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Enthalapy of Formation - HELP

  1. Oct 16, 2006 #1
    Hey guys this is my 1st post here...Need help with a question thats been troubling me for a long time...

    A student determined experimentally that 2.06 kJ of heat energy were released when 0.365 g of copper reacted with excess chloride. From these data, the enthalapy of formation of copper (II) chloride would be...
    a. -206 kJ/mol
    b. -1.09 kJ/mol
    c. +2.71 kJ/mol
    d. +165 kJ/mol


    please help because its frustrating me - when i picked b. my teacher said its WRONG !!!! I don't get it at all. I think area it is focussing on is Hess's Law! Please help me!!! :confused:

    Thanks in advance,
    Me
     
  2. jcsd
  3. Oct 16, 2006 #2

    mrjeffy321

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    How did you arrive at answer b? What was your logic, if any?

    -Write out the chemical reaction.
    -Find the molar ratio of Cu reacted to CuCl2 produced.
    -Convert the given number of grams of Cu into moles of Cu.
    -Use the mole ratio calculated earlier to find the number of moles of CuCl2 produced.
    -Find a relationship between the heat energy measured to be given off and the number of moles of CuCl2 produced.

    Remember that the enthalpy of formation of a substance in its pure, elemental, state is zero.
    Copper metal [Cu (s)] and Chlorine gas [Cl2 (g)] are both in their elemental states in this reaction, therefore the only thing that has a non-zero enthalpy of formation would be the CuCl2.
     
  4. Oct 17, 2006 #3
    I still cant get the right answer i am fumbeling around -358????
     
  5. Oct 17, 2006 #4
    -Write out the chemical reaction.
    Cu(s) + Cl2 (g) ----> CuCl2

    -Find the molar ratio of Cu reacted to CuCl2 produced.
    1:1

    -Convert the given number of grams of Cu into moles of Cu.
    0.365/63.55 = 0.0057435 mol Cu

    -Use the mole ratio calculated earlier to find the number of moles of CuCl2 produced.
    0.0057435 mol Cu

    -Find a relationship between the heat energy measured to be given off and the number of moles of CuCl2 produced.
    ??????????????????????????
     
  6. Oct 17, 2006 #5
    Sorry Guys I worked on it over and over and over again but couldn't get the RIGHT ANSWER - b/c THE RIGHT ANSWER ISN'T THERE!!! It should be - 358 kJ/mol . I showed my work to me intructor and she did the problem with me - realizing that she had messed up!!!
     
  7. Oct 17, 2006 #6

    mrjeffy321

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    For others' benifit,
    The change in enthalpy in the reaction is the sum of the enthalpies of formation of the products minus the sum of the enthalpies of formation of the reactions.
    Since both Cu(s) and Cl2 (g) have a zero enthalpy of formation, we know the only thing which is left to cause the final change in enthalpy is the CuCl2.
    Enthalpy of formation of CuCl2 - 0 = enthalpy of formation of CuCl2.
    We measured the change in enthalpy of the reaction to be -2.06 kJ and you have previously calculated that 0.0057435 mol [CuCl2] produced in the reaction. We are looking for a value of enthalpy per mol not per 0.0057435 mol, so dividing the measured delta H by the number of moles of CuCl2 gives is the change in enthalpy per mole of CuCl2 which we also know to be the enthalpy of formation of CuCl2.

    So you had it right earlier but you just didn’t trust your answer since it wasn’t one of the choices.
    If something like this happens again on a homework question you might try checking your calculated answer with a reference table to see how close you got. If your answer is sufficiently close to the reference table's value then this will give you more confidence when questioning the teacher. (of course this solution isn’t practical when it comes to taking tests obviously)
     
  8. Oct 18, 2006 #7
    How did you get the -2.06 kJ? Is there a formula to find that answer? I keep seeing these numbers, but I don't see how you get that number.
     
  9. Oct 18, 2006 #8
    Enthalpy help too...

    How did you get the -2.06 kJ? Is there a formula to find that answer? I keep seeing these numbers, but I don't see how toget it.
     
  10. Oct 18, 2006 #9

    mrjeffy321

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    I got the value of -2.06 kJ from the question.

    The question stated,
    The key phrase being,
    "2.06 kJ of heat energy were released".
    Since the heat was released the sign on the delta H value must be negative to make it an exothermic reaction.

    There is a formula to find the change in enthalpy of the reaction,
    (delta H) = (sum of enthalpies of formation of products) - (sum of enthalpies of formation of reactants)

    In this case, since all the reactants in the chemical reaction were in their elemental states, their enthalpies of formation were zero, leaving the only non-zero enthaly of formation that of the CuCl2...a product of the reaction, so,
    delta H = enthalpy of formation of CuCl2
     
  11. Oct 19, 2006 #10
    thank you for your reply it clarified a lot
     
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