enthalpy change

If a few drops of water is added on anhydrous copper 2 sulphate crystal, heat is evolved. The enthalpy change is negative.

But actually, I don't quite understand why heat is evoled. As far as I am concerned, ionic bond holds the ions in the dry anhydrous copper 2 sulphate crystal. When water is added, the ions become mobile. Thus, I think energy should be needed to separate the ions to mobile. Energy should be absorbed from the environment. But , why can we still feel heat evolved?

更新:

what is the new bonding formed that cause heat loss? What is ion -dipole attractions?

3 個解答

評分
  • 9 年 前
    最佳解答

    first, Hung Pan is COMPLETELY wrong.

    - ion-dipole interaction cannot be stronger than the ionic bond. instead, hydration of ions is exothermic.

    still, dissolution of ionic compounds can be exo- or endothermic, depending on relative enthalpy of hydration AND breaking of lattice of ions.

    - only a little water is added. salt is NOT dissolved. instead, this only hydrates the ions inside, giving water of crystallization.

    Brian may be correct, but he's not really explaining the situation. he just say "this reaction is exothermic because the reverse reaction is endothermic".

    but WHY this reaction is exothermic? he didn't explained.

    here's my explanation.

    as mentioned above, ions are separated (a little bit) and energy is absorbed.

    the breaking of ion lattice is endothermic; at this point, you understand that.

    next... what happens after ions are separated?

    the naked ions have high charge density. water molecules will surround the ions, forming (dative) bonds.

    naked ions become hydrated ions, and the hydration process is exothermic.

    finally, remember that copper(II) sulphate is NOT dissolved; it's still solid copper(II) sulphate-5-water, the hydrated salt.

    the hydrated ions recombine to give new lattice for this "new" salt. the recombination is exothermic as lattice is formed.

    surely these two lattice energy may not be the same, but overall speaking, there's net energy release. thus temperature increases.

    lattice of anhydrous ion ---(a little water)---> separated, hydrated ions ---(recombination)---> lattice of hydrated ions

  • ?
    Lv 7
    9 年 前

    If you still donot understand why water is added to anhydrous CuSO4 to evolve heat . This is an exothermic reaction , with -enthalpy . Think how to form anhydrous CuSO4 from its aqueous solution . You heat it up to drive away the water so CuSO4 is higher energy cpd when is dissolved again to a lower energy stable cpd , the difference of it in energy is given out in form of heat .

  • 9 年 前

    Polar sovlents such as water can dissolve some ionic compounds. e.g NaCl can be dissolved in H2O . The polar sovlent helps to break the ionic bonds by the formation of a large no. of ion-dipole attractions . these attractions make the solvated ions stable in the polar sovlent . if the ion -dipole attractions formed are stronger than the ionic bonds between ions breaken , the reaction is then exothermic .

    p.s. the solubility is mainly governed by the standard enthalpy change of solution . more negative the enthalpy change , move favourable the dissolution the ionic compound is .

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