DISTICNTION IN O-LEVEL CHEM- HOW!!ISTRY
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To UltimaOnline: I am coaching a very good student these days. She is brilliant and hard working. I want her to get a world distinction in O-level Chemistry. So I need your advice on what I should do besides covering the syllabus and past papers.
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Originally posted by Kahynickel:
To UltimaOnline:
I am coaching a very good student these days. She is brilliant and hard working. I want her to get a world distinction in O-level Chemistry. So I need your advice on what I should do besides covering the syllabus and past papers.
Every student is unique, and responds differently to different approaches. As her tutor, you're in the best position to assess her learning style and tailor optimally to her needs.
One further point : unlike A levels, in which going beyond the syllabus can be helpful (eg. I teach Uni level stuff to my JC tudents, helps to give them an advantage), for O level Chemistry going beyond the syllabus is significantly less helpful, and getting a distinction is more a matter whether the student has thoroughly memorized everything the O level syllabus requires, and whether the student is careful enough not to make any careless mistake (including not reading the question carefully, not using all the data given, calculation errors, etc) that would cost the student his/her position on the treacherous bell-curve. -
Consider the following question: Choose from the following compounds to answer the questions below BaSO4 CH4 C2H4 C3H8 CO2 CaCO3 CF3Cl K2Cr2O7 MgSO4 NaCl ZnSO4 Each compound can be used once, more than once or not at all. Which compound (a) is responsible for ozone depletion, .................................................................................................................................... (b) is formed by the bacterial decay of vegetable matter, .................................................................................................................................... (c) is used to remove sulfur dioxide in flue gas desulfurisation, .................................................................................................................................... (d) is an insoluble salt, .................................................................................................................................... (e) is orange in colour, .................................................................................................................................... (f) decolourises aqueous bromine? .................................................................................................................................... The above compounds can be used to answer these question. Now I have told my student that, besides making a track record of all the worksheets of relevant topics, make a list of the compounds whom she is encountering during my lectures. For example i have recently covered ionic and covalent bonding and i have used plenty of examples which to her are new entities such as MgO, AlF3, CH4, CF3Cl, N2, HCN, CO2 and many more. Therefore as soon as the studies progresses she will be acquainted with a large supply of these new compounds specially when electrolysis will be taught. At the end of the syllabus I will ask her to consider each entity and recall where it has found applications like the question I have quoted. Although it will be quite cumbersome for a student to do that but I think this technique will surely work in not only retrieving the factual information regarding a topic but enhancing her memory skill and her ability to use chemicals. What do you think Sir UltimaOnline?
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Originally posted by Kahynickel:
Consider the following question:
Choose from the following compounds to answer the questions below
BaSO4 CH4 C2H4 C3H8 CO2 CaCO3 CF3Cl K2Cr2O7 MgSO4 NaCl ZnSO4
Each compound can be used once, more than once or not at all.
Which compound
(a) is responsible for ozone depletion,
....................................................................................................................................
(b) is formed by the bacterial decay of vegetable matter,
....................................................................................................................................
(c) is used to remove sulfur dioxide in flue gas desulfurisation,
....................................................................................................................................
(d) is an insoluble salt,
....................................................................................................................................
(e) is orange in colour,
....................................................................................................................................
(f) decolourises aqueous bromine?
....................................................................................................................................The above compounds can be used to answer these question.
Now I have told my student that, besides making a track record of all the worksheets of relevant topics, make a list of the compounds whom she is encountering during my lectures. For example i have recently covered ionic and covalent bonding and i have used plenty of examples which to her are new entities such as MgO, AlF3, CH4, CF3Cl, N2, HCN, CO2 and many more.
Therefore as soon as the studies progresses she will be acquainted with a large supply of these new compounds specially when electrolysis will be taught. At the end of the syllabus I will ask her to consider each entity and recall where it has found applications like the question I have quoted. Although it will be quite cumbersome for a student to do that but I think this technique will surely work in not only retrieving the factual information regarding a topic but enhancing her memory skill and her ability to use chemicals.
What do you think Sir UltimaOnline?
That's certainly a good technique Kahynickel, for students of higher caliber that are able to work with this, such as your student. However, it is noteworthy that using this technique, you will inevitably have to go beyond the 'O' level syllabus, to explain for instance, why compounds such as AlCl3 has a lower-than-expected melting/boiling point for a what most 'O' level students would consider as an ionic compound : because it is more covalent than ionic (despite being a compound consisting of a metal and non-metal), in turn because of the smaller electronegativity difference between Al and Cl, and because the Al3+ ion has high charge density and thus can attract the readily polarizable Cl- anionic electron charge cloud (because Cl- is large and hence its valence electron charge clouds are relatively weakly electrostatically attracted by the +vely charged nucleus).Generally, Cambridge will try to avoid such problematic compounds in setting 'O' level questions. But if you're going all the way with your technique, then you shouldn't shy away from teaching your student beyond the syllabus : just explain to her such (going beyond the syllabus) is helpful for her understanding and to indirectly prepare her in advance for 'A' levels, but she needn't stress over memorizing such (beyond the syllabus content), since it's unlikely for Cambridge to ask directly on such content.In other words, as long as your student is up for it, then learn for the sake of truly learning, rather than for the sake of memorizing for getting paper qualifications.One more point regarding electrolysis at 'O' levels :I teach my 'O' level students to be able to understand the concept and to use the Data Booklet's standard reduction and oxidation potentials, to work out which species is reduced at the cathode, and which species is oxidized at the anode.
At 'O' levels, the term "discharged" is often used (eg. "Cu2+ instead of Fe2+ is preferentially discharged at the cathode since Cu is less reactive than Fe" is a statement commonly taught at 'O' levels), which is not as comprehensive or as correct as "reduced" or "oxidized".(In addition, at 'A' levels, this topic is also poorly taught in most JCs, which wrongly teach 'A' level students the use of the ridiculously erroneous term "eeee-nought" or "eeee-naught" when it should be pronounced as "E-standard" and symbolizing either "standard cell potential" or "standard reduction potential" or "standard oxidation potential" or "standard electrode potential" all 4 terms actually having different definitions in electrochemistry).I teach my 'O' level (and 'A' level) students to use the terms "reduction potential" and "oxidation potential" and to write "reduced at cathode" or "oxidized at anode" instead of "discharged" (which technically means 'to remove a charge', and can therefore only be technically used on ionic species and not molecular species).For instance, in the purification of copper, I teach my 'O' level (and 'A' level) students to read (and over time, internalize and memorize not the values, but the relative order of reduction and oxidation potentials, and reactivities) from the Data Booklet (graphic above), that :At the anode, Cu is preferentially oxidized to Cu2+, and less reactive metals such as Ag is not oxidized to Ag+, because the oxidation potential of Cu to Cu2+ is more positive (under standard conditions : -0.34V) than the oxidation potential of Ag to Ag+ (under standard conditions : -0.8V). Consequently, less reactive metals such as Ag, are left unoxidized at the anode and fall (due to greater density of metals compared to the aqueous electrolyte solution) to the bottom of the setup as anodic sludge, which is later processed to extract the more valuable unreactive metals.At the cathode, the Cu2+ is preferentially reduced to Cu, and the ions of more reactive metals such as Fe2+ (which were previously oxidized at the anode together with copper) are not reduced to Fe, because the reduction potential of Cu2+ to Cu is more positive (under standard conditions : +0.34V) than the redution potential of Fe2+ to Fe (under standard conditions : -0.44V). Consequently, ions of more reactive metals such as Fe2+, are left unreduced at the cathode, and remain in the electrolyte solution.In this way, impure copper at the anode is transferred to form pure copper at the cathode.Tell your 'O' level student that the reduction and oxidation potentials given in the Data Booklet are indicative of the potential = tendency = propensity = inclination for a species to be reduced (ie. its reduction potential) or oxidized (ie. it's oxidation potential).For 'A' level students :Standard Cell Potential = Standard Reduction Potential @ Cathode + PLUS + Standard Oxidation Potential @ Anode.The formula often-taught in JCs of "e-nought cell = e-nought reduction - e-nought oxidation" or "e-nought cathode - e-nought anode" or "e-nought right - e-nought left" are all technically wrong, even though they give the same correct answer as the correct formula I've described above.If you prefer a minus sign to be used in the formula (which doesn't make as much sense as a plus sign, since how well a company performs is the sum of how well each department in the company performs; likewise the potential of a Galvanic/Voltaic cell should similarly be the sum of the potential of its two compartments the cathode (where by definition reduction occurs) and the anode (which by definition oxidation occurs) remember Red Cat riding An Ox), then the correct formula should be :Standard Cell Potential = Standard Reduction Potential @ Cathode - minus - Standard Reduction Potential @ Anode.This formula doesn't make as much sense for another reason : by definition of anode only oxidation occurs, therefore we should be looking at oxidation potential @ anode, not reduction potential.The JCs are doing a dis-service to JC students teaching them wrongly like this.A further point of note regarding representation of 'dative' bonds at 'A' levels.
This is another aspect that is poorly taught in JCs : different JC teachers (even within the same JC) often teach this differently, and end up confusing students.
The correct (ie. most recommended, Cambridge-accepted) version is as follows :
Physical Chemistry :
Kekule or Lewis structures :
Once the dative bond is shown (ie. with the arrowhead), there should NOT be any more lone pair at the base of the dative bond.
Dot-&-Cross structures :
Do NOT show the dative bond arrowhead in dot-&-cross structures. Instead, show the dative bond as a dot-dot or a cross-cross.
For physical chemistry (ie. Kekule or Lewis structures), note that formal charges should be shown after (not before) the dative bond is formed. However, Cambridge will accept both versions (in case you're worried). For instance, in the NH4+ ion, is the +ve formal charge on the N atom or on the H atom?
If you show formal charges after the dative bond is formed, then the N atom bears the +ve formal charge (recommended for physical chemistry).
If you show formal charges before the dative bond is formed, then the H atom bears the +ve formal charge (recommended for inorganic chemistry, specifically complex ions).
Formal charges should be shown on Kekule structures, but not for dot-&-cross structures (because the +ve formal charge looks like a cross).
Inorganic Chemistry (specifically, complex ions) :
Because the non-metal ligands are more electronegative than the central metal ion, it makes a lot more sense to show the formal charges before the co-ordinate dative bond is formed. (Alternatively, think of it as showing the oxidation state / number on the metal ion, which will always be positive; since oxidation state = formal charge + electronegativity consideration).
If you show formal charges after the co-ordinate dative bond is formed, the central metal ion will bear a -ve formal charge, which does not accurately portray the true charge on the metal ion, since metals are electropositive and thus its positive oxidation state will better reflect the true charge on the metal atom.
Finally, for complex ions, it is preferred that lone pairs indeed be shown at the base of the co-ordinate dative bond, ie. on the ligand's donor atoms. Notice this presentation is in direct contrast (but with good rationale which I've just explained in the preceding paragraphs) to the preferred presentation for Kekule or Lewis structures in Physical Chemistry.
Organic Chemistry :
In Organic Chemistry, NEVER show the 'dative-bond arrowhead'. Instead, we use CURVED ARROWS to show the flow of electrons (and NOT the dative-bond arrowhead).
In Organic Chemistry, we can still use the term 'dative bond' loosely for the sake of convenience, eg. "a nucleophile donates a dative bond to an electrophile" is still an ok statement, but in terms of showing the mechanism, we use a curved arrow instead of the dative-bond arrowhead, and we show the mechanism in two (or more) separate stages.
In this sense, the 'dative-bond arrowhead' of physical and inorganic chemistry, is really just a mini-mechanism, combining two stages (in an organic chemistry mechanism) into one diagram.
In Organic Chemistry, formal charges must ALWAYS be shown, while partial charges must be shown WHENEVER RELEVANT (eg. whenever neutral nucleophiles, bases or electrophiles are involved).
Finally, be aware that the curved arrow in Organic Chemistry mechanisms MUST have at its base, either a lone pair or a bond pair. Every year, many students errorneously (perhaps because they were poorly taught in their JCs) draw the curved arrow originating from a -ve formal charge or a -ve partial charge or the (nucleus of the) atom itself. Sacrilege!
In Organic Chemistry, curved-arrow electron-flow mechanisms can be either for chemical reactions (ie. sigma bonds are broken and/or formed) or for resonance (ie. only pi bonds are broken and/or formed). For chemical reactions, use double-half (equilibrium) arrow or normal arrow to link together the different stages. For resonance, use double-headed (resonance) arrow to link together the different resonance contributors (also called resonance structures).
The resonance hybrid, or true version of the molecule, is the weighted (based on factors including electronegativity) average of the resonance contributors, and has partial charges and partial double (or triple) bonds.
For instance, the resonance contributors of benzene have alternating double and single bonds. The resonance hybrid has partial double bond character between all adjacent C atoms, and usually represented by a continuous circle within the benzene ring (as commonly taught in JCs).
One annoying bugbear regarding the benzene ring, which I find many JCs teach errorneously, is the often quoted statement in JC lecture notes that "the Kekule structure of benzene is wrong".
Nonsense! The Kekule structure of alternating double and single bonds, is perfectly CORRECT for the resonance contributor of benzene. What the statement should have been, is "the non-resonance model of benzene is wrong". A Kekule structure can be correctly shown for either the resonance contributor (ie. alternating double and single bonds), or for the resonance hybrid (ie. the continuous circle within the benzene ring). Both are Kekule structures!
But I don't blame JCs for teaching 'wrongly', because (JC teachers and the MOE-SEAB syllabus setters figure that) most JC students can't handle (or aren't interested in) the truth. So without teaching students about resonance (for which because the H2 syllabus doesn't direct test on, and for which the JCs already struggle to finish the syllabus, and for which many JC students are not interested in truly understanding chemistry but are content with blindly memorizing), it's no wonder the (all things considered, still) errorneous statement "the Kekule structue of benzene is wrong" is still taught in many JCs. But it's still misleading, and students are still poorer (in their understanding of Chemistry) for it. -
Woah.
I feel completely out of place now as a H1 Chemistry student, haha. -
Originally posted by Dejomel:
Woah.
I feel completely out of place now as a H1 Chemistry student, haha.
Tis a real pity you take H1 instead of H2. Chemistry should always be taken at H2, never at H1. Because H2 Chemistry (alongside H2 Physics) is a prerequisite for a majority of Science courses in the Uni, while H1 Chemistry (or H1 Science in general) is useless for use as prerequisites.It's not too late, you can always take H2 Chemistry as a private candidate after you leave JC. In fact, while you're at it, it's enriching to take up triple H2 Science (as a private candidate), for the fun of it!
