Structure & Bonding_試題分析 2

DSE/Sample Paper/1B/7

Complete the table below by

(a) drawing a three-dimensional diagram for the structure of each solid substance (3 marks), and

(b) giving an explanation of whether the solid substance is an electrical conductor. (3 marks)

Solid substance	Three-dimensional diagram for the structure of the solid substance	Explanation of whether the solid substance is an electrical conductor
Diamond
Graphite
Caesium chloride

考評局的DSE Sample paper問題較簡單直接，但介紹這題的目的想一次提醒大家要識畫鑽石、石墨及二氧化矽的巨型共價結構之餘，亦希望大家留意DSE只會考的兩種ionic compounds︰Sodium chloride & Cesium chloride(氯化銫)的巨型離子結構不一樣。

(考試唔會出，純粹滿足大家求知慾)

之前介紹CsCl的結構較簡單，如果該結構繼續伸延就會得出以下的三維圖︰

 (Source: Wikipedia)

由此得知每粒銫離子和8粒氯離子組鍵、而每粒氯離子亦和另外8粒銫離子組鍵，反而在NaCl中，每粒鈉離子和6粒氯離子組鍵，反之亦然︰

每粒離子組的鍵越多、化合物就越穩定，所以CsCl的組合方式會較穩定。但為甚麼NaCl又不砌出同樣結構。

根據Group I，銫在鈉之下，電子層較少，所以銫離子亦比鈉離子大。而Cs⁺和Cl^-的大小差不多，所以當銫被8粒氯包圍時氯離子之間會有足夠的空間，不會形成很強的排斥力︰

但由於鈉離子較細，如果組成類似結構時氯離子之間變得更緊密，排斥力增加令結構反而更不穩定。所以如果鈉只被6粒氯包圍，氯離子之間的排斥力較低，結構會比較穩定。)

剩下的結構和解釋在前文已載，不再詳述。(Graphite的圖案來自前文製作版本，考生屆時需以圓圈列明碳原子位置，而且畫兩層即可)

Solid substance	Three-dimensional diagram for the structure of the solid substance	Explanation of whether the solid substance is an electrical conductor
Diamond		Diamond has neither delocalized electrons nor mobile ions, thus it is an insulator.
Graphite		Graphite contains delocalized electrons moving throughout each carbon layer, so it is an electrical conductor.
Cesium chloride		Cesium and chloride ions are immobile in solid state, so it is an electrical insulator.

DSE/14/1B/1

Graphite is a form of carbon and has a layer structure. Graphite is an individual single layer of graphite. Their structures are shown below:

(a) Thin sheets of graphene can be easily peeled off from graphite using adhesive tape.

(i) Explain why graphene can be easily peeled off. (1 mark)

和用鉛筆寫字時石墨脫落的情況十分相似︰

Carbon layers in graphite are held only by weak van der Waals force that can be easily peeled off. (註︰膠紙的黏力也是范德華力)。

(ii) Explain whether graphene can conduct electricity. (1 mark)

Graphene possesses delocalized electrons. (講左N次，無野再補充)

(iii) Draw the electron diagram for a molecule of the compound formed by complete combustion of graphene, showing electrons in the outermost shells only. (1 mark)

中四中五的同學未必認識Complete combustion精確的意思，但既然Graphene只有Carbon，而Combustion涉及氧氣，同學可推斷Complete combustion of graphene會產生二氧化碳︰

(b) Based on the fact that graphene can be easily peeled off from graphite, a student concluded that graphite should have a low melting point due to its layer structure, Explain whether you agree with this conclusion. (1 mark)

雖然Carbon layers之間的范德華力很弱，但要令石墨溶化(準確來點應是氣化)就應該連碳原子之間的共價鍵都拆埋，但強硬的共價鍵會令石墨的「溶點」很高︰

Not agree. Graphene layer has of a giant covalent structure.  / A large amount of energy is needed during melting to destroy the large amount of strong covalent bonds between atoms. 
(個人認為一係答第二個point一係兩個point都寫埋出黎，淨係講石墨係giant covalent structure其實解釋唔到問題核心)

(c) Fullerene (such as C₆₀) is another form of carbon. Briefly describe the structure of C₆₀, and suggest why it is soluble in some organic solvents. (3 marks)

如果要形容C₆₀的結構可以很複雜，單是碳原子的排列方法和之間的共價鍵已經有排講。但既然要以其結構解釋它能溶於有機溶劑之中，就可以倒轉諗，先想甚麼結構的物質才溶於有機溶劑，知道是Simple molecular structure之後就再想為甚麼C₆₀是簡單分子，這才清楚明白要講出C₆₀是球體形(而不是無限伸延的layer structure)︰

C₆₀ has a spherical shape.

C₆₀ has a simple molecular structure.

The weak van der Waals force between C₆₀ molecules are compatible to the molecules in organic solvent.