Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Understanding Entropy,Enthalpy,Spontanity with example

  1. Oct 9, 2011 #1
    A reaction is Spontaneous means what??a reaction is favorable?
    let me give an example:-By taking a Balloon and blowing air into it we are imparting Energy to the Balloon as the KE of the air molecules so was this reaction spontaneous? or the reverse is spontaneous i.e leave the balloon and let the balloon sink??
    and if the energy(enthalpy) of a system increases the entropy too which contradicts the statement of thermodynamics
    and r both enthalpy and entropy referred,of system or surrounding?
    in the first case the energy of the system is increasing and also the randomness and in second case both r decreasing??
    Its really confusing
     
    Last edited: Oct 9, 2011
  2. jcsd
  3. Oct 9, 2011 #2

    Ken G

    User Avatar
    Gold Member

    Spontaneous just means that at the end of the process, the system in a sense lives in a larger space of possibilities than it did at the beginning of the process, so the end state is favored over the initial state just based on probability issues. A classic example is if you put a hot body into contact with a cold body. Given the total energy there, the system has access to a class of configurations in which the heat is shared more economically between the two bodies-- when the two have the same temperature they have access to a larger space of possible configurations than when their temperatures are different.

    So if the system is in some sense constantly randomly sampling all the possible configurations that are accessible given the total energy, it will spend a lot more time in a configuration belonging to the class with equal temperature than in any of the classes with unequal temperature. So we say that the bodies come to the same temperature "spontaneously"-- it's just what systems do that have access to lots of possibilities. That's what we expect any large closed system in good thermal contact with itself to do.
     
  4. Oct 9, 2011 #3
    Hello ND, welcome to Physics Forums.
    Hopefully after you have been here a while you will be able to change your name.

    Wood burns quite readily, but how often does the table your computer is sitting on burst into flames?

    Of course it doesn't - this reaction (the combustion of wood) is non spontaneous - you have to help it along ie ignite the wood.

    On the other hand place a spoonful of sugar in your coffee. The sugar dissolves rapidly.
    This reaction - the solution of sugar in water is spontaneous.
    This reaction is driven by a large increase in entropy.

    Does this help understanding spontaneity (notice the spelling) ?

    Now does water run uphill?

    Not in a river perhaps, but suppose the water is in a central heating system, being heated by a boiler.

    The boiler is adding enthalpy to the water, and the hot water spontaneously rises to the top of the heating system and drive the process.

    Do you know the thermodynamic equations that link the properties you quote?

    ΔU = TΔS - PΔV
    ΔH = TΔS + VΔP
    ΔG = ΔU - TΔS ?

    If so we can discuss examples in relation to them.

    go well
     
    Last edited: Oct 9, 2011
  5. Oct 9, 2011 #4
    thanks guys,but i guess a relevant explanation according to balloon thing would be good,it would be good if u can relate theoretically both enthalpy and entropy
     
  6. Oct 9, 2011 #5
    Discussion is a two way process and preferably in English, so it would also be good if you took an interest in what was said to you.

    Your balloon example does not make sense and is not a reaction so you will need to explain it further.
     
  7. Oct 10, 2011 #6
    ok,well i guess "Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation"
    The KE of the particles of the system is changing...isnt it,work is done on the baloon?
    correct me if i am wrong in any point.......
     
  8. Oct 10, 2011 #7
    and in the water thing...when heat is supplied the enthalpy does increase and also the entropy as the KE of the molecules increase?so how does it explain spontaneity?
     
  9. Oct 10, 2011 #8
    I think perhaps you are going too far too fast and mixing up concepts.

    In particular you are mixing up Thermodynamics and the Kinetic Theory and other ideas from mechanics.
    They are linked, yes but they are not the same and either could exist without the other.

    I did ask some questions to help formulate an answer but you did not bother to answer them so I will try going right back to basics.

    You have mentioned the word reactions. We normally talk about these in relation to chemical reactions. They can be considered as thermodynamic processes. ‘Process’ has a wider meaning is the more general term used in relation to thermodynamics. It includes mechanical and other phenomena.

    One of the fundamental drivers of physical processes is energy. For the moment I shall just say that Enthalpy is a measure of a certain type of energy.

    Processes may be spontaneous or non spontaneous and I have thought of a better example as in my sketches.

    I hold a ball on a slope with my finger, as in the first sketch.

    If I release the ball there are three scenarios.

    1) The ball rolls downhill. This is the spontaneous process. It will commence of its own accord as soon as I let go and continue until it is stopped. That is it is self sustaining.

    2) The ball rolls uphill. This is a non spontaneous process and will only occur if I push it. Further I will have to keep pushing to keep it going uphill. It is non self sustaining.

    3) The fairy tale – the ball leaves the slope and flies away.

    The spontaneous process is self starting (=spontaneous) and self sustaining because the ball is moving towards a position of lower energy – potential energy in this case.
    Moving uphill means it is moving to a position of higher energy.

    Now suppose there is a small step in the slope, as in the second picture.

    The ball will not start to roll down the slope even after I remove my finger, until it has been lifted over the step.
    This means it has to be pushed a small way uphill first, in the direction of increasing energy. Once it is over the step the process is self sustaining.
    This small energy input to start an otherwise self sustaining process is called activation energy and is one reason why many processes that appear energetically favourable do not occur in real life.


    I think that is enough to start. Please confirm you are still with me.
    Yours is (I hope) is a serious question about a serious subject that deserves better consideration than cellphone textspeak.
     

    Attached Files:

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Understanding Entropy,Enthalpy,Spontanity with example
  1. Entropy and enthalpy (Replies: 38)

Loading...