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

Endothermic absorb energy How do they begin in the first place?

  1. Mar 29, 2012 #1
    Endothermic absorb energy... How do they begin in the first place??

    PLEASE, BEFORE ANSWERING I would like you to hear me out. >>>

    Im really confused as to the phenomenon of why endothermic reactions between mixing of two substances just spontaneously happen.. giving them temporary "cold" properties...

    Thermal neutrality and the urge for everything to equalize in temperature is completely intuitive to me, so when i think of something getting hot, obviously energy was introduced to the system... In endothermic reactions however, this is not necessarily the case.

    Obviously one the reaction happens, it has gone to a state of less energy... It is absorbing the room temperature heat and from your fingers when touching.

    i understand how putting a source of heat between two substances can cause an endothermic reaction, because your introducing outside energy to start with.. they say when bonds are being broken, it costs energy. But what energy is being displaced to being the endothermic reaction, before it gets real cold..

    how do bonds just spontaneously break apart and loose energy when they are mixed like metal in a salt solution for example.

    Could someone explain this in a precise clear manner, cause i think the hardest part of the question is communicating my misunderstanding, I couldn't get the answer I wanted from my chem teacher, she kept saying entropy causes this, naturally. I know what entropy is, i am still confused, however.

    Thank you for reading, I would very much appreciate some insight.

    ammonium nitrate cold packs for example... they just magically get cold.

    this is my observation so far: these spontaneous endothermic reactions when substances are mixed are because they are at an unstable state? and with the cost of a little bit of (room temperature) heat, they break apart into more stable molecules/atoms?

    The want to go from unstable to stable at the cost of a tiny bit of heat energy from its surroundings make sense to me, but im not sure..
    Last edited: Mar 29, 2012
  2. jcsd
  3. Mar 30, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Re: Endothermic absorb energy... How do they begin in the first place??

    I could be wrong, but I don't think endothermic reactions result in the product of the reaction being in a state of LESS energy. That energy isn't lost, it has to go somewhere, so it should go be bound up in the molecules, putting them in a higher energy state. If this energy is taken from the heat of the substance, it would result in a lowering of the temperature.
  4. Mar 30, 2012 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Re: Endothermic absorb energy... How do they begin in the first place??

    The main idea to keep in mind is that there are three tendencies that drive reactions in one direction or the other:

    1. A tendency to go in the direction of lower internal energy
    2. A tendency to go in the direction of smaller volume (in the presence of external pressure on the system)
    3. A tendency to go in the direction of higher entropy

    When the 3 tendencies favor different directions, you have to calculate the Gibbs Free Energy for the products and reactants; whichever has a lower G "wins" in terms of reaction direction:
    Gibbs Free Energy = G = U + PV - TS
    In an endothermic reaction, the U+PV terms alone would favor the reactants. But, the products have a sufficiently higher entropy so that overall the products are the favored direction for such a reaction. That "-TS" term (-Temperature·Entropy) is what makes G smaller for the high-entropy products than for the low-entropy reactants, even though heat must be transferred into the system for this to happen.

    Hope this explanation makes some sense -- I probably left out some details in the interest of keeping things fairly brief. If you have more questions, feel free to ask.
    Last edited: Mar 30, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook