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Enthalpy problem

  1. Aug 25, 2010 #1
    1. The problem statement, all variables and given/known data

    I am working on a lab that has me a little confused:

    I diluted 5.5g of NaOH into 200ml of water to disassociate the sodium ions. In the equation below you can see my temperature results.

    Write the net ionic equation for the reaction, and note the value of ΔH.

    NaOH ---> Na + OH

    2. Relevant equations

    ΔH = m•ΔT•Q
    ΔH = (200 g + 5.5 g)(final temp - initial temp)(4.18 J/g°C)
    ΔH = (205.5 g)(41 °C - 26 °C)(4.18 J/g°C)
    ΔH = 12.9 kJ

    3. The attempt at a solution

    Because it's an exothermic reaction (It created heat....), I know ΔH should be negative. Why is it coming up positive? I'm lost here... I'm pretty sure I've done everything correctly. Thanks!
    Last edited: Aug 25, 2010
  2. jcsd
  3. Aug 26, 2010 #2


    User Avatar

    Staff: Mentor

    Don't worry. You have correctly calculated amount of heat that was absorbed by the solution - it was gained by the solution, so it was 'lost' by the NaOH. Just change the sign.

    Beware, correct reaction equation is

    NaOH -> Na+ + OH-

    you can't ignore ions/charges.
    Last edited by a moderator: Aug 13, 2013
  4. Sep 23, 2010 #3
    So would the correct answer to the question be negative? I'm a little confused. I understand the logic, but can I just arbitrarily change the symbol like that?
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