STPM化学专区

morphv02c01

原帖由 Dicardo 于 27-8-2009 11:32 AM 发表
so if it only written 'aqueous' , we can assume it is dilute ?

aqueous IS NOT equal to dilute

RachelT

我刚做完一个实验，titration of KA1,KA2,KA3。
想问有没有人知道为什么这个实验不需要用indicator？？
我好想知道答案。。
有人写因为KMnO4本身会变色，所以可以act as indicator，但是这个答案是错的耶。。
所以正确答案是？？？？

idontwant2b

有些印象，是不是关于那个dissociation的？

morphv02c01

你的titrant和titrate是什么?

jenyeh90

想問下哦，一個REACTION,
我要如何知道他是REVISIBLE OR NT?

EG:
A + B ﹣﹣﹣﹥ C    OR
A + B ﹤﹣﹣﹥ C

darksider

这我们要看那个reaction的entropy change.如果entropy change =0 ,那个reaction是reversible.

我们在stpm没有学到ｅｎｔｒｏｐｙ。

jenyeh90

那寫equation的時候要怎么辦？

darksider

原帖由 jenyeh90 于 2009/11/16 12:17 AM 发表
那寫equation的時候要怎么辦？

原来你问这个。。。如果那个arrow是有两个箭头的，那就是reversible reaction
如果一个箭头就irreversible.

A <-----> B

phoebie

请帮我解答

1.如果一个complex ion with 4 coordination number....我要怎样分他是tetrahedral 还是square planar???
plzzzzzzzzzzzzzzzz.....thxxxxxxxxxxxx

四月一日的小皮

low spin state和high spin state。
Tetrahedral and square planar complexes are the most common.
Small ions and/or large ligands prevent high coordination numbers (Mn(VII) or Cr(VI)).

Many d0 or d10 complexes have tetrahedral structures (only consider bonds).
exp MnO4- and [Ni(CO)4]

In stpm，you nd to know about square planar complex。

phoebie

楼上的。。。谢谢。。。可是我不明白耶。。。。有没有简单点的？？？

darksider

原帖由 phoebie 于 2009/11/19 01:55 PM 发表
楼上的。。。谢谢。。。可是我不明白耶。。。。有没有简单点的？？？

Why are they square planar?
When we talk about molecular geometry, we first consider their axial position (imagine it as y-axis going across the central atom) and the equatorial position (on the same plane as the central atom)

we know the VSEPR states that electron pairs will repel each other so that they are as far from each other as possible (minimum repulsion).

If there are four covalent bonding on the equatorial position and that the central atoms have 2 lone pairs on its axial position, its molecular shape will look like a square, thus square planar (square on the same plane) since lone pair is very small and cannot be seen.

So why don't the lone pair stay at the equatorial position? Why don't the covalent bond (dative bond) stay at the axial position?
To answer this u need to know in what kind of arrangement will they get minimum repulsion.
If you exchange one of the lone pair with one of the bonding pair, it will distort the square shape on the equatorial position, since repulsion btw lone pair -bonding pair > repulsion btw bonding pair- bonding pair, the square shape will be altered into a rhombus. This gives to non-minimum repulsion so it isn't possible.

The lone pairs must stay on the axial position in order for the repulsion to be minimum. Remember, electron pairs in the axial position will not have effect on the electron pairs in the equatorial position.

idontwant2b

原帖由 darksider 于 20-11-2009 12:57 AM 发表


Why are they square planar?
When we talk about molecular geometry, we first consider their axial position (imagine it as y-axis going across the central atom) and the equatorial position (on the  ...

我们老师说这种low spin / high spin / axial position 的东西不需要读，是真的吗？

darksider

原帖由 idontwant2b 于 2009/11/20 12:29 PM 发表


我们老师说这种low spin / high spin / axial position 的东西不需要读，是真的吗？

不会叫我们解释。但是如果要明白molecular geometry的话，就要懂，不然就一定要背。。。背这个比明白容易。。。

可以参考这个http://intro.chem.okstate.edu/1314F97/Chapter9/VSEPR.html
有animation,比较容易明白。

你可以那PCl5来当例子，过后你exchange a lone pair with one of the bondng and think about which bonding pair should be substituted to give minimum repulsion. (see-saw)

Then try substitute 2 lone pair and think they should replace which. (T shape)

Allison

PbO2 + conc.HCl 的product是什么？

darksider

原帖由 Allison 于 2009/11/20 03:10 PM 发表
PbO2 + conc.HCl 的product是什么？

PbCl4 and H2O... PbO2 is amphoteric... just treat it like an alkali when it reacts with acid.

ginny

Zaitsev's rule & Saytzeff's rule is same?

海涵

请问
pH + pOH = 14
里的pH & pOH 代表什么？

darksider

pH = -log [H+] , pOH = -log [OH-]
pH is a measure of the concenration of H+ ions.

H2O <----> H+ + OH-
Kw = [H+] [OH-] = 10^(-14) , ionic product of water
你写的equation是从以上那个derive 出的

meiling

是不是Kw 一定= 10^(-14)
but [H+] 不一定= [OH-]