Originally posted by Kahynickel:I am making some new questions for AS organic chemistry. I have a reservation regarding option 3 of the following question.
I think it to be incorrect since the stereoisomerism cannot be guessed by just looking at the molecular formula therefore the answer is both 1 and 2 are correct- B is the answer. My students thought that option 3 is also correct. Question is given here…...
A compound, X, has the formula C6H12.
Which of the following piece of information is true about X.
1 It is an alkene
2 It is a cycloalkane
3 it shows stereoisomerismWhat do you say??
Yes, you're correct.
Option 3 should be rephrased to "it (some alkenes) may be capable of stereoisomerism".
Originally posted by Kahynickel:Thank you for the help.
I want to ask that option 3, if we rephrase it as you are saying as “capable of stereoisomerism”, should be true or unclaer in the context of this question since this cannot be deduced from molecular formula. Now I kept this option 3 as incorrect and students are treating it as “correct”. Is placing of “option 3” is correct??
As you correctly said earlier, you can't tell by the molecular formula whether or not this molecule is capable of stereoisomerism, but the way this (poorly phrased) question is phrased, seems to imply that as long as there is any molecule (among many possible molecules with this formula) that fits the option "it is capable of stereoisomerism", then the student should include this option 3.
In other words, if the question is "Based on the molecular formula, which of the following might be true?", then the answer is 1, 2 and 3.
But if the question is "Based on the molecula formula, which of the following must definitely be true?", then the answer is none of the above.
But if the original AS question is exactly as you phrased it, "Which of the following piece of information is true about X?", then this is such a poorly phrased question that the answer can either be 1, 2 and 3 or none of the above.
I suggest you teach your students by explaining to them that this is a poorly phrased question, and that the answer can either be 1, 2 and 3 or none of the above. As long as your students understand this, then they'll have to make their best guess as to what Cambridge (or whatever examining authority) wants as the correct answer.
Your unceasing and timely assistance has enabled me to make questions which covers maximum areas of the syllabus.
As you know that CIE A2 papers are being mixed with AS chemistry so students needs to apply their concepts to the Applications of chemistry. In this respect I made the following question in June 2010 session which covers the support booklet part Ionic Liquid and linked it to AS concept of Ideal gases.
I need your assistance that whether the question is of CIE standards or any way i can modify the question.
QUESTION
A supercritical fluid (SCF) is a gas that is compressed and heated so that it shows properties of a liquid and a gas at the same time. The fluid is neither a gas nor a liquid and is best described as intermediate to these two extremes. A supercritical fluid retains solvent power approximating liquids as well as the transport properties common to gases. They have replaced organic solvents in various applications. The table shows some of SFCs and their critical conditions.
|
Fluid |
Critical temperature (C) |
Critical pressure (bar)
|
|
Carbon dioxide |
31.1 |
73.8 |
|
Water |
374 |
221.2 |
|
Ammonia |
405 |
113.5 |
Supercritical CO2 (ScCO2) is the most widely-used fluid on large-scale. It has found applications in the extraction of foodstuff, removal of caffeine and as cleaning agent.
(b)(i) By referring to the table suggest one advantage for the large-scale of ScCO2 as compared to ScH2O and ScNH3.
(ii) At room temperature CO2 is not an ideal gas. Suggest in terms of Kinetics Molecular Theory how this behaviour is changed in ScCO2.
b) Give the name of one gas which shows ideal behaviour
c) Besides CO2, water is the other commonly applied supercritical solvent. Suggest one limitation of using supercritical water as solvent as compared to ScCO2 in the separation of organic compounds.
Originally posted by Kahynickel:Your unceasing and timely assistance has enabled me to make questions which covers maximum areas of the syllabus.
As you know that CIE A2 papers are being mixed with AS chemistry so students needs to apply their concepts to the Applications of chemistry. In this respect I made the following question in June 2010 session which covers the support booklet part Ionic Liquid and linked it to AS concept of Ideal gases.
I need your assistance that whether the question is of CIE standards or any way i can modify the question.
QUESTION
A supercritical fluid (SCF) is a gas that is compressed and heated so that it shows properties of a liquid and a gas at the same time. The fluid is neither a gas nor a liquid and is best described as intermediate to these two extremes. A supercritical fluid retains solvent power approximating liquids as well as the transport properties common to gases. They have replaced organic solvents in various applications. The table shows some of SFCs and their critical conditions.
Fluid
Critical temperature (C)
Critical pressure (bar)
Carbon dioxide
31.1
73.8
Water
374
221.2
Ammonia
405
113.5
Supercritical CO2 (ScCO2) is the most widely-used fluid on large-scale. It has found applications in the extraction of foodstuff, removal of caffeine and as cleaning agent.
(b)(i) By referring to the table suggest one advantage for the large-scale of ScCO2 as compared to ScH2O and ScNH3.
(ii) At room temperature CO2 is not an ideal gas. Suggest in terms of Kinetics Molecular Theory how this behaviour is changed in ScCO2.
b) Give the name of one gas which shows ideal behaviour
c) Besides CO2, water is the other commonly applied supercritical solvent. Suggest one limitation of using supercritical water as solvent as compared to ScCO2 in the separation of organic compounds.
Hi Kahynickel,
I am grateful to you for these moments of attention you are bestowing upon me. I have no hesitation in disclosing my answers at this forum because for this particular question and for all which are in pipeline.
In fact, this is just one question from the mock paper 1 I prepared for June 2010 session. I have the whole paper (10 questions) so is it possible for you, if you like, that I e-mail you the whole paper alongwith the answers. I am definitely sure that you will be pleased to witness the questions. [You will have to send your e-mail via Private message]
Otherwise I will answer this question on this particular forum.
Thank you very much.
Originally posted by Kahynickel:I am grateful to you for these moments of attention you are bestowing upon me. I have no hesitation in disclosing my answers at this forum because for this particular question and for all which are in pipeline.
In fact, this is just one question from the mock paper 1 I prepared for June 2010 session. I have the whole paper (10 questions) so is it possible for you, if you like, that I e-mail you the whole paper alongwith the answers. I am definitely sure that you will be pleased to witness the questions. [You will have to send your e-mail via Private message]
Otherwise I will answer this question on this particular forum.
Thank you very much.
Hi Kahynickel,
Ok then. I am not including the prologue of the question. Here are the answers.
(i) By referring to the table suggest one advantage for the large-scale of ScCO2 as compared to ScH2O and ScNH3.
ScCO2 requires low temperature and low pressure which requires less electricity and less time than Sc NH3 and Sc H2O which requires high temperaures needs larger amount of electricity and longer time to produce this temperature.
At room temperature CO2 is not an ideal gas. Suggest in terms of Kinetics Molecular Theory how this behaviour is changed in ScCO2.
at
At low temperature CO2 has van der Waal's forces so does not behave as ideal gas. At high temperature the vander Waal's forces would be overcome CO2 will become a super critical fluid. [I am not taking into account the high pressure because gases become ideal at low pressure]
(
(iii) Give the name of one gas which shows ideal behaviour. Helium
Besides CO2, water is the other commonly applied supercritical solvent. Suggest one limitation of using supercritical water as solvent as compared to ScCO2 in the separation of organic compounds.
Organic compound may react with ScH2O (Hydrloysis) as it is reactive while ScCO2 is non-polar overall and less reactive than ScH2O.
All good questions and good answers.
One suggestion though :
The question
At room temperature CO2 is not an ideal gas. Suggest in terms of Kinetics Molecular Theory how this behaviour is changed in ScCO2.
could be modified to avoid the problem of pressure contradicting temperature, in terms of ideal vs non-ideal gas behaviour.
For instance, the question could be modified to :
Describe and explain in terms of Kinetic Molecular Theory, how the behaviour of CO2 is altered, when the temperature and pressure are changed from room temperature and pressure, to the supercritical temperature and pressure.
The answer could be along the lines of :
how the high temperature ensures that supercritical CO2 has gaseous properties.
and
how the high presure ensures that supercritical CO2 has some liquid properties.
Originally posted by Kahynickel:Ok then. I am not including the prologue of the question. Here are the answers.
(i) By referring to the table suggest one advantage for the large-scale of ScCO2 as compared to ScH2O and ScNH3.
ScCO2 requires low temperature and low pressure which requires less electricity and less time than Sc NH3 and Sc H2O which requires high temperaures needs larger amount of electricity and longer time to produce this temperature.
At room temperature CO2 is not an ideal gas. Suggest in terms of Kinetics Molecular Theory how this behaviour is changed in ScCO2.
At low temperature CO2 has van der Waal's forces so does not behave as ideal gas. At high temperature the vander Waal's forces would be overcome CO2 will become a super critical fluid. [I am not taking into account the high pressure because gases become ideal at low pressure]
(iii) Give the name of one gas which shows ideal behaviour. Helium
Besides CO2, water is the other commonly applied supercritical solvent. Suggest one limitation of using supercritical water as solvent as compared to ScCO2 in the separation of organic compounds.
Organic compound may react with ScH2O (Hydrloysis) as it is reactive while ScCO2 is non-polar overall and less reactive than ScH2O.
I am very sorry for late reply, I was busy in students.
Thank u very much for these suggestions. I am thinking on some other lines I will discuss with you.
As you said that you are specific for singapore A-levels rather than CIE. Do you think its better than CIE.
Originally posted by Kahynickel:I am very sorry for late reply, I was busy in students.
Thank u very much for these suggestions. I am thinking on some other lines I will discuss with you.
As you said that you are specific for singapore A-levels rather than CIE. Do you think its better than CIE.
No problem at all.
The Singapore-Cambridge 'A' level papers are still set by Cambridge University, same as the Cambridge International Exams (CIE). Just a slight difference in syllabus emphasis.
The Singapore-Cambridge 'A' level H2 Chemistry syllabus :
http://www.seab.gov.sg/aLevel/2013Syllabus/9647_2013.pdf
One (non-academic) difference, is the availability of exam papers. The Singapore-Cambridge 'A' level papers are not available online, unlike the CIE papers (since they're intended for an international audience) which you can buy (with credit card) from Cambridge's website.
(Incidentally, you can also buy the International Baccalaureate exam papers from the IB website (with credit card). Their Chemistry papers are good practice for any 'A' level Chemistry student).
A year or two ago, you (Kahynickel) asked me how to obtain the Singapore-Cambridge 'A' level papers, and I sent you the Singapore Examinations and Assessments Board (SEAB) website, which had the email addresses of authorized publishers.
http://www.seab.gov.sg/aLevel/aPublications.html
Were you ultimately able to obtain any of the Singapore-Cambridge 'A' level papers?
No.
They wroth 18 dollars but when shipped to Pakistan they costed me 118 dollars due to shipment charges. Recently I contacted my relative who occasionally visit Singapore and procided him the ISBN of the books I am interested in.
As for Singapore syllabus vs the CIE:
1- The syllabus content for CIE is lengthy including the Suuport booklet for A2 in CIE.
2-Entropy, Free gibb's energy are not taught in CIE.
The following question appeared in the Prelim papers for A-level 2010-2011 sinagapore which i downloaded from internet.
Q. Straight chain alkanes undergo cracking under high temperature and in the presence of a catalyst. Which of the following does not represent a correct cracking reaction?
A C10H22 C6H14 + 2C2H4
B C13H28 C7H16 + 2CH3CH=CH2
C C7H16 CH3CH=CH2 + CH(CH3)3
D C11H24 C4H10 + CH2=CH2 + C5H10
The answer is C.
In catalytic craking the molecule undergoes rearrnagement to a larger extent than in thermal cracking cracking therfore why C is not correct?
Originally posted by Kahynickel:The following question appeared in the Prelim papers for A-level 2010-2011 sinagapore which i downloaded from internet.
Q. Straight chain alkanes undergo cracking under high temperature and in the presence of a catalyst. Which of the following does not represent a correct cracking reaction?
A C10H22 C6H14 + 2C2H4
B C13H28 C7H16 + 2CH3CH=CH2
C C7H16 CH3CH=CH2 + CH(CH3)3
D C11H24 C4H10 + CH2=CH2 + C5H10
The answer is C.
In catalytic craking the molecule undergoes rearrnagement to a larger extent than in thermal cracking cracking therfore why C is not correct?
You're right, Kahynickel. This question is flawed. The Singapore 'A' level syllabus doesn't include cracking (either thermal or catalytic), hence whoever set this question wanted the Singapore student to assume no rearrangement.
Thank you very much for again answering my query.
You're totally welcome, Kahynickel.
reaction ralative rate
RCH3 to RCH2Cl 1
R2CH2 to R2CHCl 7
R3CH to R3CCl 21
Using this information, and considering the number of hydrogen atoms of each type
(primary, secondary or tertiary) within the molecule, predict the relative ratio of the
two possible products J and K from the chlorination of 2-methylpropane. Explain your answer.
CH3 CH3 CH3
CH3 CH CH3 yielding CH3 C Cl CH3 CH
CH3 CH3 CH2Cl
2-methylpropane J K
SOLUTION:
ratio J / K = 21 / 1
EXPALNATION: According to the information given tertiary hydrogen atom will be repalced in 2-methylpropane to produce J in higher quantity where as there is only ONE primary replaceable hydrogen in the minor product K. Hence the ratio would be
J:K = 21 : 1
Is this the right expalnation or am I missing something UltimaOnline?
Originally posted by Kahynickel:reaction ralative rate
RCH3 to RCH2Cl 1
R2CH2 to R2CHCl 7
R3CH to R3CCl 21
Using this information, and considering the number of hydrogen atoms of each type
(primary, secondary or tertiary) within the molecule, predict the relative ratio of the
two possible products J and K from the chlorination of 2-methylpropane. Explain your answer.CH3 CH3 CH3
CH3 CH CH3 yielding CH3 C Cl CH3 CH
CH3 CH3 CH2Cl
2-methylpropane J K
SOLUTION:
ratio J / K = 21 / 1
EXPALNATION: According to the information given tertiary hydrogen atom will be repalced in 2-methylpropane to produce J in higher quantity where as there is only ONE primary replaceable hydrogen in the minor product K. Hence the ratio would be
J:K = 21 : 1
Is this the right expalnation or am I missing something UltimaOnline?
This question requires the student to consider both factors in his/her calculations : number of H atoms substitutable and stability of alkyl radical intermediates.
To get J :
(number of H atoms substitutable) x (stability of alkyl radical intermediates)
= 1 x 21 = 21
To get K :
(number of H atoms substitutable) x (stability of alkyl radical intermediates)
= 9 x 1 = 9
Hence ratio of products J to K = 21 / 9 or 7J : 3K
If Cambridge asks "what factors affect product distribution in free radical substitution?" the student should qualitatively explain both factors.
But if Cambridge asks "what is the relative percentages of the expected product distribution?", then :
If the Cambridge exam question does not provide data for the relative stabilities of alkyl radical intermediates (which can be phrased as "relative rates of reaction to generate primary, secondary and tertiary alkyl halides" as seen in the original question above), then the student need only give a quantitative answer based on calculations that only consider "number of H atoms substitutable".
If the Cambridge exam question does provide data for the relative stabilities of alkyl radical intermediates (such as in the original question above), then the student needs to consider both factors "number of H atoms substitutable" and "stability of alkyl radical intermediates" by multiplying the values associated with both factors, to obtain the true product distribution.
Most schools only teach the simpler factor of "number of H atoms substitutable", but this is misleading because in practice, both factors (which you would notice tend to contradict each other : tertiary alkyl halides have the least no. of H atoms to substitute away, but yet tertiary alkyl radicals are the most stabilized by hyperconjugation from the alkyl groups) are equally important and teaches the student that in Chemistry, just as in Real Life, it is myopic and does oneself injustice, to simply consider one side of the story or one particular viewpoint only; one must consider all sides of the story or all relevant viewpoints to understand and appreciate the truer, bigger, more beautiful picture of the Universe and of Life.
Thank you.
This question appeared in CIE A2-level in Nov 2011 session. This type of question was previously asked in 1994.
Would you please let me know is there any site or in this forum I may find these types of questions. I have ordered the Sinagapore A-level papers (TYS). Do they have these type of questions?
Originally posted by Kahynickel:Thank you.
This question appeared in CIE A2-level in Nov 2011 session. This type of question was previously asked in 1994.
Would you please let me know is there any site or in this forum I may find these types of questions. I have ordered the Sinagapore A-level papers (TYS). Do they have these type of questions?
You're welcome.
Interesting. It seems together with the Singapore-Cambridge 'A' level H2 Chemistry exams, the CIE Chemistry papers are also increasing in difficulty.
Actually, the Singapore-Cambridge 'A' level H2 Chemistry exams only started getting really difficult in 2010 and 2011 (and no doubt, the upcoming 2012 paper is expected to be even more lethal).
So buying the Singapore 'A' level TYS won't really satisfy your need for more difficult questions, as all the other papers before the 2010 and 2011 papers, are of normal difficulty.
What you're looking for (assuming your interest is in more difficult 'A' level Chemistry exam questions), is the Singapore JCs (there are 20 Singapore JCs in total) Preliminary exam papers, eg. for the past 3 years, 2009, 2010, 2011. But there isn't an authorized publisher for Singapore Preliminary exam papers.
What you can do however, is to google search for the Singapore Preliminary exam papers online (eg. on Scribd.com) and/or search out private individuals on the internet who sell softcopies of these exam papers over the internet.
I downloaded the Preliminary exam papers for 2010 spanning over 1400 pages. I occasionally read them. I am still searching for more difficult questions from scribd and other sites.
Your thread 'A' Level H2 Chemistry Qns (Part 2) lI " is full of stnadard chemistry stuff. I am trying to get as much as possible from that sea of knowledge.
Originally posted by Kahynickel:I downloaded the Preliminary exam papers for 2010 spanning over 1400 pages. I occasionally read them. I am still searching for more difficult questions from scribd and other sites.
Your thread 'A' Level H2 Chemistry Qns (Part 2) lI " is full of standard chemistry stuff. I am trying to get as much as possible from that sea of knowledge.
The 2011 Preliminary exam papers are slightly more difficult and interesting (compared to the 2011 papers), as a reaction to the tougher 2010 Singapore-Cambridge papers.
While my thread includes exam questions asked by others (usually students, but some tutors / teachers as well) from this and other forums, it's really just a compilation of my own comments on the chemistry concepts relevant to the exam questions.
A fellow chemistry teacher/tutor associate of mine, Mr Chong, also has a webpage in which he compiles 'A' level chemistry exam questions and notes. Incidentally, our webpages contain links to each other's webpages, for visitor's convenience.
http://alevelchemistrysg.blogspot.com/
CCl4 vs SiCl4
UltimaOnline posted:
The Cl atoms in SiCl4 consequently have a larger magnitude of partial negative charge (compared to in CCl4), which results in a greater degree of van der Waals repulsion (here : lone-pair lone-pair repulsion). Hence SiCl4 has a lower boiling point compared to CCl4.
Both molecules are symmetrical. Why are you not taking into account the larger vander Waal's forces in case of SiCl4 due to the larger number of elctrons as compared to CCl4?
Are you saying that the difference of just 6 more electrons in SiCl4 are not going to make it having a higher boiling point than CCl4.? Please explain.
Can you mention other similar cases?
Originally posted by Kahynickel:CCl4 vs SiCl4
UltimaOnline posted:
The Cl atoms in SiCl4 consequently have a larger magnitude of partial negative charge (compared to in CCl4), which results in a greater degree of van der Waals repulsion (here : lone-pair lone-pair repulsion). Hence SiCl4 has a lower boiling point compared to CCl4.
Both molecules are symmetrical. Why are you not taking into account the larger vander Waal's forces in case of SiCl4 due to the larger number of elctrons as compared to CCl4?
Are you saying that the difference of just 6 more electrons in SiCl4 are not going to make it having a higher boiling point than CCl4.? Please explain.
Can you mention other similar cases?
As you know, in Chemistry just as in real-life, there are multiple factors in every situation, and commonly enough, some of these factors are contradictory.
Yes, the greater number of electrons and greater molecular size is indeed in favour of SiCl4 having stronger van der Waals over CCl4.
At the same time however, the greater magnitude of partial negative charges on the Cl atoms of SiCl4 is also in favour of CCl4 having stonger van der Waals over SiCl4.
So which outweighs which? We humans can argue all we want based on theoretial principles, but in the end, the Universe has already decided (by mathematics) which outweighs which, and we humans have to carry out experiments to find out what the Universe had already decided : in this context, experiments have proven that CCl4 has a higher boiling point over SiCl4, and therefore we humans can deduce that the second factor outweighs the first factor.
Another similar example, would be that of comparing the bond angles of NH3 versus NCl3. There are two contradictory factors in this regard :
Because the N-Cl bond lengths are significantly greater than N-H bond lengths, one might expect the inter-atomic-nuclei repulsions, and thus bond angles, to be smaller for NCl3 compared to for NH3.
But because Cl has a significantly larger atomic radius and has lone pairs, while H has a significantly smaller atomic radius and has no lone pairs, one might expect the inter-lone-pair repulsions, and thus bond angles, to be larger for NCl3 compared to for NH3.
So which factor wins? As it turns out, experimental evidence has shown that the bond angles of NCl3 and NH3 are almost identical, the former 107.1 deg and the latter 107.8 deg. And therefore we humans can deduce that both factors cancel each other out almost equally.