Help with some chemistry Q's.

Discussion in 'The Okie Corral' started by USMC Medic, Mar 13, 2010.

  1. USMC Medic

    USMC Medic Corpsman Up!

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    1) Why is the standard of enthalpy of formation of H2(s) not zero?

    2)What volume of a 6.00 M KOH solution must be diluted to 200mL to make a 1.50 M solution of KOH?

    3)The value of Delta H for the reaction is -126kJ. The amount of heat that is released by the reaction of 25g of Na2O2 with water is ____kJ.
    2Na2O2(s) + 2H2O(l) ---> 4NaOH(s) + O2(g)


    It is a shot in the dark, but there are some smart people on here. I need to know how to do these calculations for the quiz on Monday. Thanks for the help.
     
  2. AltiDude

    AltiDude

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    6M/1.5M =4 so 200ml/4 = 50ml of 6M KOH
     

  3. RHVEtte

    RHVEtte

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  4. USMC Medic

    USMC Medic Corpsman Up!

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    What equation did you use to find this out?
     
  5. devildog2067

    devildog2067

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    You find the molar mass of Na2O2, convert the amount given in grams to mols. That number divided by 2 (the coefficient in the balanced equation) times the delta H.

    Heat is a "product" of the reaction just like anything else, you have to balance it.

    EDITED TO ADD: Let me say that I think the reaction "What equation did you use" is exactly the WRONG one. Not a dig against you or anything, but you should be trying to understand the science, not memorize equations. If you really get what it means to balance a given chemical reaction, you wouldn't have to ask that question. Plus of course, the answer is different for any given chemical reaction.
     
    Last edited: Mar 13, 2010
  6. devildog2067

    devildog2067

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    Because H2 is not "standard-ly" (s)...
     
  7. ElevatedThreat

    ElevatedThreat NRA Member

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    The Delta H for the reaction is "per mol" of reactants consumed, i.e., for 1 molecular weight of the reactants.

    So if you figure out how many molar weights of the reactants there are in the 25 gram weight being asked about, then you can just multiply the number of mols times the Delta H per mole.

    Molar weight of Na2O2 = (2 x 23grams/mole) + (2 x 16grams/mole) = 78 grams per mole,

    78grams/mole x 2 (because it is 2Na2O2 being consumed in the reaction as written) = 156 grams total

    25grams/156grams= 0.1602

    0.1602 x (-126kJ) = (-20.19kJ)

    -ET
     
  8. lanternlad

    lanternlad Mythmatician

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    Pertaining to your first question. Hess' Law does say that enthalpy (delta H) of gas H2 is zero since that's its natural state. H2 in a solid form isn't "natural" at ambient temperature so its enthalpy formation would be equal to the amount of energy it takes to change that solid Hydrogen to its natural state. I would suggest looking up Born-Haber cycle to walk you through the individual steps. Enthalpy formation is simply the addition of all the thermodynamic changes of each step of the equation. B/c I agree with DevilDog2067 - understand the process, which is why I explained it out) so you can do any equation.

    (This was written by my wife, the Ph.D chemist, and not me, the science stupid artist.)
     
    Last edited: Mar 13, 2010