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Homework Help: Thermodynamics help!

  1. Nov 9, 2004 #1
    Alright, so we did three labs, and for some reason I just cannot get lab B. It's so dang frustrating, too. What we did was mix 50 mL of 1.00 molar hydrochloric acid with 2.00 grams of sodium bicarbonate.

    We made our own calorimeter, they are styrofoam cups, and their specific heat is 16.7. Anyway, our 'heat evolved' (final heat) was 18.7 degrees, and the HCl started at 21.5 degrees. I think I calculated the lab heat alright:

    specific heat of water/HCL * mass of hcl (grams) * change in temp + 16.7(some heat of the cup) * change in temp

    4.18 * 0.05 * -2,8 + 16.7 * -2.8
    To get -47.3452....

    Please tell me if there is an error there.

    Anyhoo....now to get the theoretical heat, im just...i dunno...missing something huge. Black means solid, red means liquid, green means gas, blue means aqueos.

    HCl + NaHC0_3 -> NaCl + CO_2 + H_2O

    Then (and I think I might be doing something wrng here too) I split them up so I can look at their heats of formation:

    H + Cl + Na + HCO_3 -> Na + Cl + C0_2 + H_2O
    Now, I figured since both chlorines are aqueous, they cancel out, and hydrogen, CO_2 and H_2O all have 0 heat of formation.

    So I figured HC0_3 is -691.1 (from a table), Na is -239.7 (from a table), and my theoretical heat is 451.4 KJ/mol.

    Where did I go wrong?
  2. jcsd
  3. Nov 9, 2004 #2
    Can anyone help me with this? I need it for tomorrow!
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