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

Are exothermic reactions spontaneous?

  1. Apr 5, 2010 #1
    The reaction between
    2KOH + H2SO4 -> K2SO4 + 2H2O
    is exothermic...

    However is this reaction spontaneous as well? If so then why?
    the concentration that i used was:
    KOH - 2M
    H2SO4 - 1M

    (I writting a lab report on the "course of this reaction"....spontaniety of the reaction is a part of my observation table...so if the reaction is spontaneous, will it be visible to naked eyes? )
  2. jcsd
  3. Apr 6, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Hi PhyChemMath, welcome to PF. At constant temperature and pressure, spontaneity is determined by whether the Gibbs free energy change of the reaction is negative. Exothermic processes aren't always spontaneous; consider freezing, which is exothermic but definitely not spontaneous for water at temperatures about 0°C.
  4. Apr 6, 2010 #3
    hey thanks!
    yea i get the gibbs free energy concept but in this case it is the formation of salt...
    we titerated KOH(25ml) with H2SO...adding 1ml of H2SO4 each time..

    now i get what you are saying about whether not every exothermic reaction being spontaneous .. i didnt really see any visible signs of the reaction being spontaneous..

    also if i were to calculate the delta G value i need a delta H value...will the delta H value be the enthalpy of neutralisation? ???
  5. Apr 6, 2010 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

  6. Apr 6, 2010 #5
    Spontaneous means that it happens or that it would happen, in those thermodynamic conditions, if catalyzed. Non spontaneous means that it couldn't happen, in those thermodynamic conditions, even if catalyzed.
    Spontaneous: DeltaG < 0.
    Non spontaneous: DeltaG > 0.
    Equilibrium: DeltaG = 0

    1) H2 + (1/2)O2 --> H2O
    is spontaneous at standard conditions, but it doesn't happen, unless you catalyze it (example: a little spark).

    2) 2KOH + H2SO4 --> K2SO4 + 2H2O
    is spontaneous at standard conditions and it happens without other means. You can infer the fact it happens from temperature increase (for example) because this reaction is exothermic. If you are not convinced, try to use KOH 6M and H2SO4 6M (be careful).

    3) 1g of ice at 0°C + 1litre of water at room temperature --> ice melts
    is spontaneous and it happens, but it's not exothermic, it's endothermic.

    4) Ba(OH)2*8H2O + 2NH4SCN --> Ba(SCN)2 + 2NH3 + 8H2O
    is spontaneous and it happens, and it's endothermic at well:

    5) A dilute solution of NaCl in water( standard conditions) --> NaCl(solid) + H2O is NOT spontaneous and infact it doesn't happen. If it happened it would be exothermic.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook