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why ice melts even tho' it's endothermic rxn

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  • 13-04-2009 1:45pm
    #1
    KittyeeTrix
    Registered Users Posts: 7,239 ✭✭✭


    Could anyone give me a answer to this question,

    Describe why ice melts spontaneously at 0 degrees celcius even though it is an endothermic reaction

    Any help answering this clearly would be much appreciated....:)


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    #2
    Carsinian Thau
    Closed Accounts Posts: 2,054 ✭✭✭Carsinian Thau


    I hope I'm not overlooking something but:

    If the ambient temperature around ice is greater than 0C then heat can "flow" into the ice. Thus causing it to melt. This is in line with it being an endothermic reaction and hence, explains why it is a spontaneous rxn.


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    #3
    Professor_Fink
    Registered Users Posts: 861 ✭✭✭Professor_Fink


    fluffyorganic1 wrote: »
    Could anyone give me a answer to this question,

    Describe why ice melts spontaneously at 0 degrees celcius even though it is an endothermic reaction

    Any help answering this clearly would be much appreciated....:)

    Eh, it's not an endothermic reaction. It's not a reaction at all. It's a phase transition. The phase transition has an energy penalty associated with it (hence the latent heat). It melts by absorbing heat (energy) from the enviroment as long as the environment is warmer than the melting temperature of the ice.


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    #4
    KittyeeTrix
    Registered Users Posts: 7,239 ✭✭✭KittyeeTrix


    Professor_Fink wrote: »
    Eh, it's not an endothermic reaction. It's not a reaction at all. It's a phase transition. The phase transition has an energy penalty associated with it (hence the latent heat). It melts by absorbing heat (energy) from the enviroment as long as the environment is warmer than the melting temperature of the ice.


    That's how the question comes up in my 1st year physical chemistry section......
    They want us to talk about enthalpy and entropy, and to rationalise the melting of the ice alluding to the large increase in entropy..
    I find the phys. chem section a nightmare
    I haven't a clue what to say.
    Really just want a nice simple paragraph that I can get my head around enough to semi-understand and be able to memorise:)
    Would be so grateful if you could help pleeeeeeeeez


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    #5
    Professor_Fink
    Registered Users Posts: 861 ✭✭✭Professor_Fink


    fluffyorganic1 wrote: »
    That's how the question comes up in my 1st year physical chemistry section......
    They want us to talk about enthalpy and entropy, and to rationalise the melting of the ice alluding to the large increase in entropy..
    ...
    Would be so grateful if you could help pleeeeeeeeez

    Ok, well I'm afraid the question seems to be wrong. First of all, while the melting of ice absorbs heat (and hence energy) from the environment it does not actually decrease the temperature of the ice. As I have mentioned, this is because it is a second order phase transition rather than a chemical reaction. The process is actually isothermal (i.e. it keeps the temperature constant through out) rather than either endothermal or exothermal. It is not, however, adiabatic, meaning that there is a net change in energy. Ice is in a more ordered state (due to it's crystal structure) than liquid water, so the transfer of heat from the environment to the ice increases the enthropy of the ice (while decreasing the entropy of the environment), eventually melting it.

    Does anyone actually use enthalpy for anything? It's always struct me as rather pointless.


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    #6
    napapa
    Closed Accounts Posts: 149 ✭✭napapa


    Professor_Fink wrote: »
    Ok, well I'm afraid the question seems to be wrong. First of all, while the melting of ice absorbs heat (and hence energy) from the environment it does not actually decrease the temperature of the ice. As I have mentioned, this is because it is a second order phase transition rather than a chemical reaction. The process is actually isothermal (i.e. it keeps the temperature constant through out) rather than either endothermal or exothermal. It is not, however, adiabatic, meaning that there is a net change in energy. Ice is in a more ordered state (due to it's crystal structure) than liquid water, so the transfer of heat from the environment to the ice increases the enthropy of the ice (while decreasing the entropy of the environment), eventually melting it.

    Does anyone actually use enthalpy for anything? It's always struct me as rather pointless.

    Whats your take on entropy and ordering of liquids?


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    #7
    Capt'n Midnight
    Moderators, Recreation & Hobbies Moderators, Science, Health & Environment Moderators, Technology & Internet Moderators Posts: 90,762 Mod ✭✭✭✭Capt'n Midnight


    napapa wrote: »
    Whats your take on entropy and ordering of liquids?
    If I'm ordering a beer, it had better be cold. :)


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    #8
    Professor_Fink
    Registered Users Posts: 861 ✭✭✭Professor_Fink


    napapa wrote: »
    Whats your take on entropy and ordering of liquids?

    Well, liquids have higher degrees of freedom than solids, so the phase transition from liquid to solid inevitably involves an increase in entropy. You can work it out fairly easily from the structure.


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    #9
    napapa
    Closed Accounts Posts: 149 ✭✭napapa


    Professor_Fink wrote: »
    Well, liquids have higher degrees of freedom than solids, so the phase transition from liquid to solid inevitably involves an increase in entropy. You can work it out fairly easily from the structure.

    Lets say we take a simple unimolecular conformational change from isomer a to b, exhibits 1st order kinetics. We do arrhenius plots and derive the change in entropy parameter delta S in 3 separate solvents using Eyrring equation.

    If solvent A has a positive value lets say 100 JK-1mol-1

    Solvent B has lower value 20JK-1mol-1
    and
    Solvent C has negative value -50JK-1mol-1

    What can we say about the solvent effect on the system?

    Appreciate any help with this?


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