STPM化学专区

truelove2u

会容易？
我觉得超级难。。=.=

而且essay突然间没有给test pad paper了~
给一本“书”来给我们填答案。。=.=
麻烦

海涵

请问CH3CH=CH2 的overlapping of orbital 怎么画？

鸟人

有谁有全部organic里的reaction,condition,observation,reagant的资料？

silent91

当equilibrium的时候，enthalpy change 是不是等于零？

四月一日的小皮

When equilibrium is achieved, the enthaply change will become constant but no become 0, since it may be activation energy.
So,the enthalpy change in both forward and backward direction are the same in term of magnitude.

Remember the following conditions.
When in equilibrium:
1/ Rate of both forward and backward condition are the same.
2/ Concentration of reactant and product remain constant.
3/ E.m.f = 0V.(So you can find equilibrium constant using Nerst Equation)
4/Dynamic,always flow.

silent91

When equilibrium is achieved, the enthaply change will become constant but no become 0, since it may ...
四月一日的小皮 发表于 12-1-2010 09:19 PM

   谢谢解答！！感激不尽！！！

Lov瑜瑜4ever

请问下一下这几个functional group的英文名字叫什么？

a) -CHO
b) -C double bond O-
c) 为什么aldehyde和ketone的molecule里面的H atom无法与adjacent的分子形成hydrogen bond而alcohol却可以？？

孤风幻影

请问下一下这几个functional group的英文名字叫什么？

a) -CHO
b) -C double bond O-
c) 为什么aldehyde和ketone的molecule里面的H atom无法与adjacent的分子形成hydrogen bond而alcohol却可以？？
Lov瑜瑜4ever 发表于 13-1-2010 10:07 PM

a) formyl group
b) carbonyl group

Lov瑜瑜4ever

a) formyl group
b) carbonyl group
孤风幻影 发表于 14-1-2010 12:45 AM

谢谢了

语曦

回复 447# Lov瑜瑜4ever

the hydroxyl group of alcohol....function:dissolve in water molecules

Lov瑜瑜4ever

回复  Lov瑜瑜4ever

the hydroxyl group of alcohol....function:dissolve in water molecules
语曦 发表于 14-1-2010 01:52 PM

那么aldehyde和ketone的分子间做么却无法形成hydrogen bond？

四月一日的小皮

electronegativity of Carbon,C is smaller than oxygen.
So, oxygen acquire stronger negative partial change compared to carbon when shard with hydrogen.
And because of that, instanteneous dipole-induced dipole occur between hydrogen atom of an alcohol molecules(which acquire stronger partial +ve charge) and oxygen atom of adjacent alcohol molecules(which acquire stronger partial -ve charge),resulting hydrogen bond to be formed in alcohol.
In aldehyde and ketone,hydrogen is bonded to carbon with a small electronegativity difference.
This resulting weaker partial charge is formed and it is not enough for adjacent aldehyde/ketone molecules bind together to form hydrogen bond.

correct me if wrong.

外星護法

When equilibrium is achieved, the enthaply change will become constant but no become 0, since it may ...
四月一日的小皮 发表于 12-1-2010 09:19 PM

请问什么是equilibrium quotient, Q ??

Lov瑜瑜4ever

electronegativity of Carbon,C is smaller than oxygen.
So, oxygen acquire stronger negative partial  ...
四月一日的小皮 发表于 14-1-2010 04:52 PM

那除了N，O和F以外
还有什么element是能和H形成hydrogen bond？

四月一日的小皮

请问什么是equilibrium quotient, Q ??
外星護法 发表于 14-1-2010 08:45 PM

Equilibirium quotient,Q is same as the definition of the equilibrium constat,K, but it is differ to the K in term of state.
For equilibrium constant(K), the system is in equilibrium state.
But for equilibirium quotient(Q), the system is either equilibrium or non-equilibirium state.
In short, we can say, when the system is in equilibrium,Q = K
Normally, equilibrium quotient is used to predict whether the system achieved equilibrium state or not.
For example,
                                                
The equilibrium constant for the oxidation of sulfur dioxide is Kp = 0.14 at 900 K.                                                

2 SO2(g) + O2(g) → 2 SO3(g)

                                                
Initially,the reaction vessel is filled with SO3 at a partial pressure of 0.10 atm and with O2 and SO2each at a partial pressure of 0.20 atm, in condition of same temperature.
So, We will calculate the equilibrium quotient by using all data given above.
Then,

Q=(PSO3)2 / (P02)(PSO2)2

  =(0.1 atm)2 / (0.2atm)(0.2atm)2
       = 1.25 atm-1

  =/= 0.14 atm-1

K = Q when system is in equilibrium state.
But in here, since K =/= Q,the system is not in equilibrium state.
So, reaction still occur either increasing concentration of reactant or concentration of product until equilibrium state is achieved.

But how we know the direction of the reaction shift?
By using Q,we can estimate which direction the reaction will go to achieve equilibrium
Generally,
Q < K     ====>Meaning there is more reactant than product.
Q = K     ====>Meaning equilibrium is established.
Q > k     ====>Meaning there is more product than reactant.

Using La Chatelier's Principle, we can know the direction of reaction shift.
When [reactant]>[product], reaction will shift to right.More product will be formed.
When [reactant]<[product], react will shift to left. More reactant will be reformed.

For the above example,
Since Q > K, meaning there is more SO3 compared to O2 and SO2.
In order to achieve equilibrium,the reaction will proceed to shift to left direction.
Thus,[SO3] decrease while [O2] and [SO2] increase.

Correct me if i'm wrong.
Please do not use my word, some terms in here just use for explanation.

那除了N，O和F以外
还有什么element是能和H形成hydrogen bond？
Lov瑜瑜4ever 发表于 15-1-2010 09:40 AM

Generally，in stpm，这三个（N，O，F）是最常出现在题目，也是最effective的。
不过还有其他元素的，但通常不会在stpm出现。（可以在网上找到）
因为前３０个元素（H to Zn）在stpm比较注重。
通常多出现在关于physical和inorganic的题目。
好像bonding，periodic properties等等。
当然其它元素也会出，好像Cu，Fe这些。

Lov瑜瑜4ever

为什么hydrocarbon chain in alcohol越长
对hydrogen bond的形成所起的阻碍作用就越强？

四月一日的小皮

回复 456# Lov瑜瑜4ever


General term:
When alcohol getting longer,for example,let use ethanol and heptan-1-ol/1-heptanol.
ethane is C2H5OH and heptane is C7H15OH.
remember alkane group is insoluble in water as it is hydrophobic.
In here,we have 2 different effect:hydrophobic and hydrophilic effect.
hydrophobic=>dislike water
hydrophilic => like water
So,the more the carbon atom in a chain,the stronger the hydrophobic effect,resulting hydrophobic effect predominate which cause the formation of hydrogen bonding in heptan-1-ol become weaker and become less soluble(or even not soluble);In case of ethanol,since carbon chain is short,resulting weakhydrophobic effect,so the hydrophilic effect predominate and ethanol issoluble in water.

More accurate term:
you should know about electron-donating group and electron-accepting group
electron-accepting group is a group/molecules/element that attract electron away to itself in electron density/cloud of a shared atom/molecules.for example,halogen group(Cl-, Br-,I-),benzene deriative/branch(ph-), nitro group(NO2-)and the most common in organic chemistry is the carbanion.
electro-donating group is analogous to the electron-accepting group.
It donate electron or give away electron into electron density/cloud of a shared ato/molecules.
for example,alkyl group,amine group(NH2-),and yes,carbocation.

Next,you need learn inductive effect and mesomeric effect or resonance effect.

I remeber the definition of my note for inductive effect:
Permenant shift of electron density to the atoms or molecules in high electronegativity to form polar bond in sigma bond.
And for mesomeric effect:
Shift of electron density in the pi bond of a molecules.

There are a long story to tell.So, read by yourself. It is very important topics.

---

---

+ve inductive effect of an alkyl group depend on the length of thr carbon chain in alkyl group.
So,the longer the carbon chain in alkyl group,the stronger the +ve inductive effect.
As +ve inductive effect increase,more electron is contributed into electron cloud shared by carbon and oxygen in C-O bond.This resulting increasing electron density and then reduce charge density in C-O bond.C-O bond become less polar and the difficulty to form hydrogen bonding is increased.
Remember that hydrogen bonding is formed due to instanteneous dipole-induced dipole.so,polarisation occurs.



Correct me if i'm wrong,cos' i feel i did some mistakes here.
I will re-edit later.

Lov瑜瑜4ever

回复  Lov瑜瑜4ever


General term:
When alcohol getting longer,for example,let use ethanol and  ...
四月一日的小皮 发表于 15-1-2010 04:52 PM

"This resulting increasing electron density and then reduce charge density in C-O bond"

Is this mean the higher the electron density/electron cloud,the lower the charge density?
Which chapter can I find this content?
Now I am waiting to go in Form 6...

四月一日的小皮

回复 458# Lov瑜瑜4ever

increasing electron density because of electron contribution by electron-donating group.
reduce charge density cause chemical bond become stronger.

U can go check organic chemistry.
It would be great if u can understand all organic chemistry stuff.
It is not only can help you earn alots of marks in exam, but yeah it is quite fun to learn,more than just what you learn in form 5.^^
try look at this site
www.chemguide.co.uk

if u can finish 1/2 of syllabus in STPM before lower 6,why no go for participate K3M tis yr? It is more than STPM in advanced lvl.,although somekind of weird. U cn even participate when u r in upper 6.

Good to hear people join form 6 if the system didnt changed much.

Lov瑜瑜4ever

回复  Lov瑜瑜4ever

increasing electron density because of electron contribution by electron-dona ...
四月一日的小皮 发表于 15-1-2010 06:03 PM

噢噢噢
请问下你今年也是stpm生？
那个K3M是什么东西来得？
STPM会改称semester制度？