RI_Prelim_P2_ANS

Raffles Institution 2015 Year 6 Preliminary Examination Suggested Answers to H2 Chemistry Paper 2 1 (a) H2O2 acts as an oxidizing agent to oxidise Co(II) to Co(III) and itself reduced to H2O. (b) Preparation of the 100 cm3 crude mixture 1. Weigh out accurately about 1.45 g of Co(NO3)2 using an electronic balance and transfer this to a 250 cm3 beaker. 2. Measure out 45 cm3 of concentrated ammonia using a 50 cm 3 measuring cylinder and add this to the beaker. Stir with a glass rod to mix. 3. Measure out 25 cm 3 of hydrogen peroxide using a separate 50 cm 3 measuring cylinder and using a dropper, add this to the mixture dropwise/slowly. 4. Cool the mixture in an ice bath or carry out step 5 in an ice bath. 5. Measure out 30 cm 3 of concentrated nitric aci d using a separate 50 cm 3 measuring cylinder and using a dropper, add this to the mixture dropwise/slowly with stirring. 6. Remove the mixture from the ice bath, allow the mixture to sit for 10 mins for complete reaction. Precipitate the product from the crude mixture 7. Using a 100 cm 3 measuring cylinder, transfer 100 cm 3 of ethanol to the 250 cm3 beaker containing the crude mixture. 8. Allow the mixture to sit for 10 mins for complete precipitation. Separate the precipitated product from the mixture 9. Filter the precipitated product using vacuum filtration/gravity filtration and wash the residue with ethanol to remove the impurities. Recrystallisation 10. Transfer the residue into a 50 cm3 (or 100 cm3) conical flask with 2 boiling chips. 11. Add about 5 cm 3 (small amount) of ammonium nitrate solution into the conical flask and place it on the heating plate. (or a hot ammonium nitrate solution can be prepared prior to addition by placing it on a heating plate) 12. When the mixture is boiling and the solid does no t dissolve, add ammonium nitrate slowly to the mixture until the solid just dissolves. 13. Remove from heating plate and allow it to cool to room temperature slowly before cooling in an ice bath. Crystals will grow as the mixture is cooled. 14. Filter the cooled mixture using vacuum filtration/gravity filtration and wash the residu e with cold ethanol. 15. Dry the residue under the infra-red lamp.

(c) The concentrated nitric acid (or concentrated ammonia) is corrosive. Wear gloves and goggles when handling the chemical / do the experiment in a fumehood. OR Inhaling the corrosive fumes of concentrated ammonia . Carry out the experiment in a fumehood. OR Ethanol is flammable. Any heating should be done using a water bath and not with a naked flame. 2 (a)(i) sp (ii) The  bond in C=O bond of R-N=C=O is made up of sp sp2 head-on overlap whereas the  bond in C=O of CH3COCH3 is made up of sp2sp2 head on overlap. The sp sp2  bond in C=O bond of R -N=C=O therefore has a higher s character, hence leading to more effective overlap of orbitals and shorter bond length. (iii) Geometric (ci