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Thread: Exo/Endo without information?

  1. #1 Exo/Endo without information? 
    Forum Senior Booms's Avatar
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    sorry about the terrible title I couldn't think of a more descriptive one without making the message irrelevant

    Ok in AS chemistry we are doing enthalpies and Exo/endothermic reactions, I understand this and can do the equations and work it out etc
    What I can't do is get the answer the right way round for example I will write the enthalpy change of a reaction as 286 and the answer is -286

    now I know there is a way to work this out correctly and teachers and peers have explained repeatedly how to do so and I just don't get it, for some reason it will not click
    So after lots of searching (I even posted a thread here but got no answers) I discovered I can get it right! if I know whether the reaction is exothermic or endothermic


    Now this is great except I don't always get the information to work out if it's exo or endo, and I don't have the lifespan to memorise every reaction

    so is there a way to work out if a reaction is exothermic or endothermic without any information save for the equation and state symbols

    So hypothetically
    NaOH + PbS2 -> H2O + NaPbS (I made it up so don't compain it's not balanced or even exists) is it exo or endo?


    It's not how many questions you ask, but the answers you get - Booms

    This is the Acadamy of Science! we don't need to 'prove' anything!
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  3. #2  
    Forum Ph.D.
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    If the reaction does not take place how can it possible be either endo or exo thermic? How about zerothermic?


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  4. #3  
    Forum Masters Degree organic god's Avatar
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    i don't really understand, the point is you work out if a reaction is exo or endothermic by doing the calculation.

    i'm guessing the way your currently working out the enthalpy of the reaction is by an appplication of hess's law.

    so your given something like enthalpies of formation of the species involed in the reaction and then you fiddle the signs to work it out.

    simply. reactants go to elements in standard states = -(formation enthalpy)
    elements go to products +(formation enthalpies).

    then just add up all the terms you have and this will give you the overall enthalpy of reaction.
    everything is mathematical.
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  5. #4  
    Reptile Dysfunction drowsy turtle's Avatar
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    If you are referring to Hess's Law Cycles, think of it as a graph showing total enthalpy of the system.

    Taking the enthalpy of formation as a 'ground state' so to speak, then put the reactants and products in relative to the enthalpy of formation. If the enthalpy of the reactants is lower than the enthalpy of the products, relative to the formation enthalpy, then the system has gained enthalpy and so is endothermic. If the enthalpy of the products is lowest, the reaction has caused the system to lose enthalpy as heat, so it is exothermic.
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