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    • Thread Starter

    Hey there all, 2 questions for ya that i cant quite answer

    1. When the enthalpy change of a reaction is endothermic, why doesn't the product of the reaction simply reform into its reactants again? Is this because the product is kinetically stable, but energetically unstable?

    2. A reaction occurs exothermically in an open vessel, resulting in a negative enthalpy change. Now during this reaction, hydrogen gas was evolved, and as it dispersed into atmosphere, it would have to do work right? Does this "work" affect the enthalpy change in any way, or is it part of the total enthalpy change?

    These two things are really bugging me, that even my chemistry teacher can't answer grrr..In the second question, I'm trying to make that link between thermodynamics and enthalpy...

    Any help much appreciated


    1. Generally, the increase in entropy will outweigh the positive enthalpy change so the products will be more stable than the reactants. Have you come across the concept of Gibbs' free energy yet?
    This takes into account both entropy and enthalpy changes via the formula:
    delta G = delta H - T.delta S

    where G is Gibbs free energy, H is enthalpy and S entropy. If delta G is negative then the reaction is energetically feasible.

    2. As the vessel was open, the gas would do work in expanding against air pressure. (If it was in a closed container, there would be no change in volume so no work done.)
    A formula linking enthalpy with changes in pressure, volume is:

    delta H= delta U + p.delta V
    where H is enthalpy, U is internal energy (a measure of the potential energy between particles and their kinetic energies), p is pressure, and V is volume.
    So the value for enthalpy change of a reaction contains this factor of work having been done (i.e the heat change would be a higher value if in a closed container)

    a good book which explains this is: Advanced Chemistry 1: Physical and Industrial by Philip Matthews.
    • Community Assistant
    • Study Helper

    Community Assistant
    Study Helper
    ahhh entropy - the secret of life...

    All systems move towards an increase in entropy because it's statistically the most likely.

    Entropy is a function of the available energy and the number of particles i.e. and increase in either gives rise to an increase in entropy.

    Endothermic changes reduce the available energy (reducing the entropy) but make up for it by increasing the number of particles, or degrees of freedom of the particles. The overall entropy change of the universe MUST be positive.
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