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UltimaOnline SG

TYS 2010/P1/33

why is option 3 correct? Like how will osmosis increase entropy?

Analogy :

2 rooms, linked by only 1 door (the partially permeable membrane).

Room A only got girls (solute).

Room B only got guys (solvent).

Both rooms (ie. the system) have low entropy.

But now, if some guys from room B with higher guy (solvent) potential crosses the door (partially permeable membrane) to room A with the lower guy (solvent) potential, there will be in increase in disorderliness in room A (coz now got mix of guys and girls), and thus there is a positive entropy change in both room A and the overall system.

UltimaOnline SG

Reminder :

If no one attempts to post the answer to MrWorry's question (ie. why the reaction occurs despite apparently being thermodynamically non-feasible), then we'll just move on and it'll be your (ie. genuine JC students taking the A levels this year) loss. After all, it might come out for this year's A levels.

Tell you what, I'll reveal the correct answer after at least 3 pple have attempted it. Some of you probably already know the answer, but perhaps just can't be bothered to register an account to make a post.

How about the original poster himself, MrWorry?

tjyj SG

With regards to mrworry's qn on the reaction between CuSO4 and KI,

1. E cell= -0.39V < 0, hence, according to the data booklet this reaction is not feasible.

2. However, Cu2+ gets reduced to form Cu+. Cu+ is precipitated to form CuI. By LCP, position of equilibrium shifts to the right. Therefore, E Cu2+/Cu+ is more positive than +0.15 as stated in data booklet. So this reaction is feasibl.

UltimaOnline SG

With regards to mrworry's qn on the reaction between CuSO4 and KI,

1. E cell= -0.39V < 0, hence, according to the data booklet this reaction is not feasible.

Excellent, Tjyj. Your answer should get full marks from Cambridge, but to play safe, also do mention that "as a consequence of the CuI(s) precipitation, [Cu+] decreases" then your Le Chatelier statement, and finally the conclusion that "hence the cell potential under these non-standard conditions (where CuI(s) precipitates out), becomes positive, and the reaction becomes thermodynamically feasible."

Flying grenade SG

Topic : atomic radius

Hi! This is from singapore cambridge nov a level exam year 2010,

Paper 1 qn 1

Qn: which element has the largest atomic radius?

So suggested answer is potassium, as the valence electrons of the atom of K experience the weakest nuclear charge effect among the 4 options above

And Kr is the atom which valence electrons experience the strongest attraction to the nucleus, as nuclear charge effect increases across the same period while shielding effect remain approximately constant across the same period, hence effective nuclear charge increases across the period

My question is , why not Kr ?

From Data booklet :

Atomic radii /nm :

Na : 0.186

Ar : 0.192

Why cant we extrapolate this observation and apply it for period 4?

From Wikipedia :

In order of atomic radius, covalent radius, van da waals radius :

Na : empirical: 186 pm, 166Âą9 pm, 227 pm

K : empirical: 227 pm, 203Âą12 pm, 275 pm

In order of covalent radius, van da waals radius :

Ar : 106Âą10 pm, 188 pm

Kr : 116Âą4 pm, 202 pm

Thank you (:

https://en.m.wikipedia.org/wiki/Sodium

https://en.m.wikipedia.org/wiki/Potassium

https://en.m.wikipedia.org/wiki/Argon

https://en.m.wikipedia.org/wiki/Krypton

UltimaOnline SG

Topic : atomic radius

Hi! This is from singapore cambridge nov a level exam year 2010,

Paper 1 qn 1

Qn: which element has the largest atomic radius?

So suggested answer is potassium, as the valence electrons of the atom of K experience the weakest nuclear charge effect among the 4 options above

My question is , why not Kr ?

From Data booklet :

Atomic radii /nm :

Na : 0.186

Ar : 0.192

Why cant we extrapolate this observation and apply it for period 4?

From Wikipedia :

In order of atomic radius, covalent radius, van da waals radius :

Na : empirical: 186 pm, 166Âą9 pm, 227 pm

K : empirical: 227 pm, 203Âą12 pm, 275 pm

In order of covalent radius, van da waals radius :

Ar : 106Âą10 pm, 188 pm

Kr : 116Âą4 pm, 202 pm

Thank you (:

https://en.m.wikipedia.org/wiki/Sodium

https://en.m.wikipedia.org/wiki/Potassium

https://en.m.wikipedia.org/wiki/Argon

https://en.m.wikipedia.org/wiki/Krypton

It's not a fair comparison, because K (being a metal) doesn't have van der Waals radius, while Kr (being unreactive) *only* has van der Waals radius. You could say it's Cambridge's fault for setting this problematic question, but for A level purposes, since in the H2 syllabus, you focus only on the concept of increasing electronegativity and ionization energy across a period, instead of the different types of atomic radii (specifically covalent radius versus van da waals radius) which isn't really in the H2 syllabus, thus Cambridge (reasonably) expects you to just apply the concept of increasing electronegativity and ionization energy across a period, and that's it. Don't need to care about covalent radius versus van der Waals.

Other than atomic radius, Cambridge can test you on comparing the *ionic* radii of iso-electronic species, eg. K+ vs Cl- (both isoelectronic with Ar atom). That's about it. No covalent vs van der Waals radius.

Flying grenade SG

Thank you

Trend in Atomic radius of period 3 elements use electronegativity and ionisation energy to explain, not effective nuclear charge/distance of valence e- and nucleus??

How can i follow this thread so i can have a email notification instead of manually checking here?

UltimaOnline SG

Thank you

Trend in Atomic radius of period 3 elements use electronegativity and ionisation energy to explain, not effective nuclear charge/distance of valence e- and nucleus??

How can i follow this thread so i can have a email notification instead of manually checking here?

The concepts are related, as to explain the electronegativity and ionisation energy trends, you need to talk about nuclear charge, shielding effect, atomic radius, etc.

Just check the Chem threads here every other day or so (anytime a post is made in a thread, the thread is bumped up on the forum page, so you'll know if it's active or inactive). And btw, you've already got enough posts as a new member, so feel free to start your own thread when you've got a new qn to ask. Just title the thread [H2 Chem] - *insert chem topic here* so others will know.

Flying grenade SG

A level 2014 paper 1 qn 29

Isnt treating a c=c double bond with aq bromine will result it to be a halohydrin ( a br bonded to one C, OH bonded to other C).

Treat Br to the phenol group will result it attached to the ortho position

So wouldn't the max number be 3 br atoms?

UltimaOnline SG

A level 2014 paper 1 qn 29

Isnt treating a c=c double bond with aq bromine will result it to be a halohydrin ( a br bonded to one C, OH bonded to other C).

Treat Br to the phenol group will result it attached to the ortho position

So wouldn't the max number be 3 br atoms?

If diluted soln, then 3. If concentrated soln, maximum 4 (Br2 added across double bond) is possible. Cambridge knew many students would think as you did, hence they deliberately (and too kindly) left out 3 as a possible option.

Flying grenade SG

Becos the para position is occupied on paracoumaric acid, br cannot attach there

Flying grenade SG

But qn state it is aqueous bromine, not concentrated

Then, can i form dihalide from c=c by using conc br2, instead of X2 in ccl4, dark

Do you have notification when a post is being posted on your thread?

Sorry i need wait 5 minutes hence the lag post

UltimaOnline SG

But qn state it is aqueous bromine, not concentrated

Then, can i form dihalide from c=c by using conc br2, instead of X2 in ccl4, dark

Do you have notification when a post is being posted on your thread?

Sorry i need wait 5 minutes hence the lag post

Aqueous can be concentrated aqueous. Different from liquid. Yes you can obtain some dihalide (previous qn stated *maximum possible*, which could be a minor product, not necessarily major), but it won't be 100% yield, so you won't get marks for using this method. You should still specify X2 in CCl4 when asked.

No notification. I just check this forum often.

Flying grenade SG

A level 2009 paper 2 qn 2g)

How do we know this zn2+ +2oh -> zinc hydroxide ?, ( cant find this in cs toh study guide): )

And why cant or can, instead write this hydrated [Zn(H2O)6]2+ form?

Do we need to know/memorise strength of ligands so that we would know which ligand displace which ligand?

E.g. Zn (OH)2 + NH3 -> [Zn (NH3)4 ]2+ + OH-

(Sorry didnt balance and didnt write state symbol )

Is the NH3 ligand always stronger than H2O ligand? As it displaces H2O ligand from cu2+ complex

UltimaOnline SG

A level 2009 paper 2 qn 2g)

How do we know this zn2+ +2oh -> zinc hydroxide ?, ( cant find this in cs toh study guide): )

And why cant or can, instead write this hydrated [Zn(H2O)6]2+ form?

Do we need to know/memorise strength of ligands so that we would know which ligand displace which ligand?

E.g. Zn (OH)2 + NH3 -> [Zn (NH3)4 ]2+ + OH-

(Sorry didnt balance and didnt write state symbol )

Is the NH3 ligand always stronger than H2O ligand? As it displaces H2O ligand from cu2+ complex

In absence of OH-(aq), you write the hydrated [Zn(H2O)6]2+ form. With OH-(aq), you get Zn(OH)2(s) ppt.

No need to memorize ligand strength or ligand field strength (google if you don't know the difference). Just apply common chemistry sense (eg. ligand strength, nucleophilic strength and Bronsted-Lowry basicity strength, all parallel each other because all 3 are functions of Lews basicity strength, with some exceptions you needn't care about at A levels), and most importantly, follow the clues given in the question. If there's a colour change upon adding a reagent, it implies ligand substitution aka ligand exchange has occurred.

On a related note, whether the coordination complex generated has 4 or 6 ligands, is a complex matter (pun intended), and the underlying reasons cannot be fully covered at A levels. For A level purposes, try to familiarize (no need to blindly memorize 100+ coordination complexes, their geometries and colours, etc) yourself with the dozen or so complexes more commonly encountered at A levels. For this purpose, Chang Kim Seng & Jeanne Tan's book, and George Chong's book, will be more useful than CS Toh's book (which is summarized lecture notes). Your own JC should have given you a list as well.

Tips : if the ligands are small and neutral, eg, H2O ligands, there will usually be 6 ligands, ie. octahedral geometry. If the ligands are large, eg. Cl ligands, there will usually be only 4 ligands, ie. tetrahedral geometry, due to steric hindrance or van der Waals repulsions. If the ligands are small but have high anionic charge densities, eg. OH- ligands, there will usually be only 4 ligands, ie. tetrahedral geometry, due to anionic electrostatic repulsions.

The notable special case (which the H2 syllabus specifies you need to memorize), is the deep-blue coloured and with square planar geometry, tetraamminecopper(II) ionic coordination complex, [Cu(NH3)4]2+. The reasons why Cu2+ only accepts 4 NH3 ligands, and why the ionic complex's geometry is square planar instead of tetrahedral, is because of the JahnâTeller effect and other reasons too complex to be dealt with at A level purposes. For A level purposes, other than the few commonly encountered coordination complexes you need to memorize (their colours and geometries), simply follow the clues which will be provided by Cambridge in the question.

Yes, NH3 ligand is stronger than H2O ligand, but don't forget molarity (ie. concentration) also determines the position of equilibrium in the Kstab or Kf equation (google it), and therefore determines which ligands substitute or exchange away which ligands. In other words, a weaker ligand with a higher molarity, can substitute away a stronger ligand with a lower molarity. Which is why the Cambridge question will specify, "added in excess" to ensure the ligand substitution occurs, due to the molarity affecting the position of equilibrium.

Flying grenade SG

Thank you so much omg (:

2014 planning part c)

Qn : will the max temp rise vary if you use double the volumes of original acid and metal hydroxide solutions?

Conflicting answers from various tys publishers

One of the answer is : temp will rise. Based on q=mcđşT,

Making deltaT the subject of equatuon, delT = q/mc.

Since vol doubled, m is doubled, hence delT is halved?

Is this wrong cos the 'm' in this case is not water, density might not be 1gcm-3, so cannot equate volume to mass?

Nt too sure

2nd answer posited :

There will be no change in max temp rise

Although the amount of heat released will be doubled, there will also be twice the amount of water formed to absorb the heat

Help!!

Flying grenade SG

What affects Nucleophilic and electrophilic strength?

Is it How e- deficient an ephile is, and how delta + positive charge on the nucleophile?

Does the electronegativity of an electrophile play part too?

UltimaOnline SG

Thank you so much omg (:

2014 planning part c)

Qn : will the max temp rise vary if you use double the volumes of original acid and metal hydroxide solutions?

Conflicting answers from various tys publishers

One of the answer is : temp will rise. Based on q=mcđşT,

Making deltaT the subject of equatuon, delT = q/mc.

Since vol doubled, m is doubled, hence delT is halved?

Is this wrong cos the 'm' in this case is not water, density might not be 1gcm-3, so cannot equate volume to mass?

Nt too sure

2nd answer posited :

There will be no change in max temp rise

Although the amount of heat released will be doubled, there will also be twice the amount of water formed to absorb the heat

Help!!

It's true that double the moles of H2O is generated, and hence double the heat is given off. But since the volumes of the water solvent (which absorbs the heat) in acid + alkali, is doubled, this also means the total (doubled) heat given off is also spread out and absorbed over the total (doubled) volume of water solvent, and thus the maximum change in temperature will still be the same.

UltimaOnline SG

What affects Nucleophilic and electrophilic strength?

Is it How e- deficient an ephile is, and how delta + positive charge on the nucleophile?

Does the electronegativity of an electrophile play part too?

You're asking a generic conceptual qn, not a specific Prelim paper or TYS paper qn (which is what these forum threads are for), which is worrying as it's indicative that at this late juncture so near the A levels, your conceptual foundation is weak, which means either you haven't been studying or following your school tutorial / lectures (either due to laziness, poor time management, BGR problems, family problems, medical problems etc), or your school Chem teacher(s) sucks, or your Chem private tuition tutor sucks.

Edited to add : on hindsight and to be fair to Mr Flying Grenade, Singapore JCs don't really teach their H2 Chem students much about the factors that affect the nucleophilicity or electrophilicity of a species, do they? They just tell you to memorize the lecture notes (which doesn't explain much about nucleophilicity or electrophilicity, I assume?). To be fair to Singapore JCs and the Singapore education system as well, they're themselves under severe (time, manpower, logistical and resource) constraints, so they can't be expected to cater to every unique student his/her every unique need, which is why there's still a useful role for good quality private tuition providers (eg. BedokFunland JC) to contribute positively to Singapore's academic excellence. And so ok fine, it's not Flying Grenade's fault for asking such a question, I may have been a little harsh.

I'll oblige this once, but in future (eh only a couple of days left to A levels leh!), for such generic conceptual qns, you should Google and Wikipedia these conceptual matters out yourself, or look up reputable online sources of Chemistry such as Khan Academy, Jim Clark's ChemGuide, Rod Beavon's Chemistry, and/or study CS Toh's / Chan Kim Seng & Jeanne Tan's / George Chong's / Jasmine Wong's A level H2 Chem books, and/or Owl Cove's A level study notes (written and shared by A level students in previous years), which are admittedly more for final month memorization & revision (month check : yup it's now almost November liao), rather than for learning concepts from scratch, which should have been done in earlier months in school with your school teacher, or with your private tutor during tuition.

Nucleophilicity and Electrophilicity is affected by several factors :

- Magnitude of charges and charge densities, whether formal charges or partial charges, which is affected by electronegativity (eg. is the negative formal charge on a C atom or O atom?), % s orbital character in hybridized orbital (affecting charge density), atomic/ionic radius, shielding effect, induction and resonance.

- Availability of vacant orbitals on electrophile (addition-elimination mechanism requires lower Ea than SN1 & SN2 aliphatic nucleophilic substitutions, which is one reason why in Periodicity of H2 syllabus, SiCl4 readily undergoes hydrolysis, while CCl4 is hydrolysis resistant; the availability of 3d orbitals in Si (but not C) creates the potential for addition-elimination hydrolysis mechanisms with lower activation energy).

- Availability of lone pair on nucleophile, which is affected by electronegativity, % s orbital character in hybridized orbital (affecting lone pair availability), atomic/ionic radius, shielding effect, induction and resonance.

- Steric hindrance by bulky groups (either on electrophile reducing electrophilicity, or on nucleophile reducing nucleophilicity and increasing Bronsted-Lowry basicity instead) around the electrophilic or nucleophilic atom.

- Viability and stability of leaving group upon elimination (in nucleophilic aliphatic and acyl substitutions ; for electrophilic aromatic substitutions the incoming electrophilic substitutes away a proton, to restore aromaticity and resonance stabilization energy), which is affected by electronegativity (eg. is the negative formal charge on a C atom or O atom?), % s orbital character in hybridized orbital (affecting stabilization of charge), atomic/ionic radius, shielding effect, induction and resonance.

- Endothermicity of bond dissociation enthalpy and its activation energy, or (in other words) the strength of the covalent bond required to be cleaved (be sure to quote Data Booklet values whenever relevant), which is in turn affected by % s orbital character in hybridized orbital (affecting strength of covalent bond, eg. sp2-sp3 bonds stronger than sp3-sp3 bonds), atomic/ionic radius and period of element (effectiveness of overlap, whether head-on for sigma bonds or side-on for pi bonds, is greatest between 2nd electron shell orbitals and other 2nd electron shell orbitals, and much less effective between 2nd and 3rd/4th/5th electron shell orbitals, or 3rd/4th/5th electron shell orbitals and other 3rd/4th/5th electron shell orbitals, regardless of whether hybridized or not), electronegavity & induction (increasing polarity of covalent bond increases it's ionic character and thus strengthens it, which is why CO triple bond is stronger than NN triple bond in Chemical Bonding, or why Al3+ and Mg2+ salts are acidic in Periodicity and Acid-base Equilibria, due to decreasing polarity and thus weakening of O-H bond in water ligands) and resonance (the C-X bond in aryl & vinyl halides have partial double-bond character in their resonance hybrid, hence aryl & vinyl halides are resistant to hydrolysis).

- % s orbital vs p orbital character of hybridized orbitals, as mentioned above, affects availability of lone pair on Lewis base (ie. nucleophile and/or Bronsted-Lowry base; eg. why amines are more nucleophilic & basic over imines, and imines over nitriles; for amides the resonance factor takes precedence), affects strength of covalent bond required to be cleaved (and hence activaiton energy required), and affects stability of charge (both in nucleophile / Bronsted-Lowry base, as well as in the leaving group after elimination).

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The above is just a brief summary off the top of my head (see how I made links across different topics in the syllabus? to do that you gotta deeply understand chemistry, not just blindly rote-memorize school notes without understanding, which is a common criticism of the Singapore education system). But JC students may not fully understand or appreciate all the points I've raised, which is fine (because while everything I said is relevant and useful for the H2 exam, some of it are optional elaborative points which you don't have to mention to get the marks).

To all JC studens reading this : At this late juncture in time (a couple of days before the A level exams), don't panic, just focus on the main syllabus essentials as required by the TYS (basically you *must* memorize the entire CS Toh A level study guide, and do all the TYS papers from 2010 to 2014), and do Prelim Papers for practice only after you've completed the TYS. Any specific TYS or Prelim paper qn, in which you don't understand or agree with the given answer, you guys (we've hundreds of silent lurkers!) can ask here in this forum (as Flying grenade has done for the 2014 planning part c qn above). I won't type out the full worked solution or calculations for any qn, I'll only comment and advise after you've included the given (Prelim or TYS) answer, and specify what you don't understand or agree about it.

Flying grenade SG

Thank you!!!!

Why is the delT argument wrong /not valid?

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