Chemistry Questions

用戶147780

2011-01-02 2:54 am

Account for each of following facts.[8M]

(a)Sodium chloride has a high melting point.

(b)Carbon dioxide and silicon(Ⅳ) oxide are both oxides of GroupⅣelement.
They are both covalent compounds.Yet carbon dioxide is a gas at room conditions,while silicon(Ⅳ) oxide is a high-melting solid.

(c)Nitrogen has a very low melting point(-210℃) and boiling point(-196℃).
This means very little energy is required to change nitrogen from solid to liquid and then to gas,However,even at high temperature(e.g.3000℃), only a small proportion of nitrogen molecules are decomposed into atoms.

I don't know how to answer it,help me please!
Thanks a lot!

回答 (1)

知識就是力量

2011-01-02 5:16 am

✔ 最佳答案

(a) NaCl has a giant ionic structure, which the ions are held by strong ionic bonds. A large amount of heat is required to separate them. So the melting point is high.

(b) So we can conclude at the normal atmospheric pressure, carbon dioxide has a lower boiling point than silicon(IV) oxide. This is because carbon dioxide is just held by weak van der Waals' forces. However, silicon(IV) oxide has a giant covalent structure, which is held by strong covalent bonds. So just less heat is required to separate carbon dioxide molecules while a large amount of heat energy is required to separate silicon(IV) oxide.

(c) Nitrogen is held by weak van der Waals' forces. So very little energy is required to separate the discrete molecules between them. So melting point and boiling point is low. However, to separate the nitrogen atoms inside the nitrogen molecules, that means we need to separate their strong triple bonds between them, a very large energy is required to separate. This explains why even at very high temperature, only a small proportion of nitrogen molecules are decomposed into atoms.

參考: Knowledge is power.

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