ASRJC 2022 Inorganic QA Notes

1 Anderson Serangoon Junior College Inorganic Qualitative Analysis Notes CONTENT No. Description 1. Qualitative Analysis 2. Manipulative Skills 3. Recording of Observations 4. Appearance of Unknown 5. Solubility of Salts 6. Testing of Ions 7. Separation of Ions 8. Test for Gases 9. Heating 10. Qualitative Analysis Notes Name: ___________________________ Class: ___________________________

2 1. Qualitative Analysis Qualitative Analysis is a method used to identify ions in compounds based on chemical reactions that involve:  Evolution of gas  Formation of precipitate  Colour change 1.1 Safety In addition to the general lab safety rules, the following should be observed at all times when implementing qualitative analysis experiments:  Students must wear safety goggles.  Do not touch chemicals with your hands. Wash off immediately any chemicals that comes into contact with the skin.  Note the smell of a gas by wafting it gently to the nose. Do not take in deep breath of gas.  Dispose wastes carefully. Dilute concentrated acids and alkalis before pouring them away and flush the sink with plenty of water. Dispose solid waste into waste bins, not the sink.  A test-tube holder must be used when tests involve: o warming o adding concentrated acids, alkalis and flammable reagents. Such experiments should be carried out in the fume cupboard.  When warming the mixture or burning, direct the mouth of test-tubes away from people.  Turn off the Bunsen flame when not in use. 2. Manipulative Skills 2.1 Quantity of Chemicals  Collect sufficient quantities but not in excess. The use of large quantities of a chemical often leads to a waste of time as it takes a longer time to dissolve. In addition, this may lead to wrong results, e.g. if too much of lead iodide is used, its solubility in hot water will not be observed.  The following quantities should be used, unless otherwise specified: o Solid: Enough to fill the hemisphere at the bottom of a clean and dry test-tube. The solid should be transferred by means of a clean and dry spatula into the bottom of the test-tube. o Solution: ~1 cm depth (unless otherwise specified) of a clean test-tube.

3  When testing the solubility of precipitate in excess reagent , pour out the excess precipitate until about 1 cm depth is left, before adding the excess reagent. Figure 1. How to test for solubility of precipitate formed in excess reagent 2.2 Handling of Reagents When adding reagent, dropper or tip of squeeze bottle should be held just above the mouth of the te st-tube. The tip of the dropper/bottle should not touch the sides of the test-tube. This is to avoid contamination of the bench reagents.  Add the reagent dropwise and shake until no further change in mixture or when reagent is in excess. 2.3 Gravity Filtration technique 1. Fold filter paper: or 2. Place the filter paper in the funnel and wet it with a little deionised water. 3. Carefully pour the mixture from the test tube into the filter funnel. 4. Retain the filtrate and residue for further analysis if necessary. Add reagent to unknown Pour away excess until about 1 cm depth is left …. ….before adding excess reagent, dropwise, to the remaining ppt A ppt forms

4 3. Recording of Observations  Indicate at which stage a change occurs by writing the observations along side the tests performed. Record them immediately after each test in ink . Answers should include details such as: o Colour of salt or solution o Colour of precipitate (ppt) & its solubility in excess reagent o Colour change of solution o Colour of residue and filtrate after filtration o Colour of solid, before and after heating o Solubility of solid o Chemical test results for gas  “Clear” is different from “Colourless”. A clear solution may still be coloured. For example, aqueous Cu(NO)2 is a clear blue solution.  “Effervescence” vs “Evolved”: o “Effervescence” is for gases formed as bubbles during the reaction. o “Evolved” is when gases are produced but no bubbling is seen.  Making deduction: o Make logical deductions based on the nature of the test reagents used. o Deduction is consequential – ions that have been eliminated in the earlier tests need not be considered again in later deductions. o Deduction should include the word ‘present’ or ‘absent’. o Deductions should also be made for negative results, e.g. no ppt, no colour change, no gas evolved etc. o DO NOT write ‘no change or no observation’ for negative results.  For a test with more than one step, align your observations and deductions with the corresponding steps in the test or state clearly at which stage the observations were made. E.g. Test Observation Deduction To 1 cm depth of FA 1, add aqueous potassium manganate(VII) purple KMnO4 did not decolourise. absence of reducing agent. To 1 cm depth of FA 1, add aqueous ammonia, then add aqueous ammonia in excess white ppt observed ppt is soluble in excess NH3. Al3+, Mg2+ or Zn2+ may be present Zn2+ is present  Acceptable short form: ppt (precipitate)

5 4. Appearance of Unknown The appearance of the unknown in its solid form or in solution often provides a clue to its identity. However, further chemical tests are needed to arrive at the final deductions of the unknown. This is because there are several possibilities for a given appearance or a mixture of unknowns could be given. E.g. a mixture of FeCl3 and CuCl2 solution appears green. Appearance Solid Solution Pink hydrated Mn2+ salts, hydrated Co2+ salts Mn2+ (pale pink), MnO4– (in very dilute solution), Co2+ Brown FeCl3, Mn(OH)3, MnCO3, anhydrous CuCl2, CuBr2, Ag2O Fe3+, I2 in I– (I3–) Reddish-brown Cu2O, Cu2Fe(CN)6, Fe2O3, Fe(OH)3, Ag2CrO4 Br2 in CCl4 Orange K2Cr2O7, Cr2O72–, Br2 water Yellow hydrated FeCl3, BaCrO4, K4Fe(CN)6, AgI FeCl3, Fe3+, CrO42–, Fe(CN)64–, very dilute I2 in I– Green hydrated Fe2+ salts, hydrated CuCl2, CuCO3, Cr2O3, MnO, hydrated Ni2+ salts, NiCO3 Fe2+ (very pale green), Cu2+ (in concentrated solution), Cr3+ (bluish-green), Cr(OH)63–, MnO42–, Ni2+ Blue hydrated Cu2+ salts, anhydrous Co2+ salts Cu2+, Cu(NH3)42+ (deep blue), Ni(NH3)62+ Black CuO, CuS, FeO, FeS, MnO2, NiO, NiS, Co2O3, CoS, Ag2S, I2 (crystalline) Purple KMnO4 MnO4–, I2 in CCl4, Cr3+ (in concentrated solution) Red K3Fe(CN)6 Grey with metallic lustre metals in powder form

6 5. Solubility of Salts in Water Salts of Soluble Insoluble NH4+, Na+ & K+ all - NO3 all - NO2 all Ag+ SO42 all Ba2+, Ca2+ & Ag+ SO32 NH4+, Na+, K+ all Cl, Br and I all Ag+ CO32 NH4+, Na+, K+ all OH NH4+, Na +, K + (Ca2+ fairly soluble) all 6. Testing of Ions  The Qualitative Analysis Notes (see page 20 21) will be available during Practical assessment. Note: while the observations are available in the Qualitative Notes (Data Booklet), you may be asked to explain the chemistry involved using chemical equations as well as relevant concepts from e.g. periodicity, electrochemistry and transition metal chemistry.  List of possible cations & anions are: Cations: NH4+ Mg2+ Al3+ Ca2+ Cr3+ Mn2+ Fe2+ Fe3+ Cu2+ Zn2+ Ba2+ H+ Anions: CO32─ NO3─ NO2─ SO42─ SO32─ Cl─ Br─ I─ Experiments may be conducted involving ions other than those listed above to draw conclusions of a general nature (e.g. whether they are oxidising or reducing, etc.) 1. Always check out the physical appearance of unknown solid/solution which often provides a clue to the detection of the ions present. E.g.: colour of a solid or solution. (Refer to section 4) 2. Do not perform other experiments except to test for a gas when app