I have a 'quickish' Chemistry question

Discussion in 'Community Discussion' started by fireshot91, Nov 28, 2009.

  1. fireshot91 macrumors 601

    fireshot91

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    #1
    So, in my textbook, it says "The energy required to separate one mole of the ions of an ionic compound is referred to as lattice energy. The strength of the forces holding ions in place is reflected by the lattice energy. The more negative the lattice energy, the stronger the force of attraction"


    So, the second/third sentence is saying the lower the lattice energy, the stronger the bond? Would it not make sense if it was the other way around? Like, the more energy required to separate an ion from something means the bond is stronger, no?
     
  2. -aggie- macrumors P6

    -aggie-

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    #2
    It's saying the "more negative the lattice energy." This means it requires even more energy to separate the ions (by adding energy). It's basically calculated by multiplying the two opposite charges and dividing by the square of the distance between the two ions, which results in a negative quantity. However, older textbooks defined lattice energy as a positive quantity, so the confusion is understandable.
     
  3. fireshot91 thread starter macrumors 601

    fireshot91

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    #3
    Wait to me, any energy is positive...Like, there can't be negative energy :confused:.

    So it's a negative number? Meaning, the lower the value of it, the stronger the bond?
    Even with a negative number, it would make sense to make it closer to 0=stronger the bond, and farther away=weaker the bond..I think, if I said that correctly.
     
  4. -aggie- macrumors P6

    -aggie-

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    #4
    Look at the equation I wrote out above.

    Also,

    LE = (a constant) * q * -q / r^2.

    where q and -q are the two ions held together and r is the distance between them. As they get closer together, the bond is stronger (harder to separate them). As the charges increase, the force of attraction is stronger.
     
  5. TuffLuffJimmy macrumors G3

    TuffLuffJimmy

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    #5
    No the further from zero the more energy. Think of it as positive energy and negative energy.
     
  6. fireshot91 thread starter macrumors 601

    fireshot91

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    #6
    Err, that's what I'm saying right? :confused:

    Like, lets talk about positives right now. Closer to 0 the ionization energy, the easier it is for the electron to come out of the atom, right? Since the energy required is lower, the actual energy is lower. The higher the energy, the more "work" is required, so obviously the energy is higher.

    Now, for Lattice Energy, it's saying that the higher the energy, the easier it is to separate the mole of the ion? (As a positive Lattice Energy)
    For the negative Lattice Energy, you all/the textbook is saying the farther away from 0/the lower the number is, the bond is STRONGER. And since the bond is stronger, it would take MORE energy to separate the mole of the ion, and more is always a higher number right?
     
  7. feelthefire macrumors 6502a

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    #7
    It's easier to think of it this way, even though this is not technically correct:

    If you think of energy in the negative sense, you're thinking about what you would have to PUT IN to that bond in order to bring the energy to zero (break the bond). Just like a negative bank account, you have to put something in.

    What you're measuring with a lattice energy is (in a simplified sense) the energy advantage gained by a molecule forming a crystal structure with other molecules. Because the lattice energy is the energy of formation of an inherently more stable compound, it is always going to be negative--energy is RELEASED when the crystal is formed.

    To add to the confusion, it was often defined in reverse in older textbooks, meaning all the numbers were positive--however if your book is modern, all lattice energies will be negative for anything that can form a crystalline lattice.

    Thus, the larger the NEGATIVE number, the more energetically favorable the lattice formation is. The smaller the ions used in forming the crystal, the more negative the number will be. There's a few other trends that aren't that interesting.

    Someone correct me if any of this isn't correct, as it's been several years since I took inorganic chemistry.
     

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