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Homework Help: Calculating heat of a reaction

  1. Mar 28, 2012 #1
    1. The problem statement, all variables and given/known data
    I'm doing report for a Thermochemistry lab and I need to calculate the molar enthalpy of formic acid, ΔH°2.

    Two reactions are taking place

    1) H3O + OH <-> 2H2O with ΔH°1 = -58400J/mol and some q1
    2) HCOOH + OH <-> H2O + HCOO and we need to calculate ΔH°2.

    I also know the pH is 1.96 before the neutralization took place.

    2. Relevant equations

    q = nΔH°
    qsystem = q1 + q2 = -qsurroundings = -(CcalΔTcal + CvVacidΔTacid +CvVbaseΔTbase)...........[2]

    3. The attempt at a solution

    So my general approach is:::

    First calcualate what q1 is because I know the pH.

    [H3O] = 10-1.96
    [H3O] = 0.011 M
    => moles of H3O = Vacid*[H3O] = 0.05L * 0.011M = 0.00055 mol H3O {Is this right?}

    Then, q = nΔH° = 0.00055mol * 58400 J/mol = 32.12 {does this make sense?}

    Now I calculate q2 using equation [2]
    Ccal = 76 J/°C
    ΔTcal = 6.2°C

    Cv = 4.16
    Vacid = 50mL
    ΔTacid = 6.2°C

    Vbase = 50mL
    ΔTbase = 5.2°C

    q2 = 2842.4J {correct?} Now, ΔH°2 = q2/moles of HCOOH

    How do I find the moles of HCOOH? I'm told that "some of the formic acid dissociated and some of the OH reacted with H3O, so you need to determine the mole sof HCOOH left over after the reaction of H3O and OH"

    How do I do that? and am I right so far?
  2. jcsd
  3. Mar 29, 2012 #2


    User Avatar

    Staff: Mentor

    It won't hurt if you will tell us what you really did, what are q1 and q2, and what are you expected to determine. We don't know how to read your mind, and what you wrote so far is not enough, it is chaotic and leaves a lot ambiguity. Some things you did are wrong for sure, no matter what you were trying to calculate.
  4. Mar 29, 2012 #3
    Ignore that whole question, I have a whole new question:

    if you have a galvanic cell set up like:

    Pt | 0.1M Fe3+, 0.1M Fe2+ || 0.1M AgNO3 | Ag

    What does it mean to write a balanced cell reaction equation and how do I do it?

    I know that the + electrode was Ag(s) so that means that the cathode was Ag meaning Ag was reduced from Ag2+. So is the answer Pt(s) + Ag2+(aq) -> Pt2+(aq) + Ag(s)???
    Last edited: Mar 29, 2012
  5. Mar 30, 2012 #4


    User Avatar

    Staff: Mentor

    Please don't pot new questions in old threads, start a new one.

    Pt is just an electrode material, it doesn't take part in the reaction.
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