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Does temperature decrease

  1. Mar 10, 2008 #1
    One mole of certain gas is confined in a container. Suppose every molecule has decomposed and given rise to- two new molecules. Let's assume (for this purpose ) it requires negligible amount of energy for the molecule to decompose (or not any energy at all). So what will we observe?
    The amount of matter has been doubled by the process, so pressure should increase- that's obvious. What happens to the temperature?
    Does it stay the same OR increase?
  2. jcsd
  3. Mar 10, 2008 #2
    I think I have got it. If the two newly formed molecules have the same mass then the pressure should not increase rather the temperature is halved. It is because temperature is something proportional to kinetic energy. . When a molecule is splitted to two new similliar molecule then the energy is divided between them each having halve of the original (energy is conserved). So temperature falls nothing happens to pressure.
    Am I right? Please help.
  4. Mar 10, 2008 #3
    I would say yes.
  5. Mar 10, 2008 #4


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    It depends on the process and what the constraints are. You have an endothermic reaction that increases the number of moles of gas. You can:

    - Decompose by heating slowly, which increases pressure at constant temperature
    - Decompose by increasing volume slowly, which decreases temperature at constant pressure
    - Combine activities or conduct the process irreversibly, which could result in a combination of changing temperature and pressure.
  6. Mar 10, 2008 #5


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    The total number of moles changes and you have an entropy change if you can recall there's an equation S=Q/T

    Also note that such a reaction is going to have an enthalpy component due to the changes in bonds.
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