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Teachers’ Day

Mr Khemistry

Teacher’s Day haul this year:

Incredibly, the student who gave me the bubble tea i’ve only ever taught her ONE lesson before.

The tuition centre staff who baked cookies for me i only got to know her last month in person.

The students who wrote the card, i only taught them 2 lessons before.

Moral of the story – Teach without expecting anything from students in return. You might just be
pleasantly surprised 🙂

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Organic Deductive Questions

Mr Khemistry

Organic Chemistry

Deductive Questions – Side Reactions

When you have only one C=C in the organic molecule and question says that when the compound is heated with acidified manganate (VII), 3 fragments are obtained. This means that there was actually TWO cleavage of bonds.

But shouldn’t it be only two fragments since the oxidative cleavage only occurs at the C=C bond?

The clue lies in the often overlooked detail that whenever you have heat with acidified aqueous medium, you can also have acidic hydrolysis of esters/amides. So the initial molecule might have ester or amide functional groups on top of alkene functional group.

Same goes for iodoform test, where you use iodine with heat in a basic medium. Main reaction is the iodoform reaction, but basic hydrolysis can also occur.

Be sure to watch out for these “hidden” side reactions!

Clue: Solvent

Halogenoalkanes reacts with KOH in aqueous and alcoholic medium to form two different products.

This is one of the most common questions in organic chemistry, where different solvents will affect the type of reaction happening.

In this case, RX will undergo elimination of HX in alcoholic medium but undergo nucleophilic substitution in aqueous medium.

Another twist could be RX undergoing nucleophilic substitution reaction but giving two different products. This could arise when the solvent can also act as a nucleophile. eg ethanol can give ethoxide

More tips here

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Third Term swings into gear!

Mr Khemistry

Reminder: Third Term is here.

If you haven’t been putting in 100{8445fa0408f68c331c03e03f10d6a7bff33fb7168cc52e9b2191ddf5ec3671a6} into your studies, now’s the time to start!

Remember to pace yourself though, studying late into the night makes for unproductive school day the next day.

Weather’s also erratic and there’s some flu clusters around in schools.

Please have at least 2L of water and 6-7 hrs of sleep everyday. Not forgetting that you need your daily servings of fruits and vegetables.

Junk food is sometimes unavoidable, but if you exercise regularly and follow the above, then you should keep healthy.

All the best gearing up for Prelims/Promos!

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Random Musings

Mr Khemistry

The Random

Recently, i received a colorful brochure in my mailbox of a particular pre-school/kindergarden in Central Tampines.

It is glossy with a lot of infographics and photos, about 5-6 pages long, A4 size. Being a business owner now, i thought to myself this publicity would have cost thousands of dollars. I read through the brochure carefully and what was presented within really attracted me as a parent. I thought of enrolling my son to this centre and excitedly discussed with my wife about it. Alas, the monthly fees were prohibitive to say the least. 1.5k. Per MONTH. What kind of an income does the parents have to earn to sustain this level of expenditure on one pre-school? 

It was a no-go for us as it was more than twice the fees of a typical pre-school centre. One thing that resonated with me was that coding is fast becoming a skill like “Bilingualism” was in the 70s, “Typing” in the 80s, Microsoft office in the 90s and Financial Literacy in the 2000s. With IT creeping into every aspect of our lives, it will soon become clear that coding is the third language we have to master if we are to progress beyond being a consumer in all aspects of our lives.

The Musing

To me, it’s clear that Life is just like School. Except that the consequences are less likable than a failing grade.

School: The first and most basic level is just reading, listening, memorizing facts and maybe practicing a few basic questions. Imbibing whatever you were taught.
Life: You do whatever Boss tells you to, settling into a routine of waking up, going to work, going home, sleep and repeat.

School: The second level is to ask questions, critically, after thinking through what has been told/shown to you. Teachers dig this part, it allows us to examine what we’ve learnt and try to dissect it together with the students, hopefully in the process, both learns something new.
Life: You go beyond what is needed, drawing up a schedule, pre-empting potential issues that from experience, you know might happen at various stages of work. You learn from books, courses, others who are much more knowledgeable/experienced than you are.

School: You integrate and synthesize ideas, concepts and extrapolate knowledge obtained from the teacher. Beyond academics, you rise up to take on student leadership positions, serving the student body and going behind the scenes to understand the workings of the school.
Life: You leave work after saving a sizable amount, networking with others in the corporate world. Set up your own business, learning how to be the producer instead of the consumer.

Think through and reflect what do you want to do with your life. Plan then execute. Failure to plan is to plan to fail.

If you or someone you know need help on how to pass Chemistry, please give us a call at 98537960 🙂 

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What does the units “mol/dm3” refer to in standard enthalpy change of reaction?

Mr Khemistry

Enthalpy Change of Reaction

Some of you may be thinking the enthalpy change of reaction just a general definition for any reaction but the units actually don’t refer to any specific reactants or products.

It refers to the molar quantities of all the reactants and products. For eg,

2H2 (g) + 2O2 (g) → 2H2O (l)

It means 2 moles of hydrogen reacting with 2 moles of oxygen to form 2 moles of water.

Thus, if you are to find the enthalpy change of reaction for:

H2 (g) + O2 (g) → H2O  (l)

The ΔHrxn will be half the value of the first equation.

If you are sharp, you might notice that this equation is also applicable for enthalpy change of combustion for hydrogen, enthalpy change of formation for water.

This means that some enthalpy definitions overlap and there’s nothing wrong with that 🙂

This enthalpy change of reaction is useful for equations that doesn’t neatly fall into common enthalpy definitions.

So it is important to state the appropriate equation for the particular ΔHrxn quoted!

Do remember for Energetics, it’s very important to include state symbols in your equations as different physical states has different energy levels.

More commonly asked questions here.

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Strength of Hydrogen Bond

Mr Khemistry

Strength of Hydrogen Bond

Qn: Which of these compounds has the strongest hydrogen bond, HF, H2O or NH3? How do we estimate the strength of hydrogen bond?

Ans: HF. Due to the largest electronegativity difference between H and F. Followed by H2O and then NH3.

Qn: But then why does water have the highest boiling point among them? Doesn’t this mean that water has the strongest hydrogen bond?

Ans: No. This only means water has the most extensive hydrogen bonds as it forms two hydrogen bond per molecule. HF forms only one hydrogen bond per molecule as it only has one hydrogen atom. NH3 forms only one hydrogen bond per molecule as N has only one lone pair per molecule.

So we need to differentiate between the strength of the individual hydrogen bonds and the overall extensiveness of the hydrogen bonds formed 🙂

More common questions here

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Common mistake in drawing hydrogen bond

Mr Khemistry

Common mistakes in drawing hydrogen bond

One of the most common mistakes is to write the dipole across the hydrogen bond, e.g.

A dipole exists as a pair, partial positive and partial negative. It also depicts the uneven electron density distribution in a covalent bond thus it first forms in a single molecule where
the hydrogen atom is bonded to a very electronegative element such as F/O/N. As the hydrogen is almost “stripped” bare of its electron, it is strongly attracted to the lone pair on a highly electronegative F/O/N atom. This electrostatic attraction is known as a hydrogen bond. e.g.

Another type of hydrogen bonding within the same molecule is known as Intramolecular hydrogen bonding. Which type is overcome during boiling?

More here

Come sign up for our weekly group tuition to find out! 🙂

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My Last Class T-shirt

Mr Khemistry

Thankful that i had the chance to teach such a wonderful bunch of students before i left the college. Even though i told them i didn’t need one as i wouldn’t have
any occasion to wear it. Thank you 17S305, for my last class t-shirt 🙂

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Tips for organic deductive questions

Mr Khemistry

Organic deductive questions

If the organic deductive questions gives the molecular formula, see if the number of C and H is comparable. In addition, if the molecule contains more than 6 carbon atoms, the compound is likely to contain a benzene ring.

Even if C and H is not comparable, the degree of unsaturation can tell us a lot. (Every two H lesser than the alkane general formula is one degree of unsaturation)

For example, one degree of unsaturation means that the compound either has one C=C or is cyclic. Two degree of unsaturation means
the compound could have one C=C and is cyclic, has two C=C bonds or one triple bond. Degree of unsaturation is also known as index of hydrogen deficiency.

Another vital piece of information is how many products a strong oxidative cleavage produces. If only one single product is formed with no loss in C, it means the the original compound is likely to be cyclic. If there is effervescence, the gas is likely to be carbon dioxide and the original compound contains terminal alkene. Be careful though, ethanedioic acid can also oxidize to 2 moles of carbon dioxide under strong oxidation conditions e.g. hot acidic potassium manganate (VII)

More tips here

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