Sunday, November 05, 2006
Some comments based on June 2006 Paper [Make sure u look thru the paper, similar questions may be set for A lvls.]
1) Besides using NaOH(aq) to test for presence of metal cations, it is also used to test for presence of ammonium ions. The ammonium ion (a conjugate acid of ammonia) will react with hydroxide ions to give ammonia gas and water (no heating is required as it is an acid-base reaction). Ammonia liberated will turn moist red litmus paper blue. Ammonium ion can be from an inorganic salt e.g. ammonium sulphate (NH4)2SO4 or from an organic salt e.g. ammonium ethanoate CH3COONH4.2) When using the general gas equation pV = nRT, make sure that p is in Pa, V is in m3, n is in mol and T is in K. Note that 1 atm = 101000 Pa. 3) The reactions of conOcentrated sulphuric acid with solid magnesium halides are the same as those of sodium halides. For magnesium chloride, white fumes of HCl will be produced.MgCl2(s) + H2SO4(l) -----> MgSO4(s) + 2HCl(g)For magnesium bromide, reddish brown fumes of Br2(g) are produced. Note that SO2 which is formed is a colourless gas with an irritating smell. This may not be observable as it may be masked by that of Br2(g). For magnesium iodide, violet fumes of I2(g) are produced. Note that H2S which is formed is a colourless gas with the smell of rotten egg. This may not be observable as it may be masked by that of I2(g).4) Hot, acidified K2Cr2O7 does not cause oxidative cleavage of alkene or side chain oxidation of benzene.5) A substitution reaction is one in which an atom or a group of atoms of a molecule is replaced by another atom or group of atom. For example, in the nitration of benzene, one hydrogen atom is replaced by -NO2 group.6) Oxides of nitrogen (NO or NO2) are homogeneous catalyst for the oxidation of SO2 to give SO3.NO(g) + 1/2 O2(g) ----> NO2(g)SO2(g) + NO2(g) ----> SO3(g) + NO(g) [regeneration of the catalyst, NO]Overall equation: SO2(g) + 1/2 O2(g) ----> SO3(g)7) Determination of concentration of Fe(II) ions using KMnO4 - Fill the burette with 0.0200 mol dm-3 of KMnO4 solution.
- Pipette out 25.0 cm3 of the solution containing Fe(II) ions. Acidify solution with 10 cm3 dilute sulphuric acid.
- Titrate the solution containing Fe(II) ions with KMnO4. Add KMnO4 dropwise near the end point. End point is reached when solution turns orange. Note the volume of KMnO4 used.
- Repeat the titration until consistent results are obtained (within 0.10 cm3)
- Average titre value should be taken for the calculations.
