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Number of hydrogen bonds in H3PO4

em01 [原帖由 kctsui75 於 2010-7-26 10:44 AM 發表  

Although there are only 3 hydrogens ,don't forget the O atom in the P=O
bond can form another hydrogen bond with the H atom in another H3PO4
molecule. So, your statement：[At most 3 hydrogen bonds per molecule.]
IS INCORRECT！]

桃子的回覆：
[ We count hydrogen bond between =O from molecule A and -OH from molecules B once only.

Let's considered a simpler case.
HF...HF...HF...HF...HF...
Here, we consider there to be one hydrogen bond per molecule even HF forms hydrogen bond on both sides.]

首先感謝桃子的指教及提醒！原來我忽略了 “per molecule”兩個字。在最後計算
H bond 的數目時，我們應取其平均值 (average)！現在我才明白“The total number of H bonds of a water molecule” 為什麽有人答：“The total number of H bonds of a water molecule is 4”
[ Water is unique because its oxygen atom has two lone pairs and two hydrogen atoms, meaning that the total number of H bonds of a water molecule is four]
[url]http://www.edinformatics.com/int[/url] ... /hydrogen_bonds.htm

[ because the oxygen of one water molecule has two lone pairs of electrons, each of which can form a hydrogen bond with a hydrogen on another water molecule. This can repeat such that every water molecule is H-bonded with up to four other molecules, as shown in the figure (two through its two lone pairs, and two through its two hydrogen atoms).]
[url]http://en.wikipedia.org/wiki/Hydrogen_bond[/url]

但同時亦有人答：“The total number of H bonds of a water molecule is 2” 的道理了。
The differences may be due to the use of a different method for defining and counting the hydrogen bonds.

為了避免小弟同樣的疏忽或疑惑，我引用以下的例子與各位師兄/師弟分享：

[Example 1.] H2O

The O atom can form 2 hydrogen bonds because it has 2 lone pairs of electrons.

Each H atom can form 1 hydrogen bond, and the 2 H atoms can form 2 hydrogen bonds.

Since 1 hydrogen bond links two molecules together, thus the average maximum number of hydrogen bonds is 2 (= (2 + 2)/2).

[Example 2.] NH3

The N atom can form 1 hydrogen bond because it has 1 lone pair of electrons.

Each H atom can form 1 hydrogen bond, and the 3 H atoms can form 3 hydrogen bonds.

Mole ratio N : H = 1 : 3, and thus the H atoms are in excess to form hydrogen bonds.  Averagely, only 1 H atom in each NH3 molecule can form hydrogen bond.
Since 1 hydrogen bond links two molecules together, thus the average maximum number of hydrogen bonds is 1 (= (1 + 1)/2).

[Example 3.] HF

The F atom can form 3 hydrogen bonds because it has 3 lone pairs of electrons.

The H atom can form 1 hydrogen bond .

Mole ratio H : F = 1 : 1, and the lone pairs on F atoms are in excess to form hydrogen bonds.  Averagely, the F atom only form 1 hydrogen bond.

Since 1 hydrogen bond links two molecules together, thus the average maximum number of hydrogen bonds is 1 (= (1 + 1)/2).

但小弟亦想請教大家：目前我們所討論的 H3PO4，[ What is the total possible number of H- bonds that it can form？] and [ What is the number of H- bonds per molecule？ & would you mind showing us the calculation in details?]

附圖

[url=http://en.wikipedia.org/wiki/File:3D_model_hydrogen_bonds_in_water.jpg][img=250,248]http://upload.wikimedia.org/wikipedia/commons/thumb/f/f9/3D_model_hydrogen_bonds_in_water.jpg/250px-3D_model_hydrogen_bonds_in_water.jpg[/img][/url]