熔點的比較 (20分!)

請幫忙比較下列各組物質 何者擁有較高的熔點 並說明原因 (可用路易士結構來說明原因)

原文:suggest, giving reasons, which substance in each pair is likely to have the higher melting point (Lewis structure may help your arguments)

a) HCl or NaCl

b) C2H5OC2H5 or C4H9OH

c) HF or HCl

d) H2O or CH3OH

e) KBr or CH3Br

2 Answers

Rating
  • Tiger
    Lv 5
    1 decade ago
    Favorite Answer

    I prefer showing you how to solve the problem. Don't worry, it won't be too hard.

    Briefly, ionic compounds tend to have high melting points. All you need to do is draw the Lewis structure and say ok, A has extremely higher melting temperature because "Lewis structure shows why the compound A has to be ionic".

    There are some nice and easy-understandable Lewis structure tutorials:

    <http://www.miramar.sdccd.cc.ca.us/faculty/fgarces/...

    <http://www.uwosh.edu/faculty_staff/gutow/Lewis_Tut...

    <http://chemed.chem.purdue.edu/genchem/topicreview/...

    a) HCl or NaCl

    Hydrochloric acid (HCl) <http://en.wikipedia.org/wiki/Hydrochloric_acid>

    ==> −26 °C (247 K)

    Sodium chloride(NaCl) <http://en.wikipedia.org/wiki/Sodium_chloride>

    ==> 801 °C (1074 K)

    The Lewis Structure for the Salt NaCl, shows two ions which have their (Now) outer shells of electrons filled with a complete octet. In the case of the sodium cation, the filled shell is the outermost of the 'core' electron shells. In the Chloride ion, the outer shell of valence electrons is complete with 8 electrons. The two ions are bonded with ionic bonds. <http://www.chem.queensu.ca/people/faculty/mombourq...

    b) C2H5OC2H5 or

    C4H9OH (butanol) <http://www.newton.dep.anl.gov/askasci/chem03/chem0...

    n-butanol, iso-butanol, and tert-butanol ==> -89C, -115C, and +25C respectively.

    C2H5OC2H5 (Diethyl ether) <http://en.wikipedia.org/wiki/Diethyl_ether>

    ==>−116.3 °C (156.85 K)

    c) HF or HCl

    HF(Hydrofluoric acid)<http://en.wikipedia.org/wiki/Hydrofluoric_acid>

    ==> -83.55°C

    Hydrochloric acid (HCl) <http://en.wikipedia.org/wiki/Hydrochloric_acid>

    ==> −26 °C (247 K)

    d) H2O or CH3OH

    H2O<http://en.wikipedia.org/wiki/Water_%28molecule%29>

    ==> 0 °C, 32 °F (273.15 K)

    CH3OH(Methanol)<http://en.wikipedia.org/wiki/Methanol>

    ==> -97 °C (176 K)

    e) KBr or CH3Br

    KBr (Potassium bromide) <http://en.wikipedia.org/wiki/Potassium_bromide>

    => 734 °C (1007 K)

    CH3Br(Methyl bromide)

    ==> -93.6 °C

    Source(s): from Tiger & google.com
  • 風行
    Lv 4
    1 decade ago

    基本上,比較沸點時,一般情況:共價網狀固體>離子化合物>有氫鍵的分子化合物>有極性的分子化合物>分子化合物

    a) HCl or NaCl

    前者為分子化合物,後者為離子化合物,所以沸點:HCl < NaCl

    b) C2H5OC2H5 or C4H9OH

    醚類因為立體效應較難有分子間氫鍵,醇類有分子間氫鍵,且兩者同碳數,所以沸點:C2H5OC2H5 < C4H9OH

    c) HF or HCl

    同樣是分子化合物,比較兩者分子間的作用力大小(即凡得瓦力),而HCl的分子量較大,所以凡得瓦力較大,但HF有分子間氫鍵,已知氫鍵強於一般的凡得瓦力,所以我們預期沸點大小: HF > HCl

    d) H2O or CH3OH

    水平均一個分子的分子間氫鍵數目大於甲醇平均一個分子的分子間氫鍵數目,所以預期沸點:H2O > CH3OH

    (或者從生活經驗下手,已知甲醇比水容易揮發,所以預期沸點:H2O > CH3OH)

    e) KBr or CH3Br

    前者為離子化合物,後者為分子化合物,所以沸點: KBr > CH3Br

    2006-04-06 19:11:22 補充:

    我查到的資料如下,證實HF沸點確實比HCl高HF(Hydrogen fluoride)沸點(Boiling Point) 19.4________________^^^^^ 熔點(Melting Point ) -83.55 比重(Density)  1.27 HCl(Hydrochloric acid)沸點(Boiling Point) -83________________^^^^ 熔點(Melting Point ) -112.5 比重(Density)  1.19

    2006-04-06 19:12:25 補充:

    資料來源:http://chemwww.pu.edu.tw/cheminfo/molecular-server...

    2006-04-06 19:16:46 補充:

    抱歉,洋洋灑灑寫了一大堆,結果是比較熔點,不過比較熔點與沸點的準則是差不多的,所以結果仍是一樣。即基本上,比較熔點時,一般情況:共價網狀固體>離子化合物>對稱性良好的分子化合物>有氫鍵的分子化合物>有極性的分子化合物>分子化合物

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