Chem

emmacherry

Diamond is a crystalline form of carbon.
我想問下咩叫crystalline form.

emmacherry

原帖由 Zend 於 20-4-2008 16:08 發表
amorphous and crystalline
crystalline = crystal般的
a regular lattice in which particles are packed orderly and in repetitive manner...

thanks
I want to ask also
when water is added to copper(II) sulphate,
copper (II) sulphate 係唔係變左copper(II) sulphate crystal?

emmacherry

原帖由 Zend 於 20-4-2008 17:08 發表

well, it should be CuSO4(aq)...[[tk_32]]

when it is crystallized , water of crystallization of that salt is formed .
CuSO4.5H2O
when it is heated =>it finally becomes anhydrous copper(II) sulp ...

i have juz done a mc question
which states that heat would be liberated if water is added to copper(II) sulphate..
i dunno y

emmacherry

原帖由 Zend 於 20-4-2008 18:21 發表

That should be a question of  "exothermic"/"endothermic" reaction ?

This is the question.
On heating, blue copper(II) sulphate crystals gradually change to a white powder. Which of the following statements are correct?
(1) Heat would be liberated if water is added to the white powder.
(2) On further heating, the white powder would turn reddish-brown.
(3) A chemical chagne occurs during the heating of the blue crystals.

It is obvious that (2) is wrong and (3) is correct
but i juz dun understand y (1) is correct too.
i think that the white solid left after heating is copper(II) sulphate.
and when water is added, the white solid will dissolved.
i think heat will not be evolved during the dissociation of copper(II) sulphate.

[ 本帖最後由 emmacherry 於 20-4-2008 18:29 編輯 ]

emmacherry

原帖由 Zend 於 20-4-2008 18:51 發表
Umm. first, simply speaking, according to conservation of energy, when u added water to anhydrous CuSO4, heat should be released.
and. (because u does supply energy to decompose it )


there is a term, although it is out-c, is called standard enthalpy change of solution

http://en.wikipedia.org/wiki/Enthalpy_change_of_solution
http://www.chemicalforums.com/index.php?topic=20358.0
from the site above, it shows that even it is dissolved to form a solution, the overall reaction is still exothermic and drawing a Born Harber Cycle...

sry....
i dun understand the part that is bolded

emmacherry

原帖由 Zend 於 20-4-2008 19:22 發表

你可以當成CuSO4.5H2O 有某個chemical energy
CuSO4 有另一個

CuSO4.5H2O->CuSO4 唔會自己發生, 要energy 去做decomposition
所以CuSO4 同佢product 的total 能量係多左

當你加番水, 做番form water of cr ...

但係加左水
CuSO4會form water of crystallization 咩?
佢唔係會dissovle咩??

emmacherry

原帖由 Zend 於 20-4-2008 19:49 發表
Compound: CuSO4(s)
dH0solution/kj/mol: -66.5

Compound: CuSO4.5H2O(s)
dH0solution/kj/mol: +11.7"

如果1 mol 的CuSO4 溶係infinitely amount of water(唔好問點解, 定義)
佢會release 66.5KJ mol^-1 的 ...

ok thanks
但係我最後都想問
到底我地加水落copper(II) sulphate
佢係會溶定form water of crystallization ??
因為我睇書
要form water of crystallization 係要經過一個crystallization o既程序
唔係直接dissolve 就有的

emmacherry

原帖由 Zend 於 20-4-2008 20:01 發表

第一佢唔會考
第二, 放得熱主要有四類

neutralization
precipitation
combustion (usually 係, 包括用oxidizing agent 好似O2, Cl2,F2 )
Redox, 可行果d 一般直接mix 都會放熱, cell 只係將佢(chemical ene ...

仲有係
CuSO4+ 5H2O-----> CuSO4.5H2O
同埋
CaO + H2O----->Ca(OH)2

emmacherry

原帖由 Zend 於 20-4-2008 20:08 發表

係.
但CaCl2 最特別係佢吸好多水, 連自己都溶埋- - ~

唔該晒你
真係明左好多

我想問除左conc. H2SO4 同CaCl2
仲有乜野chemical可以用黎吸水

emmacherry

原帖由 Zend 於 20-4-2008 20:12 發表

P2O5, Na2SO4, MgSO4,silicon gel, conc NaOH等

係A-level (以前=.=) 係會細分佢地的用途
CaCl2 唔會用係organic substance 的drying 方面

想問你對chem 攞a 有咩心得
paper 1 &2 要幾多分到