回應(會考化學)

回應

Maryuen:

你好,化學友..我是maryuen我又找到新的問題想請教你我想問chem 2006的past paper的mc

4(可以explain一下嗎?)

9(why it is low melting point?容易melt好嗎??那high elecrical conductivity不是更重要嗎?)

16(可以explain一下嗎?)

48(那個2nd statement的dilute acids not include dilute nitric acid??)

唔..我還想知道那個NH4+會不會是一個oxidizing agent if not in zinc-carbon cell??

對不起..我還想知道在什麼時候,一些reaction,heat would be liberated..即什麼時候是exothermic, 什麼時候是endothermic??

化學友:
4.
Which of the following combinations concerning the change of physical state of a substance is INCORRECT?

Change of physical state Process
A. liquid to gas evaporation
B. liquid to solid precipitation
C. solid to gas sublimation
D. gas to liquid condensation

B錯呀，因為一隻物質由liquid變成solid係叫做freezing。precipitation係形容chemical reaction當中有d唔溶嘅solid形成左,e.g. Fe2+(aq) + 2OH-(aq) --> Fe(OH)2(s)

9.
Which of the following properties is considered the most important one when choosing an alloy for making fuse in electric plugs?

A. low melting point
B. high electrical conductivity
C. good ductility
D. high mechanical strength

答案係B。fuse嘅作用係當electric plugs嘅current太大時斷左o個circuit，減少意外。一般o黎講current大會令到溫度高左，所以如果fuse嘅melting point低，fuse就會溶，咁樣circuit就會斷左。

16
Which of the following compounds is formed from the reaction of propene with chlorine



好Q呀，上個禮拜化學友有d學生都問左e條。maryuen，做e條最緊要將條reaction寫出o黎，睇下面:



答案係B。o拿，如果你睇返個reaction，A同埋D都錯，因為2個Cl唔會o向同一個carbon上面。而C都錩，因為2個Cl應該o向1,2位，而唔係o向1,3位。

48
1st statement: Carbon dioxide can effectively be prepared by the action of dilute sulphuric acid on calcium carbonate.

2nd statment: Carbonates react with dilute acids to give carbon dioxide.

答案係C。2nd statemento岩o架。又咁Q，化學友o個幾個學生又有問e條。maryuen，我唸你記錯左lu, metal + dil. HNO3先會出NO，carbonate + very dil. HNO3、dil.HNO3或者conc. HNO3都係出CO2 o架。

至於NH4+裡面個N好少oxidize其他o野，因為佢個O.N.已經-3(好低)，O.N.好難再decrease。至於H就可以oxidize其他o野，O.N.會由+1 decrease to 0(H+ --> H2)。不過呢化學友覺得CE chem好少會咁問。

最後，邊d reactions係exothermic邊d係endothermic，好難generalize wor。 不過，o向CE chem level大至可以將d reactions咁分類:

個表差唔多包晒會考常考嘅reactions, 你睇下幫o吾幫到你丫。