STPM化学专区

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-]

darksider

原帖由 meiling 于 2009/11/20 05:57 PM 发表
是不是Kw 一定= 10^(-14)
but [H+] 不一定= [OH-]

at 298k.

很少H and OH dissociate from water...

if [H+] =/= [OH] , then the solution is not neutral already.

海涵

那pOH is a measure of the concenration of OH- ions??

meiling

应该可以这样说吧。。
但是考试大多数是问pH的。。

morphv02c01

yup, because pKa + pH is always equal to 14

morphv02c01

pOH = -log [OH-], given the pOH or pH of a solution, you can calculate the concentration of the ion by taking the antilog of the value of pH or pOH.

海涵

concentration & molarity 一样吗？

恩儿

原帖由 phoebie 于 19-11-2009 08:47 AM 发表
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

based on 我们老师的说法，这个没有一定的arrangement.
不过他告诉我们当ligand是group的时候，大多数都是square planar的，

meiling

concentration & molarity不一样的
concentration是g dm-3
molarity是mol dm-3

星星之玲

我想请问我们每次算那个degree dissociation的C 是concentration 还是molarity?   还有可以解释一下什么是solubility product and common ion effect? 我都看不懂...惨了啦!

idontwant2b

原帖由 morphv02c01 于 20-11-2009 10:30 PM 发表
yup, because pKa + pH is always equal to 14

不是pOH+pH吗？

原帖由 星星之玲 于 21-11-2009 06:19 PM 发表
我想请问我们每次算那个degree dissociation的C 是concentration 还是molarity?  还有可以解释一下什么是solubility product and common ion effect? 我都看不懂...惨了啦!

concentration in mol dm-3 = molarity，
solubility product 就是一个salt 溶进一个solvent时，
看会不会有precipitation发生的咯。
比如：
BaSO4 -> Ba2+ + SO42-
Ksp=[Ba2+][SO4 2-]
假如不同的concentration加进去，equlibrium constant =K
假如K>Ksp，那就会有precipitation。
等楼下的人回答会比较好，我才刚学会。

而common ion effect可以说是application of Le Chatelier's principle,
你看回那课关于change in concentration的，
就比如AgCl added to aqueous HCl,
原本加入水的equlibrium = AgCl <> Ag+ + Cl-
现在加入HCl，Cl- ion is common ion (in excess)，
所以equilibrium shift to left，solubility product 就变少了=solubility减少。

correct me if i am wrong. 我才学会而已。

想问为什么一些reagent要有一些solvent呢？
比如 Br2 in CCl4 , KMnO4 in H2SO4, LiAlH4 in dry ether, NaOH in ethanol，
那是为什么？

[ 本帖最后由 idontwant2b 于 21-11-2009 06:38 PM 编辑 ]

darksider

原帖由 idontwant2b 于 2009/11/21 06:27 PM 发表


不是pOH+pH吗？



concentration in mol dm-3 = molarity，
solubility product 就是一个salt 溶进一个solvent时，
看会不会有precipitation发生的咯。
比如：
BaSO4 -> Ba2+ + SO42-
Ksp=[SO4 2-]
假 ...

To dissolve / not dissolve them. eg, Br2 (non-polar ) dissolves in CCl4(non-polar solvent
NaOH (polar) in ethanol(non-polar) ; so that the solution doesn't have OH-, and Na+ ions.

meiling

Br2 in CCl4
CCl4 是inert solvent，如果是在水里面的话，Br2会有reaction-->inorganic halogen
KMnO4 in H2SO4,
这个不一定的，有些是在alkali的solution like NaOH，只是说给acidic condition，出来的东西是acidic的
LiAlH4 in dry ether
这个是一定的，因为有水会爆炸
NaOH in ethanol
这个嘛，没有原因，记住而已，in water有不同的reaction

Weakerlife

i got ques to ask~~ as we know that.... small k=rate constant...big k=equiliubrium constant.....then what things can effect them?and how we rememeber it...coz i always 4get.....=.=感谢感谢~~

四月一日的小皮

rate constant can affect by temperature,pressure,concentration,and catalyst
equilibrium constant only affect by temperature.


***rate constant only affect by temperature & catalyst,based on Arrheniusl equation.
equilibrium constant only affect by tempearature,based on van h'off equation

[ 本帖最后由 四月一日的小皮 于 23-11-2009 04:52 PM 编辑 ]

Weakerlife

Kp呢？我看arc ahead。。。inert substances oso can effect Kc...what happen of it?

idontwant2b

不会的，
adding noble gas at constant pressure = increasing pressure, (equilbrium shift to the side which has nett reduced mole coefficient)
adding noble gas at constant  volume= no effect

the only thing that change Kp/Kc is temperature,
the rest that wont change Kp/Kc and only change position of equilbrium is pressure/concentration/adding gas at constant pressure.
the only thing that dont change Kp/Kc nor position of equilbrium is catalyst.

darksider

For equlibrium constant, it depends on van hoff equation
ln K = -(enthalpy change)/RT +c
IF A<----> B +C , enthapy change = -x kJmol^-1,
then ln K = -(-x)/RT   +  c = x / RT , if T increases, x , ln K will decrease.


for rate equation, arrhenius equation
k = Ae^(-ea / RT)

[ 本帖最后由 darksider 于 22-11-2009 01:12 AM 编辑 ]