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Why reaction between alkali metals and water exothermic??

  1. Oct 9, 2015 #1
    Why the reaction between alkali metals and water exothermic??
    I know that it produces heat to surroundings
    But, what actually happens?? Why it gives heat to surroundings??

    http://antoine.frostburg.edu/chem/senese/101/thermo/faq/exothermic-endothermic-examples.shtml says that when forming ion pairs (like ionization reaction), it is endothermic (since it needs some heat to get the electron out)

    But, the reaction between alkali metals and water is

    2M(s) + 2H2O(l) => 2M+(aq) + 2OH−(aq) + H2(g)

    Which forms ions pair..
    But, it's not endothermic, but exothermic.. Why???
     
  2. jcsd
  3. Oct 10, 2015 #2

    Borek

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    Staff: Mentor

    Cation hydration is strongly exothermic.
     
  4. Oct 11, 2015 #3

    James Pelezo

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    Gold Member

    2Nao(s) <=> 2Na+(aq) + 2e- Eo = -2.71 volts
    H2O(l) + 2e- <=> 2OH-(aq) + H2(g) Eo = -0.83 volts
    2Nao + 2H2O(l) <=> 2Na+(aq) 2OH- + H2(g) Enet = (Eredn - Eoxdn = [(-0.83v) - (-2.71v) = 1.88v (net)

    ∆ G = - nFEo = -[(2e-)(96,500C)(1.88v) = -362,840 joules = - 363 Kj

    Compares well with Heat of Formation Tables using Hess's Law
    ΔHrxn = [∑ΔHf(P) - ΣΔHf(R)] = [2mole(-240.1Kj/mole) + 2(-230.0Kj/mole)] - [-(2(285.8 Kj/mol)] = - 368.6 Kj
    4
    A common 1" diameter Parraffin wax candle will gives off about 4x more heat than in this lab study;i.e., ~ 1.2 x 10 4 Kj/mole wax.

     
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