STPM 化学问题 马拉松！

ronloh94

请问Ninhydrin是什么来的？

原帖由 banko 于 31-10-2006 02:49 AM 发表
pls list out 3 type of solid desiccant...

calsium oxide,calcium chloride。还有什么？我不确定。silica gel?

dunwan2tellu

由于出题者本身没有现身，而题目也不是很清楚，我只好另出一题。

问题 74

Compare the electric conductivity of

Li,Be,C,O

explain the trend .

紫色的黑蝴蝶

原帖由 dunwan2tellu 于 7-11-2006 15:38 发表
由于出题者本身没有现身，而题目也不是很清楚，我只好另出一题。

问题 74

Compare the electric conductivity of

Li,Be,C,O

explain the trend .

it should be,Be>Li>Cl>C

Li has an extra valance eletron so it can conduct eletric easily by losing one electron to be ions Li+,free moving ions causing electric to conduct.Be in group 2 so it has to lose two electrons so it has greater electric conductivity than Li.C is in group 14 so it is rather to pose a covalent character than ionic,they used to share electrons,no extra electrons moving around,so has no electric to conduct,CL in group 17 can uptake an electron to form an anion which is Cl-,it has free ions moving around also...

Am i correct ?please recorrect it if i was wrong...thanks!

紫色的黑蝴蝶

The products of electrolysis of brine in a mercury cathode cell are chlorine,hydrogen,and sodium hydroxide.How many moles of these products are formed when one mole of electrons is passed through the solution?

dunwan2tellu

it should be,Be>Li>Cl>C

Li has an extra valance eletron so it can conduct eletric easily by losing one electron to be ions Li+,free moving ions causing electric to conduct.Be in group 2 so it has to lose two electrons so it has greater electric conductivity than Li.C is in group 14 so it is rather to pose a covalent character than ionic,they used to share electrons,no extra electrons moving around,so has no electric to conduct,CL in group 17 can uptake an electron to form an anion which is Cl-,it has free ions moving around also...

Am i correct ?please recorrect it if i was wrong...thanks!

题目是 Be,Li,C,O 并没有 Cl

不过就算是用 Cl 的话，应该是

Be > Li > C > Cl

因为 Cl (当然不是指 Cl(-) ion ) 是 covalent compound ,不会 conduct electricity . Carbon 则需要在 high temperature 里才会  conduct electricity (semi-conductor)

lowch1

原帖由 紫色的黑蝴蝶 于 10-11-2006 12:06 AM 发表
The products of electrolysis of brine in a mercury cathode cell are chlorine,hydrogen,and sodium hydroxide.How many moles of these products are formed when one mole of electrons is passed through t ...

look at the equations

NaCl --> Na+ + Cl-
H2O --> H+ + OH-

anode: 2Cl- --> Cl2 + 2e-

cathode: 2H+ + 2e- --> H2

For every 2 moles of electrons transfered, one mole of chlorine gas, one mole of hydrogen gas, and two mole of sodium hydroxide are formed.
Therefore, when one mole of electrons is pass through, 0.5mole of H2, 0.5mole of Cl2, and 1 mole of NaCl are formed.

加分!!

lowch1

Q76

A neutral compound Z, has the empirical formula, Co(Cl)x(NH3)y. The percentage conposition by mass in the complex is as follows.
Co, 23.52%
N, 28.00%
Cl, 42.53%
H, 5.95%
When an aqueous solution containing 12.52g of Z is reacted with excess AgNO3 solution,14.35g of AgCl is precipitated. Deduce the formula of the complex Z. What is the oxidation state if cobalt in the complex?

紫色的黑蝴蝶

原帖由 lowch1 于 10-11-2006 19:53 发表
Q76

A neutral compound Z, has the empirical formula, Co(Cl)x(NH3)y. The percentage conposition by mass in the complex is as follows.
Co, 23.52%
N, 28.00%
Cl, 42.53%
H, 5.95%
When an aqueous ...

A neutral compound Z, has the empirical formula, Co(Cl)x(NH3)y. The percentage conposition by mass in the complex is as follows.
Co, 23.52%
N, 28.00%
Cl, 42.53%
H, 5.95%
When an aqueous solution containing 12.52g of Z is reacted with excess AgNO3 solution,14.35g of AgCl is precipitated. Deduce the formula of the complex Z. What is the oxidation state if cobalt in the complex?


form 4 chapter ,by using the percentages to find out the empirical formulae of the complex ions,so it would be Co:1 N;5 Cl:3 H:15

Empirical Formulae: Co(Cl)3(NH3)5

but,do we have such complex ion?No we dont have in our syllabus so only Cl- ions do not bonded to the complex as ligands would be precipatet as

AgCl---->Ag+ +Cl-

Hence,no of moles of AgCl= 143.5
                           ------    =0.10 mol
                           108+35.5
         
No of moles of Z= 12.52
                  ----- =0.05mol
                  250.4

No of moles of free Cl- in Z= 0.10 mol,hence 1 moles of Z has 0.10
                                                              ---- = 2 mole
                                                              0.05
Hence only 2 mole of free Cl-

Formulae of Z, [CoCl(NH3)5]Cl2

X+(-3)+0=0
Oxidation number of Cobalt,=+3

Hehe,...

紫色的黑蝴蝶

1.Which of the following substances would NOT decolourise bromine water?

A.F2    B.C6H5OH      C.C6H5NH2        D.FeSO4.

2.What is the nature of an aqueous solution containing [Fe(H2O)6]Cl3?

A.Reducing Agent and weak acid
B.Reducing agent and weak base
C.Oxidizing agent and weak acid
D.Oxidizing agent and weak base

Please come across with explanations!!Thanks...Muacks!!

JamesTea

原帖由 紫色的黑蝴蝶 于 10-11-2006 08:59 PM 发表
1.Which of the following substances would NOT decolourise bromine water?

A.F2    B.C6H5OH      C.C6H5NH2        D.FeSO4.

2.What is the nature of an aqueous solution containing Cl3?

A.Reduc ...

1) answer is A . F is the most electronegative element ,therefore the bond between F2 molecule are held strongly . C6H5OH is ortho and para directing compound and it will reacts with bromine water to form 2,4,6-tribromophenol which is white precipitate, therefore decolourise bromine water. For C6H5NH2 ,same as C6H5OH ,it is a orthoi and para directing compound which will reacts with bromine water to form white preecipitate of 2,4,6-tribromophenylamine . For FeSO4 , when mix with bromine water , it will form complex ion [Fe(H2O)6]2+ SO4^2- ,Bromine can acts as ligands which can displace water molecule to form complexes [Fe(H2O)5(Br)]+ . therefore it will too decolourises bromine water.


2) answer is C. oxidising agent and weak acids . [Fe(H2O)6]Cl3 inm water will dissociate to form ions , i.e. [Fe(H2O)6]3+ complex ion and Cl- . High charge density of Fe will attract the water molecule so the water molecule H - OH will be dissociated into H+ and OH- , where the complex will react with OH- to form [Fe(H2O)6(OH)]2+ , that is the oxidation number of Fe in these complexes reduce from +3 to +2 whereas H atom in water molecule is oxidised  from oxidation number 0 to +1 , that is H+ ion which gives the acidic characteristic to the solution .

[ 本帖最后由 JamesTea 于 10-11-2006 11:59 PM 编辑 ]

JamesTea

Q79 :

Ammonia is a very important ingrediant in the manufacture of fertilizers. Its synthesis from nitorgen may be represented by the following equations.

N2 (g)   +  3H2(g)   <----->   2NH3(g)

(i)  why is a high pressure used in this process ?
(ii) name the catalyst used in the process and explain its action .

紫色的黑蝴蝶

原帖由 JamesTea 于 10-11-2006 23:54 发表







1) answer is A . F is the most electronegative element ,therefore the bond between F2 molecule are held strongly . C6H5OH is ortho and para directing compound and it will reacts with  ...

but i tot fluorin is more electronegativity than cbromine so it can decolourise Bromine?...Any furthur explanation?

感激分享!!

紫色的黑蝴蝶

原帖由 JamesTea 于 10-11-2006 23:54 发表







1) answer is A . F is the most electronegative element ,therefore the bond between F2 molecule are held strongly . C6H5OH is ortho and para directing compound and it will reacts with  ...

请贴上下一题吧，不然何来马拉松
谢谢

JamesTea

原帖由 紫色的黑蝴蝶 于 11-11-2006 12:08 AM 发表



but i tot fluorin is more electronegativity than cbromine so it can decolourise Bromine?...Any furthur explanation?

感激分享!!

because of  the most elecronegative of F atom , its diatomic molecule F-F is held very tightly , it can also be expained as follows, size of F atom is very small , therefore when forming covelant diatomic molecules, the bond strength is very high , distance betwwen shared electrons and the nucleus is very short , therefore it is not so easy to dissociate or break the bond between F -F molecules

紫色的黑蝴蝶

原帖由 JamesTea 于 11-11-2006 00:04 发表
Q79 :

Ammonia is a very important ingrediant in the manufacture of fertilizers. Its synthesis from nitorgen may be represented by the following equations.

N2 (g)   +  3H2(g)   <----->   ...

Ammonia is a very important ingrediant in the manufacture of fertilizers. Its synthesis from nitorgen may be represented by the following equations.

N2 (g)   +  3H2(g)   <----->   2NH3(g)

(i)  why is a high pressure used in this process ?
(ii) name the catalyst used in the process and explain its action .

i) i think this is an exothermic reaction.High pressure will tend to reduce the volume inside the container according to Le Chartelier's Rule.So it will reduce the number of moles from 4 to 2 from the equation,the reaction now is being forwarded so will have more yield b produced in high pressure instead of low pressure.

ii)Iron.
   this isa heterogenous catalyst.Ferum provides  good adsorption surface which able to adsorb the reactants by chenisorption.N2 and H2 form temporary bonds with Fe and the N2 n H2 bond are weaken.On the surface of Ferum,H atom n N atom collide to each other to form new N-H bonds,it decreases activation energy and increase the rate of reaction(Arhenius equation). Finally the NH3 molecules leave the Ferum surface by desorption.


有何错误请纠正,谢谢!!

JamesTea

原帖由 紫色的黑蝴蝶 于 11-11-2006 12:24 AM 发表



Ammonia is a very important ingrediant in the manufacture of fertilizers. Its synthesis from nitorgen may be represented by the following equations.

N2 (g)   +  3H2(g)   <----->   2N ...

overall is acceptable but in answer (i) you should include the term "equilibrium" . So when pressure is exerted into the system , the number of moles of the reactants is higher than that of the products , according to Le Chatelier's principle, the equilibrium tends to shift to the right , that is more  ammonia is form so that the pressure of the system is restore/reduced into equilibrium OR the system is now favour the product which more yeild of ammonia gas is formed .



in (ii) during explain the functions of a catalyst. One of the very important points is that catalyst provide alternative route of the reaction where it minimise the activation energy of the reaction and then continue with your explanation of the used of heterogeneous catalytic .Also the process of the adsorption of N2 and H2 into Fe is due to  the presence of empty 3d orbitals which provide temporary formation of bond between nitrogen and hydrogen .

[ 本帖最后由 JamesTea 于 11-11-2006 12:36 AM 编辑 ]

紫色的黑蝴蝶

原帖由 JamesTea 于 11-11-2006 00:33 发表







overall is acceptable but in answer (i) you should include the term "equilibrium" . So when pressure is exerted into the system , the number of moles of the reactants is hig ...

感激分享..Muacks!!

紫色的黑蝴蝶

Which of the following can b used to distinguish between phenylmethanol and 4-methlphenol?

A Bromine water
B Iron(II) Chloride
C Ethanoyl chloride
D Dilute hydrochloric Acid

JamesTea

原帖由 紫色的黑蝴蝶 于 11-11-2006 12:39 AM 发表
Which of the following can b used to distinguish between phenylmethanol and 4-methlphenol?

A Bromine water
B Iron(II) Chloride
C Ethanoyl chloride
D Dilute hydrochloric Acid

这题的真正解释我不太清楚，我的看法如下

Answer is A . When phenylmetanol reacts with bromine water, it will decolourises the brown colour of bromine water. For 4-methylphenol , although it will decolourises also the brown colour of bromine water, but the ortho and para directing compound of phenol reacts with bromine water to form 2,6-dibromo-4-methylphenol with is a white precipitate , thus it can be used to distinguish betweeh phenylmethanol and 4-methylphenol.

紫色的黑蝴蝶

原帖由 JamesTea 于 11-11-2006 00:48 发表





这题的真正解释我不太清楚，我的看法如下

Answer is A . When phenylmetanol reacts with bromine water, it will decolourises the brown colour of bromine water. For 4-methylphenol , althoug ...

请贴上下一题吧，不然何来马拉松?