Monday, November 06, 2006
1) No units for Ar and Mr (since they are ratio of 2 masses). Relative formula mass of an ionic compound is the same as Mr of the compound; no units too. Units for molar mass is g mol-1.
2) To prove 1st order reaction by the continuous method, it is recommended that the half life method is used. Construction lines must be clearly drawn to show that the two half-lives (not necessary consecutive half-lives) are constant with the half-lives stated.
3) Autocatalysis is a type of catalysis whereby one of the products of the reaction catalyses the reaction. (See lecture notes and page 119 TYS question 6)
An example is the reaction between manganate(VII) and ethanedioate ions in acidic medium. Mn2+ formed acts as a catalyst for the reaction.
The rate of such a reaction is slow at the beginning of the reaction (since there is hardly any catalyst).
As the reaction proceeds, concentration of Mn2+ increases the rate of reaction increases.
The rate of reaction will eventually decrease as concentration of reactants decreased to very low levels despite the increasing concentration of the catalyst.
4) When calculating rate constant, make sure u give the correct units based on the overall order of the reaction. Note that rate constant and rate is always positive or zero (never negative).
5) When drawing reaction pathway diagram (or commonly known as energy profile), make sure u label the two axes with the y-axis labelled "energy". There should be two horizontal lines, one labelled "reactants" and the other "products". Include the activation energy as well as the enthalpy change of reaction if the data are provided.
6) Activation energy is the minimum energy reactants must possess in order to overcome the energy barrier to form the products.
7) A catalyst increases the rate of a chemical reaction by providing an alternative reaction pathway with lower activation energy. This results in an increase in number of particles having energy greater or equal to activation energy, thus increasing the number of effective collision per unit time. Note that a presence of a catalyst will increase the rate constant but NOT the equilibrium constant.
8) The bond in O2 is a double bond, i.e. O=O. So make sure u quote the right bond energy when calculating the enthalpy change of atomisation for oxygen.
9) Bond breaking is endothermic and bond formation is exothermic.
10) For some definitions, if u r not sure of how to write in a statement, u should give the mathematical expression instead. For example, pH is the negative logarithm to base 10 of the concentration of H+ can be given as pH = -lg[H+].
11) In the electrolysis of brine, the purpose of the asbestos diaphragm is to prevent mixing of the products (Cl2 and NaOH). Thus, it prevents Cl2 from undergoing disproportionation in NaOH(aq) to form NaClO(aq) and NaCl(aq). Note that the diaphragm cell operates at relatively low temperature, NaClO3 is not formed.
12) Sodium hydroxide is used in making of soap (via saponification) and detergent, used in refining aluminium oxide and for manufacturing pulp, paper and textile (rayon - artificial silk). Note: the use of NaOH(aq) in school and college laboratory is NOT acceptable as a large scale use.
Chlorine gas is used as a germicide (sterilisation of water), bleaching agent (for cotton, linen & wood pulp), making polyvinylchloride (PVC used in water pipes). Note: the use of chlorine in manufacturing of CFCs is NOT acceptable.
Hydrogen gas is used in the making of ammonia (Haber Process), for the hydrogenation of fats and oil (manufacturing of margarine) and in metal welding using an oxy-hydrogen flame (2000 degree celsius).
13) For reaction of Cl2 with alkene, it is important to state the physical state of Cl2 (g, aq or in organic solvent such as CCl4) since different products will be obtained.
14) When describing colour change, there is a need to state the initial colour. E.g. Orange K2Cr2O7 turns green.
15) When analysing mass spectrum, make sure that the species are singly positively charged unless otherwise stated by the question.
16) Labile protons are found in alcohols, phenol, carboxylic acids, primary amines, secondary amines, primary amides and secondary amides.
[8:10 pm] chicky
Sunday, November 05, 2006
Make sure u have yr entry proof and EZ link card / ID before reporting to the exam venue.
Don't panic and try to calm your nerves.
1) At the start of the paper, read quickly through the paper to get a feel of the demands of each question.
2) Select the question that looks most familiar and the easiest to attempt first. A good start helps to build up yr confidence. Try to order your choice of questions in this manner.
3) For Q3 either / or (Physical Chem), Q5 either / or (Inorganic Chem) and Q 8 either / or (Organic Chem), it is important look through both questions carefully before making your choice. Note that it is unlikely that u have time to change to another question after u start working on one.
4) Note that u have 15 minutes for each question. Organise your time effectively. For any parts which u dunno what to do, move on the the next part and come back to it later. Remember to leave sufficience space for questions which u hv skipped. All answers (except for graph work or diagrams such as Boltzmann distribution) must be given in blue or black ink.
5) Make sure you read the question carefully, make sure that u r answering the question. Check twice - a second spent here can save a lot of time later.
6) Do write out your answers in neatly and show your workings clearly with proper statements.
7) Do use diagrams in your explanation.
8) On completion of a question, do read through the question again to make sure that (a) you have not omitted any part of it, (b) all diagrams and graphs are properly labelled (c) final answer is given to 3 significant figures and (d) units are stated (e) signs are given for oxidation states, enthalpy changes and electrode potential.
[8:11 am] chicky
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.
