回應(會考化學)

回應1

pele159:
1. 請問寫essay 洗唔洗寫 equation ??

2. Preparation of sulphuric acid係咪 Contact process ??

3. 如果想整 Conc. Nitric acid 可以點整呀 ?

4. 請問係咪有個新方法整 soap ( acid hydrolysis of fat or oil ) ?

Fat/oil + water <=> fatty acids + glycerol

5. 整soapless detergent 係咪要 conc.H2SO4 & NaOH 呀 ??

6. 你上次答NaOH同hydrolysis裡面形成嘅carboxylic acid neutralize生成salt of carboxylic acid(即係soap) 咁我英文可以點答呀 ??

7. 請問點解 soap 係 alkaline ( 佢又冇OH-ion ) ?

化學友:
1. 寫essay 係唔洗寫 equation，因為essay係考communication skill (傳意技能)，以文字表達chemical knowledge。除非係題目要求，平日係唔使寫equation嘅，仲要記住唔好以point form寫出，真係會扣分o架。

2. 係呀，整sulphuric acid嘅industrial process係指Contact process(接觸法)。

3. 整conc. nitric acid嘅industrial process已經out c喇，不過都俾你參考吓啦!

首先係catalytic oxidation of ammonia (氨嘅催化氧化)，用Cu or Pt做catalyst。

4NH₃(g) + 5O₂(g) -->4NO(g) + 6H₂O(g)

跟住NO再同空氣中嘅O2react生成NO₂

2NO(g) + O₂(g) --> 2NO₂(g)

最後dissolve NO₂落水就製成HNO₃

4NO₂(g) + O₂(g) + 2H₂O(l) --> 4HNO₃(aq)

4. 唔係呀pele159，fatty acid係唔同soap，fatty acid係 carboxylic acid，佢唔溶於水。而soap係ionic compound，佢溶於水先至有清潔效能。

化學友估你可能同ester 嘅acidic or alkaline hydrolysis撈亂咗，無錯fat/oil係ester，佢哋係可以carry out acidic or alkaline hydrolysis。但係當fat/oil carry out acidic hydrolysis，就只會出咗隻唔溶於水嘅fatty acid，呢隻compound係唔會有cleaning power。如果fat/oil carry out alkaline hydrolysis，就會出soap。

5.係呀，petroleum fraction + conc. H₂SO₄，然後加入NaOH，就會做成soapless detergent。

6. pele159，睇吓咁樣寫O唔OK: NaOH neutralize the carboxylic acid formed to give sodium salt of carboxylic acid which is the soap.

7. 因為當soap溶咗落水，佢會同水有化學化應生忒OH-。雖然條equation out c，但係都寫比你睇下啦: RCOO^- (soap) + H₂O <=> RCOOH + OH-

回應2

Maryuen:
1. 化學友，我想知道怎麼看CH3CO2H->CH3CH2OH的oxidation no.?? oxidation no.是什麼??

2. 1998 5: how to use the test tube given? wt is the answer in the marking scheme?

3. 7aiii3: why ethanol can dissolve unpolymerized monomer (vinyl chloride)?(joint us answer)

4. 7bii3: PVC is not suitable for making electric socket since the current pass through it will produce heat and melt it??

5. 9aiii: how to write the equation with [O] for the reaction between propan-1-ol and acidified potassium dichromate solution?

6. 9avi1: State TWO observable changes when the contents of the test tube (ester) were added to the sodium carbonate solution.

can i write a thin colourless layer form on top of the sodium carbonate solution??

化學友:
1. 要計到e d oxidation no. 需要d F.6 嘅concept。 化學友記得e條係pastpaper mc，其實你可以simple d 咁睇: CH3CH2OH --> CH3COOH (ethanol --> ethanoic acid)係oxidation，所以CH3COOH --> CH3CH2OH就係相反嘅reduction。

2. d test tubes 係用o黎裝d chemicals嘅(例如2M hydrochloric acid and 2M nitric acid) 。

3. 因為vinyl chloride同埋ethanol都係organic compounds，化學友唸你老師應該教過'like dissolves like'嘅concept，所以ethanol可以dissolve vinyl chloride。

4. 正確d o黎講係防止火警時高溫會 melt 咗d PVC，咁copper wire外露會非常dangerous，引至explosion或漏電。

5. maryeun，你係唔係想問e絛equation: CH3CH2CH2OH + [O] à CH3CH2COOH ?

6. 係可以o架，因為ester係insoluble in water，而且浮o向水面。