopwatch), the number of decimal places used should be the same as that shown on the display. E.g.: mass with electric balance: 0.23g

For stopwatch (both analogue and digital), record to the nearest 1s

In all cases, do not give more or less number of decimal places.

Titration

Normally they will want you to record a rough titration and two accurate titrations. For the table:

Titration	1st	2nd
Final burette reading/cm3
Initial burette reading/cm3	0.00	0.00
Volume of ____ added/cm3

 All readings should be recorded to 2 decimal places.

The titre for the 2 accurate titrations should not differ by more than 0.10cm3.

Then, calculate the mean titre for the 2 accurate titrations. The mean should be given to 2 decimal places.

Reducing uncertainties in a titration:

·        Replacing measuring cylinders with pipettes or burettes which have lower apparatus uncertainty will lower the overall error.

·        To reduce the uncertainty in a burette reading the titre volume needs to be made larger. This could be done by increasing the volume and concentration of the substance in the conical flask or by decreasing the concentration of the substance in the burette.

Questions involving calculation

The number of significant figures of your answer should be equal to or one more than the number of significant figures of the raw value used in the calculation with the least number of significant figures.

 

Qualitative Analysis

For each chemical test, you should use about 1cm depth of each solution unless otherwise stated by the question.

Record full observation:

State if there is any colour change or if precipitate forms.

For any colour change, indicate both the initial and final colour, as well as the stage in which the change occurs if more than one reagent is added.

If precipitate forms, state the colour of precipitate and whether it is soluble in excess of the reagent added, and if it is soluble state the colour of the solution formed.

If you see any bubbles formed, it means that gas is released, and you should carry out gas tests to determine what the gas is. Only gas tests in the Qualitative Analysis Notes at the last page of question paper should be carried out. State that effervescence occurs and state the observations of the gas test which is positive and the type of gas released.

Distinguishing between Pb⁺² / Al⁺³ :

• NaOH / NH₃ reagents when used for identification of these ions give similar results (observations).
• PbCl₂, PbI₂, PbSO₄, PbCr₂O₇ or PbCrO₄, is insoluble. Use the following reagents: HCl, KI, K₂Cr₂O₇ or any other reagent that contains the ions mentioned above. The insolubility of the afore-mentioned lead compounds can help us distinguish between the Pb⁺² and Al⁺³ ions.
• Also, take care about other possible precipitates that might confuse your results. For example, BaCl_{2 (aq)} contains Cl^-1 and can be used to test for Pb⁺² ions but the presence of Ba⁺² ions makes precipitates of its own, so always use reagents which have Na⁺¹, K⁺¹, or H⁺¹ ions which always make soluble compounds, reducing the possibility of any other precipitates, except for the precipitates formed by Pb⁺².

Distinguishing between Ba⁺² / NH₄⁺¹ :

• Both give no precipitate with NaOH or NH₃ except that NH₃ is produced when warming NH₄⁺¹ with NaOH. Ba⁺² can be identified by H₂SO₄, it will give white precipitate.

Test for Manganese Mn⁺²

• Mn⁺² can be identified with NH₃ / NaOH. With these reagents, it has a white / pale brown ppt which turns brown when in contact with air and are insoluble in excess of the reagent. There would be brown residues floating on the top surface, and on the sides of the test tube, and white ppt/light brown ppt at the bottom.

Tests for Cu⁺²

• With NaOH: pale blue ppt insoluble in excess.
• With NH₃: Blue ppt which dissolves and forms a dark blue solution in excess. It will be hard to dissolve the precipitate if too much Cu⁺² are present in the test tube, so use very small quantity of Cu⁺² (less than 1 cm³) or use a lot of NH₃ (fill the  entire test tube) and shake vigorously to dissolve this precipitate.

Problems with Al⁺³ ions test with NaOH:

• The precipitate formed by Al⁺³ is very soluble and disappears very quickly. Students can easily make the mistake of not noticing any precipitate and writing down that no change occurred. Use a very tiny quantity of NaOH at first, just a few drops (put one drop, shake it lightly, then put another and so on), and a small white ppt will form floating on the surface of the solution, which would dissolve very quickly if a very small amount of NaOH is added.

 

The identification of the other cations and anions is easy. For them just refer to the salt analysis notes given at the end of the paper.

Identification of gases: 

Whenever a gas evolves, there is some sort of effervescence (bubbles form) that occurs in the solution. Whenever you notice such a thing, just put your thumb on the top of the test-tube. If the pressure builds up, there’ll be definitely some sort of gas evolving. Now the thing is that, how to identify them?

CO₃^-2 : If an acid is added, or is present in the test-tube and you see vigorous effervescence, then definitely it’s CO₂ that’s evolving. If you have time, just to counter check, test it with lime water, it will definitely turn milky.  Effervescence produced is similar to gas bubbles in coke. Generally produced when metal carbonates react with acids

 

NO₂^-1 : Whenever an acid is added, put your thumb top of the tube, and allow pressure to build up. The tube will turn pale brown and when you release your thumb and allow gas to escape, then a pale brown gas will release. If it occurs, then definitely NO₂^-1 is present in the solution. This pale brown gas is also very visible if seen in front of a white background. The gas is especially very visible when the reactants are thrown in the white sink and you will notice brown vapours in the sink easily.

 

SO₃^-2 : Another gas that is produced on addition of dilute acids is SO2 which indicates the presence of SO₃^-2 ions. SO₂ gas is colourless and acidic and is produced when dilute acid is added to sulfite SO₃^-2 ions. If a damp blue litmus paper is placed at the mouth of the tube, then it will turn red. Damp litmus paper must not touch the test tube itself as it might contain an acid. Note, that damp blue litmus paper will turn red when NO₂ gas is produced but NO₂ is pale brown and can be distinguished from SO₂. Another test for SO₂ gas is that it smells of rotten eggs or burnt matches. It can also be distinguished by dipping a paper in K₂Cr₂O₇ and then placing it at the mouth of the test tube. This paper will turn from orange to green.

 

NO₃^-1, NO₂^-1 : To test for these ions, NaOH is added followed by the addition of Al foil and heated. When bubbles start to form (vigorous bubbling), put a damp red litmus paper near the mouth of the test-tube. NH₃ is liberated if these ions are present, and it turns damp red litmus paper blue.

Always use damp red litmus paper, by making the litmus paper wet. Nothing happens if the litmus is not damp! And make sure that the litmus paper never touches the test tube, because the test tube might contain an alkali which will turn the litmus paper blue. A lot of students make the mistake of allowing the litmus paper to touch the top of the test tube, and in many cases an alkali is present in the test tube which makes the litmus paper blue. So, keep the litmus paper a fair distance (1 cm) away from the test tube.

Identification for hydrogen ( H₂ ) gas:

Metal + Acid —> Salt + H₂

Use the above equation to detect the hydrogen gas.

If you are adding metal, and a gas is produced, then you don’t necessarily need to test for Hydrogen gas, if you see effervescence, then it is obviously hydrogen. Just for confirming if you have time, test it. Hydrogen gas produces pop sound when burnt with a lighted splint. The only way it produces a pop sound when enough pressure is built up in the test tube. Put your thumb on top of the test tube and allow pressure to build up and only then light it.

Organic Chemistry:

Testing for Carbonyl compounds: ketones and aldehydes.

• Tollens Reagent: Tollens reagent is made by mixing AgNO3 and NH3. It gives a black precipitate with Aldehyde which has a silvery mirror floating on top. The observation should be that silver mirror is obtained with Tollens Reagent. Most of the time this silver mirror will not be visible, so black precipitate is enough to test for the presence of aldehyde.

 

• Fehling Solution: Fehling solution also tests for the presence of Aldehyde. Aldehyde is added to Fehling Solution and heated lightly. A red/brown precipitate is obtained

 

• 2,4 DNPH: This is an orange coloured solution, which has a strong acid in it. Be careful when using this. 2,4 DNPH has forms a yellow or orange precipitate with carbonyl compounds (both ketones and aldehydes). Remember that anything when it is added to 2,4 DNPH will turn yellow because it has a yellow colour. So, you should be looking for yellow precipitates and ignore the colour of the solution.

 

• Carboxylic Acid Test: If you add Na₂CO₃ and vigorous effervescence is observed, then the compound present is carboxylic acid.

 

• Potassium Di Chromate: It will turn from orange to green with alcohols and aldehydes, but the mixture has to be gently heated otherwise the colour change wouldn’t be visible. If it is strongly heated, then aldehyde and alcohols will evaporate. You should also not add Potassium di chromate in excess, as a lot of it will not get reduced and you will get a mixture of green and orange which would be very hard to distinguish.

 

• Potassium Manganate (VII): If it is added to a solution, and the solution then warmed in a water bath; if the purple colour of the solution disappears then in the solution either an aldehyde or alcohol is present.