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Endothermic and Exothermic

  1. Nov 19, 2005 #1
    How can you tell when a reaction is endo or exothermic? From what i understand there are two ways to know:
    1) if the enthaply(change in H) is greater than 0, it's endothermic. If the change in H is lower than 0, its exothermic.

    2) Also,when the chemical bonds of the reactant molecules are greater than the energy stored in the chemical bonds of product molecules, the reaction is exothermic. In endothermic chemical reactions, the situation is reversed: more chemical energy is stored in the bonds of the product molecules than in the bonds of the reactant molecules.

    However, when i see an example like the melting of ice this doesnt make sense:

    H20(solid) ---> H20(liquid) Enthalpy= +6.01 kJ

    The enthalpy is greater than zero, so it would be an endothermic reaction. But doesn't the reactant, H20(s), have stronger bonds than the product, H20(g), making it a exothermic reaction? a solid has stronger bonds than a liquid. someone please explain
  2. jcsd
  3. Nov 19, 2005 #2


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    If the enthalpy increases, i.e. energy is released, and the temperature increases, then the reaction is exothermic. Energy goes out.

    By convention + change in H indicates the heat is absorbed, or the reaction is endothermic.

    As for the difference between H2O (s) and H2O (l), energy must be put into the solid to make liquid (heat of fusion) and energy must be put into liquid to make gas, or vapor (heat of vaporization).
  4. Nov 19, 2005 #3


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    ice melting into liquid water takes energy.
    When solid, the water molecules are not moving very much and the molecules form nice crystals.
    As a liquid, the molecules flow much more freely and do not interact with eachother as much.
    So in order to over the intermolecular forces, you have to put energy into the system, thus it is endothermic.
  5. Nov 20, 2005 #4


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    For the H20(solid) ---> H20(liquid) Enthalpy= +6.01 kJ

    you need to specify the temperature and pressure. The reaction refers to a phase transition, the complete conversion of one phase to another. It you're at the equilibrium temperature (referring to chemical potentials), then the process is always endothermic for first order phase transitions of water (from solid up) at lower pressures, because you're trying to convert all of the solid to liquid and the system is stable. Don't get into it too much, enthalpy can be confusing.

    The bond rule that you mentioned refers to covalent bonds for the most part within the molecule; that is the bond of the reactants and product molecules/species. It's referring to the enthalpy of formation of each species.
  6. Feb 6, 2011 #5
    Undertow, you're getting confused about the intermolecular forces of water. H2O has the same bond energies whether it's in gaseous, liquid or solid form. The H-O bonds are still the same. what differs are the intermolecular forces that are holding all of the H2O molecules as a whole: i.e. dipole-dipole, LDF, Hydrogen bonding. when water is melted from ice to liquid form, the system takes out energy from the surroundings in order to do the melting. This you showed correctly with the positive delta H.
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