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Homework Help: Energy to separate one molecule of water from a single neighbour

  1. May 9, 2012 #1

    The amount of heat required to convert 1g of ice into vapour is approx. 3000J. Estimate the energy required to separate one molecule from a single neighbour, assuming that in ice, each H2O molecule has four nearest neighbours. [Mr(water) = 18)

    My reasoning:

    3000 x 2 / [1/18 mole x NA x 4] = 4.5 E-20 J

    Because each molecule is attracted to 4 other molecules -- hence the product of the number of water molecules with 4. And since I'm looking for the energy required to separate one molecule from a single neighbour, I multiplied the division by 2.

    Is this reasoning valid? Haven't we ignored the unused bonds of the water molecules at the outermost part of the ice cube? How significant is it to take into consideration those unused bonds?

    If the reasoning is wrong, could anyone please point to me the proper way of attempting the question (rather than getting a fortuitous answer)?
  2. jcsd
  3. Jun 4, 2012 #2
  4. Jun 6, 2012 #3
  5. Jun 7, 2012 #4
    I think the reasoning is almost correct. I don't understand why you multiplied by two, though. Each molecule has four neighbors. And the term you get without multiplying the two, is the binding energy between them for each neighbor.

    Also, there are no 'outermost bonds'. It is somewhat like a football structure, closed on itself, so each ice molecule is joined to four others.
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