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sklee

Physical chemistry "Reaction Kinetic" 我不是很了解。其实这课要怎么读？有什么重点（除了calculation）是要背的？恳请各位好心的大哥大姐解释整课可以吗？就当作自己也为了即将来临的大考温习这课吧^^

变态小红帽

stpm 的，我帮不到你～

但要你们加油 ～

Critical

http://www.khanacademy.org/
去这个网站点击 kinetics及其相关的部分
希望这能帮到你

[ 本帖最后由 Critical 于 12-10-2009 11:06 PM 编辑 ]

darksider

Learn how to integrate the rate equations...
consider the reaction : A <----> B

Rate of reaction,
if first order to A , then rate = k[A]^1
if second order to A, then rate = k[A]^2
if zero order to A, then rate = k[A]^0 = k

Learn to derive them (require integration), eg shown below for first order rate equation

rate of reaction = k [A]
or decrease in [A] / dt= -d[A]/dt =k[A]  where k is a constant.

-d[A]/dt = k[A] , (1/[A]) d[A] = -k dt
integrate [(1/[A]) d[A] ] = - integrate [k dt]
ln [A] = -kt + c
when t = 0, [A] = [A]o (initial concentration)
therefore, ln[A]o = c

Hence, ln [A] = -kt + ln[A]o => ln [A] - ln[A]o = -kt => ln ([A]/[A]o) = -kt
To find half life,
When t = t1/2 , [A] = 0.5[A]o
Therefore, ln ([A]/[A]o) = -kt => ln ( 0.5 [A]o / [A]o ) = -kt1/2
=> ln 0.5 = -kt1/2 ;  t1/2 = ln 0.5 / -k = ln (0.5)^-1 / k = ln2 /k


*try to derive second order and zero order rate equation yourself.

[ 本帖最后由 darksider 于 13-10-2009 02:45 PM 编辑 ]

sklee

原帖由 darksider 于 13-10-2009 02:44 PM 发表
Learn how to integrate the rate equations...
consider the reaction : A  B

Rate of reaction,
if first order to A , then rate = k[A]^1
if second order to A, then rate = k[A]^2
if zero order to A, ...

为什么我的老师说derive的不重要，他说不会出的？？除了这些，还有那些重点？过后那课也是有关系到这课的。。。。

premier66

要会看graph, table form 等， 不太会问derivation 的，不过要会知道什么是zero ,1st order 等，要会知道用什么formula ，还有要会overall order （这个会在structure question 和MCQ 出比较多），physical km只要明白theory, calculation那里 就应该会算 ， 还有不太用背的，其实一个theory 可以apply 在很多地方的

[ 本帖最后由 premier66 于 13-10-2009 09:55 PM 编辑 ]

darksider

原帖由 sklee 于 2009/10/13 07:46 PM 发表


为什么我的老师说derive的不重要，他说不会出的？？除了这些，还有那些重点？过后那课也是有关系到这课的。。。。

如果你不会derive，你就得去背那些formula. [ 背formula比derivation更加难，如果不相信的话可以试试 )

基本上如果你知道什么是y=mx+c, y=x^2 ，y=k 的curve，你就会看懂那些graph并知道他们是first order/second order or zero order。(linear , exponential, etc)

很多同学都不会derive，只会死背。

For rxn
aA + bB<------> cC,
Reaction occurs when molecules of A and molecules of B bump against each other in the right orientation and with the right energy.

generally, rate of forward reaction = k P(finding A) . P(finding B) ;

P(finding A) = probability of finding A molecules in a given solution and P(finding B) = probability of finding B in a solution and K is a arbitrary constant.
For the above equation, there is one molecule of A and one molecule of B, therefore the rate of forward rxn = k [A]^a [B ] ^b

However, in the real world, most reactions do not follow the above models.
They are classified into zero order, first order, second order and so on. The only way to know about the nature of the rate of reaction is through experiment.

For A + B ----> C

If the rxn is zero order to both A and B,
then rate of forward rxn = k [A]^0 [B ]^0 = k
This means that no matter how high is the concentration of your A and B, the reaction rate will be constant.

If the rxn is first order to both A and both B,
it implies that the rate of forward rxn = k [A]^1 [B ] ^1 = k [A][B] , which is second order overall.
If first order to A, it means when [A] doubles , the rate will double too .

If the rxn is second order to A and zero order to B, it means that the rate of rxn is dependent only on [A].
Therefore, the rate of forward rxn = k [A]^2 ^0 = k[A]^2.
if second order to A, it means that when [A] increases by double, rate of forward rxn will be 4 times its original value.

若你还是不明白什么是reaction kinetic 的话，你应该去问你的老师吧，最重要是明白它的concept。

必看
http://www.chemguide.co.uk/physical/basicrates/orders.html

可以参考以下网站
http://en.wikipedia.org/wiki/Chemical_kinetics
http://www.cheresources.com/reactionkinetics1.shtml
http://chemistry.about.com/od/stoichiometry/a/reactionrate.htm
http://www.scholarpedia.org/article/Chemical_reaction_kinetics

[ 本帖最后由 darksider 于 14-10-2009 06:46 PM 编辑 ]

sklee

你说的这些我都知道啊。。可是就是不知道自己到底还不懂什么。。读书最怕遇到不懂自己不懂什么

四月一日的小皮

请问lz是不是不会calculate还是遇到问题的时候脑突然blank了？

sklee

是这样没错。。。 尤其是calculation...

vinclam

个人觉得最重要的是你要明白  First Order, Second Order and, Zero Order and Pseudo First Order 的 differences..
Question 通常都会给你一个 Situation.. Calculate.. find what order..
Or..
given the order of reaction.. find the concentration..
Once you know what the differences, then everything will be fine..