Molecular Polarity 試題分解

1.          DSE/13/1B/2

Both BF₃ and NH₃ exist as simple molecules.

(a)    For each of these molecules, draw its three-dimensional structure.

BF₃

NH­₃

(2 marks)

(b)    For each of these molecules, explain whether or not it is polar.

(2 marks)

(c)    BF₃ reacts with NH₃ to give F₃BNH₃. Describe the bond formation between BF₃ and NH₃.

(2 marks)

(a)要畫分子結構，要考慮的都是該分子中有多少Bond pairs & lone pairs︰

	Bond pairs	Lone pairs
BF3	3	0
NH3	3	1

所以BF₃和NH₃分子的形狀分別是trigonal planar & trigonal pyramidal，每個結構一分︰

  

(b)問的正是上次講解的概念，先留意Marking Scheme的答案︰

BF₃ is a non-polar molecule. The three polar B-F bonds are svmmetricallv arranged on the same plane / dipole moments cancel out / net dipole moment is 0. (1)

NH₃ is a polar molecule. The molecule has a lone pair in its outermost shell and thus the three polar N-H bonds are not on the same plane / dipole moments cannot cancel out / net dipole moment is not 0. (1)

(Only award marks to the explanations.)

本人建議使用後者(dipole moment)的概念，Dipole moment能清楚解釋分子是否具polarity。例如在BF₃中因為3個B-F的dipole moment互相抵消，所以Net dipole moment of BF₃ molecule等於0，所以BF₃是non-polar︰

至於解釋兩個分子Polarity時第一個解釋都是以bonds on the same / different plane來判斷該分子是否polar。BF₃的解釋尚可理解，但NH₃的答法則可圈可點︰

1)

不是所有分子其polar bonds於同一個平面就等於它是Non-polar molecule。以H₂O為例，這個V-shape的分子雖然polar bond (O-H)都在同一個平面，但由於它不是對稱地排列，所以在下圖得知，左右方向的dipole可被抵消而上下方向的卻不能被抵消，所以H₂O是polar molecule︰

其他具V-shape的分子如SO₂或H₂S一樣有類似結果，所以在BF₃的情況中Marking scheme才強調” The three polar B-F bonds are svmmetricallv arranged on the same plane”，以免混淆視聽。

2)

有些分子即使其polar bonds於不同平面一樣可以是non-polar。例如CH₄，它擁有tetrahedral shape，故4個polar bonds分佈於兩個面。但由於C-H bond對稱地排列在一起，所以4個dipole都會互相抵消。可能同學覺得對稱較難理解，簡單一點說，只要中心原子沒有lone pair，該分子的net dipole就必定是0。因此Marking scheme在NH₃的情況中以分子中的polar bonds不在同一平面的說法不成立。當年以此方法解釋而獲分的同學純屬僥倖，其他考生最好用dipole moment來解釋。

(c)就要考生解釋dative covalent bond如何構成。這就要先講哪邊的原子需透過額外的電子而成為octet structure，接著解釋電子從何而來︰

In BF₃, there are three (bond) electron pairs / there is a vacant site / 6 electrons only / electron deficient in the outermost shell of the B atom. (1)

By accepting the lone pair of electrons from the nitrogen atom of NH₃ / forming dative bond with N, boron attains the stable electronic configuration of neon (a noble gas) (1) / diagram below

(Showing the lone pair is NOT a must)

DSE/SP/1B/6

A negatively charged rod was brought near a jet of water running out from a burette. The jet of water was deflected as shown:

(a)    With reference to the structure of water, explain why the jet of water was deflected.

(b)    State the effect on the jet of water if the negatively charged rod is replaced by a positively charged rod. Explain your answer.

(c)    If hexane is used instead of water and a negatively charged rod is brought near the liquid jet, would the liquid jet be deflected? Explain your answer.

(6 marks)

(a)的問題由結果開始推斷其實會更容易連到結構和分子極性的關係，水受帶負電荷的物件吸引就是因為它是polar molecule，而它是polar molecule是因為它的分子形狀是不對稱的V-shape。只不過題目想考生以結構來解釋，那就先寫水分子的結構，再寫它為何是polar molecule︰

The structure of water is non-linear / non-symmetrical (or in drawing) (1)

The polarity of the two O-H bonds cannot cancel each other / water is a polar molecule, thus water would be attracted by the electric field. (1)

(b)題中，水分子只要受到electric field影響就會被吸引過去，只不過分子受力的位置不同而已。物件帶負極時，帶partial positive charge的H atom就會向著該物件被吸引過去︰

物件帶正極時，帶partial negative charge的O atom就會向著該物件被吸引過去︰

由於在兩種情況中水分子受的力是相等，所以改變正負電荷不會影響結果。事實上，只要有electric field，分子都會旋轉令正的一邊對著電場的負方、負的一邊對著電場的正方。

The water jet will be attracted towards the rod. (1)

Water molecules will orientate themselves in alignment with the electric field so that they will be attracted. (Accept equivalent answers.) (1)

這題沒有絕對的答案，只要解釋正確。雖然話hexane是non-polar molecule，但hexane flow是否能受電場偏移亦受電場強弱影響。只要電場夠強的話甚麼分子都可以被吸過去︰

The liquid jet is attracted by the electric field. In the presence of an electric field, a net polarity will be induced in the hexane molecule. (2)

Or,

The jet is not attracted. Only a weak polarity is induced in hexane molecules. The attraction between the induced dipole and the electric field is not strong enough to cause a deflection of the liquid jet. (2)

至於DSE有關此課題的MC不算難，不在此文解釋。

最後到上回問的三條問題。由於太多英文Terms的原因，直接用英文講解︰

1.          BF₃

Mentioned above, it is a non-polar molecule owing to its symmetrical structure (trigonal planar shape).

2.          CH₄

4 bond pairs & NO lone pair, so CH₄ molecule has a tetrahedral shape:

Again, this molecule is non-polar due to its symmetrical shape that leads to cancelling out of molecular polarity / dipole moment.

3.          NH₃

Mentioned above, it is a polar molecule owing to its non-symmetrical structure (trigonal pyramidal shape).