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last SG · 2015-08-19T22:54:00+00:00

Topic : atomic radius Hi! This is from singapore cambridge nov a level exam year 2010,Paper 1 qn 1Qn: which element has the largest atomic radius?BrKKrScSo...

UltimaOnline SG

May I ask a general qns? My school tutor claimed that for energy lvl diagram when the 3d and 4s orbital is filled, 3d orbital will have a higher energy than 4s orbital but when unfilled, the 4s orbital will have a higher energy than 3d orbital. Is that true?

Yes, this is true for the transition metals, as the electrons in the inner 3d orbitals will repel the electrons in the outer 4s orbitals, raising the 4s orbital to a higher energy level than 3d. Consequently, during ionization process, electrons are removed from the outer 4s orbital before electrons are removed from the inner 3d orbitals.

Flying grenade SG

https://www.dropbox.com/s/fr20pd8kwayzf4f/Screenshot_2016-05-08-21-25-25-1.png?dl=0

Energy level of orbitals when unfilled : 3d > 4s

When filled , energy of 4s orbital > 3d

@mightybiscuits Looks like your teacher taught wrong. Or you typed wrongly. Or u copied wrongly. Or u misinterpreted. Regardless of the case, rectify the mistake, and make sure you internalise and learn the correct chemistry info. Cheers !

Flying grenade SG

Hi ultima,

I read cs toh book page 108 and understood *why* O2(g) is a *better choice* than F2(g) as an oxidiser.

But i thought of comparing oxidising strength between F2(g) and O2(g)

I know F2(g) is the strongest oxidising agent of all known chemical compounds (or is it just the strongest, common oxidising agent?)

Question : how to determine oxidising power of O2? In the data booklet, there isn't any equation for O atom , e.g. O2 + 4e- -> 2O2- , unlike F, which has a eqn F2 + 2e- -> 2F- , with a given EÂ°value.

Is oxidising power related/affected to electronegativity? Are there other factors?

2. Came across this "Why is F2 a stronger oxidising agent than Cl2 even though F2 has a less exothermic E.A than Cl2? "

https://www.quora.com/Why-is-F2-the-most-oxidising-agent-instead-of-Cl2-even-though-F2-has-less-electron-affinity-than-Cl2

But don't really understand fully

UltimaOnline SG

Hi ultima,

I read cs toh book page 108 and understood *why* O2(g) is a *better choice* than F2(g) as an oxidiser.

But i thought of comparing oxidising strength between F2(g) and O2(g)

I know F2(g) is the strongest oxidising agent of all known chemical compounds (or is it just the strongest, common oxidising agent?)

Is oxidising power related/affected to electronegativity? Are there other factors?

2. Came across this "Why is F2 a stronger oxidising agent than Cl2 even though F2 has a less exothermic E.A than Cl2? "

https://www.quora.com/Why-is-F2-the-most-oxidising-agent-instead-of-Cl2-even-though-F2-has-less-electron-affinity-than-Cl2

But don't really understand fully

Yes, F2 is a stronger oxidizing agent compared to O2 (and F2 is the strongest *elemental* oxidizing agent, but that are rare & unstable compounds that are even stronger oxidizing agents compared to F2), but since HF is highly toxic while H2O is non-toxic, and O2 is easy to store while F2 is corrosive, so overall O2 is a better choice even though F2 is a stronger oxidizing agent. It's like when you're buying a supercar, it's not just the speed which matters, how cool it looks and the price tag must also be taken into consideration.

I'll explain the following just once. For further discussion (eg. in case you still don't fully understand), go ask your school teacher or private tutor.

Oxidizing strength is primarily related to electronegativity, but secondarily to hydration enthalpy, because in the natural world, redox reactions occur mostly in the aqueous environments (ie. water being the solvent that brings the reactants together and allowing chemical reactions to occur), hence the redox potentials that are relevant are in the aqueous state.

Yes, F2 a stronger oxidizing agent than Cl2 even though F2 has a less exothermic EA compared to Cl2. First, understand why Cl2 has a more exothermic EA compared to F2 : it's because even though F is more electronegative than Cl, but the charge density of F- is too high due to small ionic radius, concordantly the inter-electron repulsions within the small F- ion are highly destabilizing. Consequently, the EA for F isnt as exothermic, as F- isn't as stable as might be imagined solely due to electronegativity (as would be the case with Cl to Cl-), as there is also a destabilizing (ie. endothermic) component to the EA process for F (but not for Cl), ie. the inter-electron repulsions of the high anionic charge density F- ion.

Next, if the EA of Cl is more exothermic compared to that of F, shouldn't Cl2 be a stronger oxidizing agent (ie. more positive reduction potential) compared to F2? Yet the Data Booklet clearly shows F2 having a more positive reduction potential compared to Cl2, and F2 is thus the stronger oxidizing agent. This is because as I mentioned earlier, we're interested in redox reactions occurring in the aqueous solvent or phase, and thus concordantly by Hess Law, reduction enthalpy = electron affinity enthalpy + hydration enthalpy ; and likewise oxidation enthalpy = ionization enthalpy + hydration enthalpy.

Because F- has a much higher anionic charge density compared to Cl-, it's hydration enthalpy is also that much more exothermic (ie. much stronger ion - permanent dipole interactions formed). And after Hess Law is applied, the overall reduction potential of F2 to F-, is still more positive compared to the reduction potential of Cl2 to Cl-, and thus in aqueous environments, F2 is still a much stronger oxidizing agent compared to Cl2.

UltimaOnline SG

I told you before : in the natural world, these redox reactions occur mostly in aqueous environments / solvents / phases. Do you think the highly unstable O2- ion is able to exist in aqueous environment? Of course not! It will kena immediately hydrolyzed to OH-, and (depending on pH) can be further protonated to H2O.

So for the oxidizing power of O2, look in the Data Booklet for the reduction potential of O2 to either OH- or H2O (depending on pH). (Ignore obscure & rare reduction products like peroxides, super oxides, ozonides, etc, unless otherwise specified by the question).

Flying grenade SG

F- (aq) can exist in aqueous environment ?

Rephrase qn : is F- more stable than O2- ?

Is it cos O2- has dinegative charge so it's very unstable?

Polyatomic ions with dinegative charge e.g. SO42- , S2O32- , S4O62- , S2O82- , can exist* due to resonance stabilisation?

Update: Can exist in aqueous form*

O2- does exist, but not in aqueous form

UltimaOnline SG

F- (aq) can exist in aqueous environment ?

Rephrase qn : is F- more stable than O2- ?

Is it cos O2- has dinegative charge so it's very unstable?

Polyatomic ions with dinegative charge e.g. SO42- , S2O32- , S4O62- , S2O82- , can exist* due to resonance stabilisation?

Update: Can exist in aqueous form*

O2- does exist, but not in aqueous form

Yes, correct.

Flying grenade SG

Why is the d(x2-y2) and d(z2) at a higher energy level than the other three d orbitals again? The two aforementioned d orbitals lie on the axes or something??

UltimaOnline SG

Why is the d(x2-y2) and d(z2) at a higher energy level than the other three d orbitals again? The two aforementioned d orbitals lie on the axes or something??

Oi. There are hundreds of websites on the internet which explain (with diagrams too!) basic A level Chemistry concepts such as these. Don't be lazy.

Flying grenade SG

Oi. There are hundreds of websites on the internet which explain (with diagrams too!) basic A level Chemistry concepts such as these. Don't be lazy.

I know repulsion

Flying grenade SG

My school wrote this : *drew carbocation intermediate* carbocation formed is more substituted (2 alkyl groups). Alkyl groups are e- donating, thus dispersing positive charge on carbocation

Is this explanation incomplete/wrong, and/or cannot get full marks when Cambridge mark?

Need write this

To get full marks, you need to draw both mechanisms (or at the very least, both the carbocation intermediates) for both products, then specify which carbocation is the more stable carbocation (ie. benzylic carbocation > allylic carbocation > tertiary carbocation > secondary carbocation > primary carbocation), then explain "The more stable the intermediate, the lower the Ea required, hence the faster the rate of reaction, hence we get more of the product, which we label as the major product." ?

UltimaOnline SG

Depends on number of marks given. But you can improve on your schoolâs answer, by adding the relevant comparison for the questionâs reaction, eg. âSince tertiary carbocation is more stable than secondary carbocation, hence the major product is xxxxx instead of xxxxx.â

Flying grenade SG

The plasma state of matter and blood plasma is diff plasma hor??

UltimaOnline SG

The plasma state of matter and blood plasma is diff plasma hor??

As different as the resonance in physics is different from the resonance in chemistry.

Flying grenade SG

A classmate of mine asked a qn in class in which the tcher was stumped and unsure how to answer : why electron shell closer to nucleus, has lower energy level?

UltimaOnline SG

A classmate of mine asked a qn in class in which the tcher was stumped : why electron shell closer to nucleus, has lower energy level?

For a simplified version (which will suffice for A level purposes), you can think of it as : to maintain your orbit further from the nucleus, you (as an electron in the outer electron shells) naturally require more energy to sustain your further orbit and resist the electrostatic pull of the positively charged nucleus.

Going any deeper like the classic scientific enigmas of "Why don't electrons fall into the nucleus?", and "Why don't the protons in the nucleus repel each other and cause the nucleus to fall apart?" (also see Cordus physics) that goes significantly beyond A levels (so we we're *not* going discuss it here, and it's a safe bet that neither your school teacher nor your private tutor will entertain you on this), and at University levels, physicists and chemists have come up with a few different theories or explanations or ways of looking at these enigmas. You may come across some of these ideas in the future if you choose to study Physics or Chemistry in the University. For now, if you're still curious (though you should of course focus on your A levels), you can google out some preliminary reading for your entertainment and satisfaction.

Flying grenade SG

Thanks so much for your reply !!!

But, an e- distance further away from nucleus experiences weaker electrostatic attraction.

Ok nvm, ur preamble will suffice !

One theory to explain why an e- cannot be found inside a atom's nucleus is because of Heisenberg's uncertainty principle.

The nucleus is extremely small compared to the size of an atom, if a e- is found within the diameter of the nucleus, the velocity of it will exceed speed of light hence impossible, hence impossible to find an e- inside a nucleus.

Flying grenade SG

One way to explain why electron shell with higher principal quantum has higher energy is :

Electrical potential Energy of a charged particle(EPE) = 1/4πε( Qproton Qe-) /r, As r increase, EPE becomes less -ve(i.e. more +ve) , indicating higher energy level. Since e- occupies that certain discrete energy level of the electron shell, only for that particular e-shell for that particular energy level, one can substitute EPE= energy of Electron shell.

Flying grenade SG

Hi ultima, hope u could help me with this,

Daniel c harris book

Qn 10-10

I copied down the final answer.

For vol NaOH added =100ml (i.e. at equivalence point,

I tried to use pH = 0.5 [pka(phenylammonium) + pka(phenylamine)]

I found ka of phenylamine by finding kb of phenylamine on the internet.

Then i took kw/kb to find ka of phenylamine.

But i dont get the answer

Hope you can help check, thanks !

UltimaOnline SG

Hi ultima, hope u could help me with this,

Daniel c harris book

Qn 10-10

I copied down the final answer.

For vol NaOH added =100ml (i.e. at equivalence point,

I tried to use pH = 0.5 [pka(phenylammonium) + pka(phenylamine)]

I found ka of phenylamine by finding kb of phenylamine on the internet.

Then i took kw/kb to find ka of phenylamine.

But i dont get the answer

Hope you can help check, thanks !

For the sake of all visitors to the forum, post the photo of the question (unless it's a TYS or Prelim paper question in which all visitors are expected to have their own access to). And your working is obviously wrong. There is no (meaningful) Ka or pKa value for phenylamine, since it is basic and not acidic. Ask yourself, what is the species present at equivalence point? Therefore, what is the relevant formula to use? What is the name given to the following formula, and exactly when do you use this formula? pH = 1/2 (pKa1 + pKa2). Your problem is you're blindly memorizing formulae without understanding them, and hence you dunno when to use which formula.

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