? 發問於 科學及數學化學 · 1 十年前

點解graphite is more energetical

ethalpy change of combustion of diamond= -395.4

ethalpy change of combustion of graphite= -393.5

點解graphite is more energetically stable than diamond?

咩係energetically stable同kinetically stable?

點解carbon does not convert from less stable allotrope to more stable one?

don't copy other"知識"to me.

更新:

Why activation energy of the conversion is too high?

2 個解答

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  • 1 十年前
    最愛解答

    點解graphite is more energetically stable than diamond?

    Both diamond and graphite have giant covalent structure.

    In diamond, the carbon atoms are sp3 hybridized. Each carbon atom is bonded tetrahedrally with 4 neighbouring carbon atoms.

    In graphite, the carbon atoms are sp2 hybridized. Each carbon atom is bonded triangularly with 3 neighbouring carbon atoms, and the p electrons are delocalized over and below the layer of atoms. There are a smaller number of C-C covalent bonds in graphite than in diamond, but the C-C bonds in graphite are stronger. This is because of the partial double bond character of the C-C bonds in graphite due to the delocalization of π electrons. Besides, there are van der Waals' forces between layers of atoms in graphite.

    Actually, the conversion of graphite from diamond only involves an enthalpy change -1.9 kJ mol-1

    C(diamond) → C(graphite) .. ΔH = -1.9 kJ mol-1

    Therefore, it can only be explained that the stability of diamond and that of graphite are COMPARABLE, as -1.9 kJ mol-1 is a very small enthalpy change.

    咩係energetically stable同kinetically stable?

    A substance is said to be energetically stable if the substance cannot change to the expected product(s) because the substance has a lower enthalpy than the expected product. In other words, the substance cannot change to the expected product(s) because the reaction is endothermic.

    A substance is said to be kinetically stable if the substance cannot change to the expected product(s) because the activation energy (Ea) is too high.

    點解carbon does not convert from less stable allotrope to more stable one?

    Although diamond is less energetically stable than graphite, diamond cannot change to graphite at room conditions. This is because the activation energy of the conversion is too high. This is said to be kinetically stable.

    2009-08-10 12:15:16 補充:

    The activation energy is high because C-C bonds are strong.

  • 冷風
    Lv 5
    1 十年前

    睇左老子的解釋, 對d terms更有清楚左

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