STPM 化学问题 马拉松！

hvaly

原帖由 dunwan2tellu 于 4-9-2006 07:19 PM 发表
问题37
State how you distinguish between chlorobenzene and 2-iodo-propane ?

是不是这样？
add acidic AgNO3(aq) into chlorobenzene and 2-iodo-propane respectively and heat.
2-iodo-propane  will undergo hydrolysis to give out I- and form yellow precipitate with Ag+(AgI) whereas chlorobenzene does not.
Ag+ + I-    --->  AgI(yellow precipitate)
如果有错误plz纠正我。。。

[ 本帖最后由 hvaly 于 10-9-2006 01:30 PM 编辑 ]

dunwan2tellu

对了！
问题 38

When aqueous ammonia is added to a solution containing Mg(2+) ions , a white precipitate is formed . Which of the following ions could dissolve the white precipitate ?

A.Na(+)
B.NH4(+)
C.SO4(2-)
D.OH(-)
E.Cl(-)

UnitedDream

问题 38

When aqueous ammonia is added to a solution containing Mg(2+) ions , a white precipitate is formed . Which of the following ions could dissolve the white precipitate ?

A.Na(+)
B.NH4(+)
C.SO4(2-)
D.OH(-)
E.Cl(-)

Answer is B . white precipitate is Mg(0H)2 , which have low Ksp . Adding NH4(+) will remove OH- ,thus equilibrim shift right , Mg(OH)2 dissolved .

這題好象是salt analysis 的耶 ﹐ 不在syllabus.............


39 .what is the condition for Sn (+4) oxidation state to be more stable than (+2) oxidation state ?

dunwan2tellu

补充 38 :

NH3 + H2O <==> NH4(+) + OH(-) ....(1)
Mg(2+) + 2OH(-) <==> Mg(OH)2 .....(2)

当 NH4(+) is added to the solution , [NH4(+)] increases , and the equilibrium of (1) will shift to the left.So [OH(-)] will be reduced and the equilibrium of (2) will shift to the left .White precipitate will dissovles .
这题不在 syllbus ? 这是 ionic equilibrium 的题目。

39 ) 一般来说 C,Si,Ge are very stable at +4 oxidation state , while Sn,Pb are stable at +2 oxidation state .Sn,Pb in 4+ oxidation state are easily decomposed when heated to form Sn,Pb 2+ .

所以我猜测是 at low temperature ?

UnitedDream

Decomposition Temperature is compare between compound stability . The more stable the ions in it's oxidation state , the more energy is required to break it's stability .This don't indicate what oxidation state is more stable ,but by it's decomposition product , we can deduce the oxidation state which is more stable .Answer is in (aq) , in aq ,+4 oxidation state is slightly more stabel than +2 , Sn(+2) is weak reducing agent in aq

你也是physic 的吧 ﹖﹖﹖ 幾時開一個physic 馬拉松 ﹖﹖﹖

[ 本帖最后由 UnitedDream 于 11-9-2006 12:05 AM 编辑 ]

dunwan2tellu

Decomposition Temperature is compare between compound stability . The more stable the ions in it's oxidation state , the more energy is required to break it's stability .This don't indicate what oxidation state is more stable ,but by it's decomposition product , we can deduce the oxidation state which is more stable .Answer is in (aq) , in aq ,+4 oxidation state is slightly more stabel than +2 , Sn(+2) is weak reducing agent in aq

原来还有这样的道理。

你也是physic 的吧 ﹖﹖﹖ 幾時開一個physic 馬拉松 ﹖﹖﹖

一次举行太多马拉松怕会体力透支。

问题 40
Compare the relative strength of acidity of the following organic acid
(i) 2-phenyl-benzoic acid
(ii)2-nitro-benzoic acid
(iii)benzoic acid

[ 本帖最后由 dunwan2tellu 于 11-9-2006 03:34 PM 编辑 ]

UnitedDream

问题 40
Compare the relative strength of acidity of the following organic acid
(i) 2-phenyl-benzoic acid
(ii)2-nitro-benzoic acid
(iii)benzoic acid

应该是 (ii)>(i)>(iii)吧 ？？我不确定phenyl group is stronger or nitro group is stronger .



41 . How to convert nitrobenzine to benzoic acid ?

dunwan2tellu

原帖由 UnitedDream 于 11-9-2006 08:40 PM 发表
问题 40
Compare the relative strength of acidity of the following organic acid
(i) 2-phenyl-benzoic acid
(ii)2-nitro-benzoic acid
(iii)benzoic acid

应该是 (ii)>(i)>(iii)吧 ？？我不确定 ...

正确的答案是 (ii) > (iii) > (i)

因为 NO2 is electron withdrawing group ,when bonded to benzoic acid will make it more stable .Equilibirum will shift to right causing more proton to be produced .
OH- bonded to benzene ring will become electron donating group due to resonance effect(原本应该是有inductive effect 所以是 electron withdrawing group 但 resonance effect > inductive effect 所以他变成 e donating group )

dunwan2tellu

41 . How to convert nitrobenzine to benzoic acid ?

这不简单。我尝试把 nitro-benzene convert 去 phenylamine(Sn +conc HCl heat) 然后再 convert 去 phenol(HNO3 , >5 celcius) ,但是行不通。提示？

UnitedDream

OH-bonded to benzene ring will become electron donating group due to resonance effect(原本应该是有inductive effect 所以是 electron withdrawing group 但 resonance effect > inductive effect 所以他变成 e donating group )

你玩我啊 ！！！！ OH group attach to benzoic acid 是叫 2-hydroxybenzoic acid  !!!!! Phenyl group is prefix for C6H5 耶 ，。:@

dunwan2tellu

原帖由 UnitedDream 于 12-9-2006 04:32 PM 发表
OH-bonded to benzene ring will become electron donating group due to resonance effect(原本应该是有inductive effect 所以是 electron withdrawing group 但 resonance effect > inductive effect 所以他 ...

不好意思，我原本是要打 2-hydroxy-benzoic acid 的，不知道怎么会打到 phenyl 去...

不过如果没记错，是 nitro group > phenyl group

[ 本帖最后由 dunwan2tellu 于 13-9-2006 11:45 AM 编辑 ]

happytomato

原帖由 happytomato 于 6-9-2006 06:34 PM 发表
不好意思，我有问题要问，past year问题来的:
C2H6O2 merupakan bahan yg dimasukkan ke dlm radiator kenderaan utk menghalang pembentukan ais ketika cuaca sejuk dan untuk mengurangkan kehilangan air ke ...

这是1994年STPM的问题来的.问题问到关于perubahan fasa因为它跟phase diagram 有关.所以我觉得答案应该是跟non-volotile solute在solution有关.就讲presence of foreign molecules orderly arrangement more difficult, then, lower freezing point等等...我只是用猜的, 我还没有正确答案.

UnitedDream

原帖由 dunwan2tellu 于 12-9-2006 02:40 PM 发表


这不简单。我尝试把 nitro-benzene convert 去 phenylamine(Sn +conc HCl heat) 然后再 convert 去 phenol(HNO3 , >5 celcius) ,但是行不通。提示？

Sorry 我忘了这题不在syllabus !! 你前面做对了 ， 当你拿到diazonium ion 的时后 ，在<5 celcius add HCN(aq) and let the temperature to raise . then you will get a phenyl group attach to CN . Then used acidic hydrolysis , you will get benzoic acid .

别生气噢 我真的\不知道不在syllabus .

dunwan2tellu

问题 42

Why Nickel can act as a catalyst in the reaction of hydrogenation between hydrogen gas and alkene ?

UnitedDream

Because Ni is transition metal , it have empty d-orbital that can form temporary bond and allow adsorption of alkene and hydrogen on it's surface .Therefore the bond between alkene and in hydrogen is weaken.The activation energy need in hydrogenation of alkene is lowered .

[ 本帖最后由 UnitedDream 于 17-9-2006 07:39 PM 编辑 ]

UnitedDream

Explain why germanium do not form catenation compounds .

dunwan2tellu

Because Ni is transition metal , it have empty d-orbital that can form temporary bond and allow adsorption of alkene and hydrogen on it's surface .Therefore the bond between alkene and in hydrogen is weaken.The activation energy need in hydrogenation of alkene is lowered .

基本上都正确，不过 STPM 要求写 Ni 是 heterogenous catalyst

Explain why germanium do not form catenation compounds

什么是 catenation compunds ? 不会又是 out of syllybus 吧？

QooLuo

catenate 的意思是： ability to form bond with its own atom

dunwan2tellu

原帖由 QooLuo 于 17-9-2006 09:41 PM 发表
catenate 的意思是： ability to form bond with its own atom

会不会是因为 Germanium 有 4 valens electron .若要 form catenate 的话就要 share 4 electron ,所以需要很大的 energy,因此行不通？

hvaly

原帖由 UnitedDream 于 17-9-2006 07:43 PM 发表
Explain why germanium do not form catenation compounds .

是不是因为germanium compound has a higher bond energy than the bond energy between atoms of germanium(catenation compound)?so it tends to form compound of germanium than catenation compound??