chem qn

pokepokee SG

hello
may i know why is pcl5 solid while pcl3 is liquid? thanks

UltimaOnline SG

hello
may i know why is pcl5 solid while pcl3 is liquid? thanks

Because PCl5 at room temperature and pressure exists as an giant ionic lattice structure of [PCl4]+ cations and [PCl6]- anions.

Test Yourself :

Draw the structure of both cation and anion, stating their electron and ionic geometries. Work out the Oxidation States of phosphorus in both ions.

Rremember that the formula for Oxidation State = Formal Charge + Electronegativity consideration. This was tested in this year's 2011 P3 Qn, "What is the OS of Br in RCONHBr?"

pokepokee SG

Thanks Smile taking A levels… formal charge is the overall charge right? Btw why does phenol form a trisubstituted product when br2(aq) is used but not when br2(ccl4) is used?

UltimaOnline SG

Thanks Smile taking A levels… formal charge is the overall charge right? Btw why does phenol form a trisubstituted product when br2(aq) is used but not when br2(ccl4) is used?

No, formal charge is the charge on the individual atom. Ionic charge is the overall charge. Ionic Charge = Sum of Formal Charges. Wikipedia out Formal Charges.

Water is strongly polar (being a protic polar solvent), and thus able to generate (by inter-electron repulsion) a stronger induced dipole in the hydrated non-polar Br2 molecule, compared to Br2 in non-polar solvents (eg. CCl4). Consequently, Br2(aq) is a more reactive (feminine) electrohile (chiobu) compared to Br2(CCl4) (a plain Jane), and accordingly, the activated phenol nucleophile (ie. the benzene ring which is made a stronger nucleophile (ie. aphrodisiac makes the guy more horny) by the phenolic OH group which donates electrons by resonance into the benzene ring, strengthening it's nucleophilicity) requires less activation energy to nuclephilically attack (ie. donate a dative bond*) to the more reactive Br2(aq) electrophile, and thus tri-substitution (tri-halogenation) can result for the more reactive (feminine) electrophile Br2(aq), in contrast to only mono-substitution for the less reactive electrophile Br2(CCl4).

* Mechanism-wise, the nucleophile (guy) shoots out it's 'balls' (lone pair) to attack the electrophile (girl), ie. donates a dative bond to the electrophile. The 'balls' are usually a lone pair for the nucleophile, with the exception of alkenes (and alkynes) benzene (and other aromatic species), whose 'balls' are not a lone pair, but a pi bond. Of course, the alkene is a stronger nucleophile than benzene, because the 'balls' (pi bonds) are localized (hence more available) in alkenes, and delocalized (hence less available) in benzene.

Similarly, amides are not basic compared to amines, because it's 'balls' (lone pair) are delocalized away by resonance to form a pi bond with the C=O carbonyl group, and are not (or at least, less) available to accept a proton.

pokepokee SG

hahaha. Your analogies are making me think sick! simply put, is it because br2(aq) is a better electrophile as it can form hbr(aq) and hobr(aq) in water? the br is positively charged and hence the extent of electrophilic sub will be greater.

UltimaOnline SG

Yes correct. Water is the nucleophile (guy) who shoots out its 'balls' (lone pair), to nucleophilically attack (ie. give its balls) to the Br2(aq) electrophile (girl), (the consequently positive formal charged O atom of water) subsequently losing a proton to the Br- (eliminated earlier when Br2 was receiving the 'balls' from the water nucleophile), thereby simultaneously generating both hypobromous acid (latin name) aka bromic(I) acid (stock name), and hydrobromic acid.

Chemistry is all about Love. Guys like girls, nucleophiles like electrophiles, positive and negative charges attract. Even covalent bonding is about electrostatic attraction (something few students realize!). The bonding electrons in a covalent bond are simultaneously electrostatically attracted to both (positively charged) nuclei of both atoms, which is how a covalent bond holds two atoms together (just as how an emotional bond holds a guy and a girl together in a relationship of love).

pokepokee SG

Thanks for the reply! Not sure if anyone had asked this before. For electrochem cell, polarity of anode is negative while cathode is positive. How come the polarity is reversed for the case of electrolytic cell?
Anode is oxidation, so electrons are released into the electrolyte hence it is negative electrode and vice versa for cathode.

UltimaOnline SG

Red Cat riding/killing/punching/hugging/kissing/loving An Ox.

Reduction occurs at the Cathode.
At the Anode, Oxidation occurs.

In an electrolytic cell, electrons flow from the power source to the electrode, where some species (usually a cation) accepts electrons to be reduced. Hence, this electrode is called the cathode. And as far as you (the species accepting the electron to be reduced) are concerned, the cathode is your source of negatively charged electrons, hence the cathode is the negative electrode, and therefore the anode is the positive electrode.

In a Galvanic aka Voltaic cell, electrons flow from the anode where oxidation occurs. Think of any oxidation half-equation, eg. Al ---> Al3+ + 3e-. See the electrons being released? It flows from the anode to the light bulb. As far as you (the light bulb accepting the electrons to light up) are concerned, the anode is your source of negatively charged electrons, hence the anode is the negative electrode, and therefore the cathode is the positive electrode.

pokepokee SG

Am i right to say that for electrolytic cell, the cathode contains a source of electrons hence it is negatively charged? And for voltaic cell, the electrons are released at the anode hence it is negatively charged?
Why cant the explanation for voltaic cell apply for electrolytic cell? I remember it as: AN OX CAR since i cant imagine a red cat Very Happy

UltimaOnline SG

The same explanation does apply to both cells. Whoever is the source of electrons, is always the negative electrode.

Re-read my explanation in my earlier post. It's just that in Galvanic cells, you're the light bulb accepting the electrons. In electrolytic cells, you're the ions in the electrolyte accepting the electrons.

The 'source of electrons' is not the definition of cathode or anode. Red Cat riding An Ox is the definition of cathode and anode. Which is why the sometimes the negative electrode is the cathode, and sometimes it's the anode.

pokepokee SG

okok. I sort of get what you mean. Gah. Im not very good at physical chem topics as i didnt take physics in upper sec. Do we have to know mass spectrum for A levels?

UltimaOnline SG

okok. I sort of get what you mean. Gah. Im not very good at physical chem topics as i didnt take physics in upper sec. Do we have to know mass spectrum for A levels?

Not for H2 Chem. But you can easily figure out mass spectrometry questions by yourself (if you wish to, or if you're forced to, eg. in a data based question).

All other spectroscopic analytic methods such as NMR, IR, UV etc, are also not in the H2 Chem syllabus.

pokepokee SG

okay Smile Lame question: can use pencil to draw energy level diagram?

UltimaOnline SG

okay Smile Lame question: can use pencil to draw energy level diagram?

Can. Pencil is acceptable as long as strokes are clear and strong.

pokepokee SG

thanks. some questions from 08 paper/p2 qn 1d
1: will a multisubstituted pdt be formed when phenol is reacted with excess br2 in ccl4?
2: Will the h2so4 present in acidic potassium dichromate react with amine to form a salt?

UltimaOnline SG

Q1. No. But if more than one phenolic OH group present, di-halogenation is possible. But this TYS qn says "suitable solvent", so you must consider aqueous solvent as well. So correct answer to this qn is tri-halogenation. Or better yet, give both answers to "case 1 : CCl4 solvent" and "case 2 : aqueous or water solvent".

Q2. Certainly, it would. Tis a common exam trick qn.

Try this :

Heat an ester under reflux with alkaline KMnO4 : what do you get?

pokepokee SG

oh.. Forgot that water is a polar solvent as well. For that qn, is it correct to give the disubstituted pdt instead of the trisubstituted one?
Hmm. The ester undergoes basic hydrolysis to produce Rcoo- and Roh?
One more qn.. For 08 paper/2 qn 3c, is it okay to use nabh4 instead of lialh4 in dry ether?

UltimaOnline SG

No. You have to give the tri-halogenated product.

Don't forget that any alcohol generated from the hydrolysis will be further oxidized as well. This is the tricky bit.

NaBH4 can only be used to reduce aldehydes and ketones, absolutely nothing else. NaBH4 is a weaker reducing agent compared to LiAlH4 because of the difference in size of their 'balls'. Translation : because Al has one entire electron shell compared to B, thus the valence electron charge clouds and hydride ions of AlH4- are more polarizable and available for nucleophilic attack, compared to that of BH4-.

Both LiAlH4 and NaBH4 cannot reduce alkenes to alkanes because LiAlH4 and NaBH4 are not gay. Guys only like girls, guys hate other guys. Because all species here (alkenes, and hydride ions from LiAlH4 and NaBH4) are all healthy, full-blooded horny males (ie. nucleophiles) pumped full of testosterone, and will only seek out attractive fertile chio-bu (ie. electrophiles) to attack. Electron-rich nucleophiles (eg. alkenes) repel other electron-rich nucleophiles (eg. H- ions from LiAlH4 and NaBH4).

pokepokee SG

okay. That qn was reduction of ketone to alcohol so i guess nabh4 can be used. Smile
See if my explanation is right:
alh4- is easier to be polarised due to the greater size of electron cloud of al as compared to b. The hydride ions are nucleophiles as they are negatively charged. Are the electrophiles the compound that are going to be reduced? Is the mechanism similar to tat of nuc sub of RX? Btw, why is reduction of alkenes not electrophilic addition?

pokepokee SG

EDIT: my qn should be is the mechanism of reduction of carbonyls similar to that of nuc addition? It shouldnt be nuc sub since there is no double bond in the pdt..