olvl chem qns

ultimatenolifer SG

got a few qns here

1) state two differences btw the hydroxide grp in NaOH and the hydroxyl grp in alcohols

2) suggest one function of the universal indicator soln that is connected to the beaker containing sugar and yeast besides testing for presence of CO2.

3) the best method to distinguish a alcohol and a carboxylic acid is using
a) bromine water
b) sodium hydroxide soln
c) sodium carbonate soln or
d) dilute sulphuric acid?

SBS n SMRT SG

got a few qns here

1) state two differences btw the hydroxide grp in NaOH and the hydroxyl grp in alcohols

2) suggest one function of the universal indicator soln that is connected to the beaker containing sugar and yeast besides testing for presence of CO2.

3) the best method to distinguish a alcohol and a carboxylic acid is using
a) bromine water
b) sodium hydroxide soln
c) sodium carbonate soln or
d) dilute sulphuric acid?

1) In O level syllabus, i do not think there is any difference, but the OH- group in NaOH could be dissociated but the OH group in hydroxyl cannot, and reactions also differ, OH- in NaOH is for neutralization and alcohols are weak acids (utima correct me if i am wrong) and can serve as a group involved in orgainic reactions (eg esterfication)

2) pH

3) (a) is for alkane and alkene, (b) Cannot observe change, unless entalpy change is measured, ehthalpy change for neutralisation for acid is nearly +57 KJ (not the strong acid value as acidic group is not fully dissociated - need to break bonds). (c) Best answer, could see effervensence, (d) Useless

UltimaOnline SG

2) pH

SBS n SMRT's answers are correct.

1) Yes, that's right. The hydroxy / hydroxyl OH group is present in the form of the hydroxide OH- ion in NaOH(aq), which is basic (ie. can accept protons to form water) and hence the solution of NaOH(aq) is alkaline (ie. pH > 7 at room temperature). The hydroxy / hydroxyl OH group in alcohol is covalently bonded to the alkyl 'R' group and has no (formal or ionic) charge, and is neutral, ie. pH = 7 at room temperature (it is not acidic because the conjugate base RO- alkoxide ion is not stabilized by being delocalized by resonance and is in fact destablized by the electron donating by induction alkyl 'R' group; it is not basic because oxygen is electronegative and holds the lone pair tightly and hence is not available to accept a proton, and the conjugate acid is unstable because electronegative oxygen does not like a positive formal charge).

In conclusion, for 'O' levels the two differences are : the OH group is sodium hydroxide is negatively charged and is basic; the OH group is alcohol has no charge and is neutral.

2) Yes, that's correct. Non-metal oxides are covalent oxides and hence are acidic oxides. Due to the greater electronegativity of oxygen compared to carbon, the carbon is partially positively charged and is hence electrophilic. It consequently invites nucleophilic attack by water molecules, forming carbonic(IV) acid upon hydrolysis :

CO2(g) + H2O(l) ---> H2CO3(aq) ---> H+(aq) + HCO3-(aq)

The protons (H+ ions) will cause the solution to be acidic and the pH to decrease below 7 (at room temperature), which can thusly be detected by the Universal Indicator solution.

In addition, when some species of yeast carry out fermentation (ie. breakdown of carbohydrates), they may also generate lactic acid, which can also be detected by the Universal Indicator solution.

http://en.wikipedia.org/wiki/Lactic_acid_fermentation

3) SBS n SMRT's answers are correct. The carbonate(IV) ions would be protonated by the carboxylic acid, forming the conjugate acid carbonic(IV) acid, which exists in equilibirum with, and hence can decompose into CO2(g) and H2O(l). Notice that carbon dioxide, being a gas, leaves the reaction mixture, pulling the position of equilibrium over to the right.

2RCOOH(aq) ---> 2H+(aq) + 2RCOO-(aq)

2H+(aq) + CO3 2-(aq) ---> H2CO3(aq) ---> CO2(g) + H2O(l)

Notice how this reaction (ie. the 2nd equation) is the exact same reaction as in our discussion on Q2, but in the opposite direction.

UltimaOnline SG

and alcohols are weak acids (utima correct me if i am wrong) and can serve as a group involved in orgainic reactions (eg esterfication)

When alcohols react with carboxylic acids with a trace amount of concentrated sulfuric(VI) acid under heat or reflux, this reaction is a

#1 - Esterification reaction, because an ester is generated as the main organic product.

#2 - Condensation reaction (NOT a dehydration reaction) because water is eliminated to join two smaller molecules (the alcohol and carboxylic acid) together into a larger molecule (the ester)

#3 - Nucleophilic acyl substitution (NOT just 'nucleophilic substitution', because those two words alone imply nucleophilic aliphatic substitution, whose mechanism is SN1 or SN2), because nucleophilic substitution is occurring on an acyl group.

#4 - Addition-Elimination reaction, the mechanism by which nucleophilic acyl substitution occurs. The nucleophile is added first (in this case the nucleophile is the alcohol, the nucleophilic atom being the partially negatively charged O atom) as it attacks the electrophile (in this case the electrophile is the carboxylic acid or acyl halide, the electrophilic atom being the partially positively charged C atom of the carboxylic / acyl group), shifting the pi bond upwards to generate a singly bonded O atom with a negative formal charge, which then reforms the carbonyl and the leaving group (in this case, protonated OH group, or OH2+) is eliminated (in this case, as water).

SBS n SMRT SG

Hi Utimaonline

Thanks for the A level explanation, it also clarifys my organic chem and ionic equilbra

But i thnk TS is asking of o level stuff leh

ultimatenolifer SG

thx to sbs and ultimaonline for ur replies. hi ultimaonline, i appreciate ur detailed answers which probably are juz at a level too profound for me to understand. nevertheless, thx again for ur prompt replies.

UltimaOnline SG

No problem, you're welcome. My posts are meant to be informative or educational to a wider audience, both 'O' levels and 'A' levels. As such, to any query I respond on, I'll give both the 'O' level answer and the 'A' level answer.

Or if I appear to give only the 'A' level answer, it is to help guide current 'A' level students (eg. SBS n SMRT) for them to give 'O' level answers to help guide current 'O' level students (eg. ultimatenolifer).

Furthermore, it is hoped that current 'O' level students will be, upon reading the higher level (eg. 'A' levels) version of the answer, become interested to read up and learn more about such knowledge, concepts and understanding, which will spur on their learning.

Afterall, this is the Age of the Internet, where the world's best encyclopedia, Wikipedia, is available on everyone's fingertips.

And notice that as years go by, more and more higher level learning is brought down. Eg. Molecular biology used to be taught only at Uni level, then years ago brought down into JC level, then more recently brought down into Secondary school level, and possibly in the future to PSLE level.

Lastly, true learning is never about passing exams or whats-in-the-sylllabus, but true learning is always about oneself and one's relationship to the universe.

Enjoy! :)

SBS n SMRT SG

Afterall, this is the Age of the Internet, where the world's best encyclopedia, Wikipedia, is available on everyone's fingertips.

Lastly, true learning is never about passing exams or whats-in-the-sylllabus, but true learning is always about oneself and one's relationship to the universe.

Enjoy! :)

Hi Utima,

about the molecular biology stuff, may i know the topics in O level that is part of it, as many of the O level biology topics are physiological in nature.

Thanks for your help in the various ways in the forum, i really apperciate it - some are albeit uni level stuff, not covered in my lectures

UltimaOnline SG

Hi SBS n SMRT

As an example, just a few years ago, transcription (DNA to RNA) and translation (RNA to protein) used to be taught only at 'A' levels. It is now part of the 'O' level syllabus.

And yes, in the same way that I hope my posts might get 'O' level students interested in taking up 'A' level Chemistry (and to want to self-learn more outside the syllabus for the joy of Chemistry), I hope that my posts, which include some University level Chemistry, might also get JC students (or chemistry based polytechnic course students) to want to self-learn more outside the syllabus for the joy of Chemistry, or to take up some Chemistry related science courses in the future when they/you go on to University.

You're totally welcome again. Remember to find what's enjoyable about your studies, allow yourself to enjoy it at a deeper level (ie. for the sake of understanding your relationship to the Universe, and not just for the sake of passing exams), and your academic life will be meaningful and fulfilling.

ultimatenolifer SG

hi ultimaonline, do u also happen to be well equipped with knowledge on o lvl biology. coz i have some qns to ask too on that lol

UltimaOnline SG

hi ultimaonline, do u also happen to be well equipped with knowledge on o lvl biology. coz i have some qns to ask too on that lol

Do feel free to ask. There are many 'O' level and 'A' level Biology students who visit this forum, and Moderator Darkness Hacker is a Biology expert.

Rest assured I'll add my comments to your Bio question threads, if I've anything at all to add, to the comments or replies from others.

SBS n SMRT SG

hi ultimaonline, do u also happen to be well equipped with knowledge on o lvl biology. coz i have some qns to ask too on that lol

i am also quite interested in biology, in fact, it is a subject i wish to major in university, so you should be able to get help, my knowledge is up to partial Uni 2

SBS n SMRT SG

Hi SBS n SMRT

As an example, just a few years ago, transcription (DNA to RNA) and translation (RNA to protein) used to be taught only at 'A' levels. It is now part of the 'O' level syllabus.

But the trend for molecular bio to be taught at primary school will not be so soon, like transcription and translation is being taught at such a superficial level in o level now, it will take time

UltimaOnline SG

i am also quite interested in biology, in fact, it is a subject i wish to major in university, so you should be able to get help, my knowledge is up to partial Uni 2

What is your intended career if you do major in Biology, SBS n SMRT?

Btw, you're in JC2 this year, yes?

UltimaOnline SG

On another forum, I replied the post of a parent who is concerned for his child, who is in Normal Academic. He was asking for advice on how to help his child improve his/her Science. I posted the following :

----------------------------------

I'm about to make a suggestion that may not work for every Normal Academic student, but does work well for some Normal Academic students. There are many reasons why a student may be streamed into Normal Academic, and it does not necessarily mean that student is not capable in his/her own way, even academically. (I'm an ex-MOE teacher, and I've seen some NA students who were even more academically capable than some Express students.)

My suggestion is this - instead of letting your child do Normal Academic papers, let him/her do Express papers, first Combined Science papers, and if he/she expresses the interest to go futher, let him/her have a go at Pure Science papers.

You can obtain the Ten Year Series (Topical would be preferable over Yearly) for 'O' levels Combined Science, and for Pure Science 'O' levels, at any Popular Bookstore.

Every student, whether Normal Technical, Normal Academic, Express or Special, is academically inclined to different extents in different subjects. If your child has a talent for Chemistry, then even if he/she struggles with Normal Academic physics (for example), he/she may be able to perform well for Express level Chemistry. This may or may not be true, depending on the individual student.

Personally, as a ex-MOE teacher now full time tuition teacher, my teaching strategy for exam preparation has always been to expose the student to at least one level higher.

I let my Normal Academic students prepare themselves using Express level Combined Science Chemistry exam papers, and let my Express level Combined Science students tackle Pure Chemistry 'O' level exam papers, and let my Pure Chemistry students try out 'A' level Chemistry exam questions, and expose my JC 'A' level students to University level Chemistry questions.

This works very well for those with talent, inclination or potential in Chemistry, regardless of which stream (eg. Normal Academic) they may find themselves currently in, for whatever reasons.

All the best for your child.

SBS n SMRT SG

What is your intended career if you do major in Biology, SBS n SMRT?

Btw, you're in JC2 this year, yes?

your ex-career, i am a J1, but the reason i have some uni knowledge because i am in biology Olympiad training

Btw, any tips?

UltimaOnline SG

your ex-career, i am a J1, but the reason i have some uni knowledge because i am in biology Olympiad training

Btw, any tips?

Sweee! MOE Teacher! :)

Biology teacher eh?

Tips ah... for Biology (and for Chem, and for any subject actually), go NUS Co Op and look for good specialist textbooks (not general textbooks like Campbell), and buy good ones that have their solution manuals available.

A couple of years ago, I recall once (at NUS Co Op) seeing an excellent Questions-&-Solutions Biology compaion book full of intriguing fascinating molecular biology & genetics challenging problems/riddles/questions, many of them similar to the Olympiad style questions, that seriously tempted me (to buy it), but since I had already decided (profession-wise) to focus on teaching (tutoring) Chemistry (rather than Biology, which I used to do many years back), so eventually I gave it a miss. But *you*, would certainly want to get it. It would definitely be useful for your Olympiad preparation!

I just checked Amazon.com, this is probably book I saw. It's titled "Molecular Biology of the Cell - The Problems Book". Check out the contents and reviews :

http://www.amazon.com/Molecular-Biology-Cell-Problems-Book/dp/0815335776/

Looking at the contents (you can click into the book and see sample questions), I have to say I'm again tempted. Lots of intriguing, fascinating problem questions. And the book comes with a CD that includes detailed solutions (to half of all the problems, I believe).

UltimaOnline SG

Ahhhhhhhhhhhhh! I've found a newer edition of the book! (It has a perfect score of 5 stars 7 out of 7 reviews! which is really rare for academic books on Amazon.com). It's kind of a companion book to the textbook by the same title. This companion book is a Problems book, filled with challenging problems. Solutions provided on CD included with the book.

http://www.amazon.com/Molecular-Biology-Cell-Fifth-Problems/dp/0815341105/

And according to the Product Description, ALL the solutions to ALL the 2000+ Olympiad-type problems are present in the CD-Rom for this edition!

Product Description :

The Problems Book helps students appreciate the ways in which experiments and simple calculations can lead to an understanding of how cells work by introducing the experimental foundation of cell and molecular biology. Each chapter will review key terms, test for understanding basic concepts, and pose research-based problems. The Problems Book has been designed to correspond with the first twenty chapters of Molecular Biology of the Cell, Fifth Edition.

• Includes the solutions to the end-of-chapter problems in the textbook Molecular Biology of the Cell, Fifth Edition.

• Contains over 2,000 problems and their solutions.

• All of the solutions to the problems in The Problems Book are on the CD-ROM that is packaged with every copy of the book.

• The problems are organized into the following categories: Terms to Learn, Definitions, True/False, Thought Problems, Calculations, and Data Handling.

The Problems Book will be useful for homework assignments and as a basis for class discussion. It could even provide inspiration for exam questions.

See Reviews :

http://www.amazon.com/Molecular-Biology-Cell-Fifth-Problems/dp/0815341105/

SBS n SMRT SG

thanks a lot

ultimatenolifer SG

on o lvl topic: energy changes

1) formation of hcl from hydrogen n chlorine is a strongly exothermic rnx. from tis, it can be deduced that

a)the temp falls during rxn

b)heat is absorbed in the rnx

c)hcl gas burns readily in air OR

d)hcl is not easily decomposed by heat

2)when hydrogen n iodine combine to form hcl, heat is absorbed. from this, we can deduce that

a) hydrogen n iodine react more rapidly at lower temps

b)the energy involved in making bonds is less than that involved in breaking bonds

c)more bonds are broken than formed OR

d)formation of hydrogen iodide uses up energy

o lvl topic: speed of rnx

1) an experiment is carried out involving magnesium ribbon and excess dilute hcl. vol of h2 is measured to find rate of rxn. Qn: can the change in the concentration of hcl be used to measure the rate of this reaction too? explain why.

2)give too reasons why the exhaust pipe of a car rusts more rapidly than the other metal parts of the car

3)zinc reacts wif an excess of dilute h2so4. describe two changes that you would observe during the rxn

4) a conical flask containing mgco3 and hcl is allowed to react, den the flask is connected to a u tube containing cotton wool b4 the hydrogen gas is allowed to be collected in a gas syringe. explain the purpose of using the cotton wool in the setup.