Enthalpy of Combustion for Magnesium

  1. I placed some magnesium ribbon in HCl and measured the temperature change. How would i find the change in enthalpy per mole of magnesium in the following reaction:
    Mg + 2HCl --> H2 +MgCl2
    If i know the following information: change in temperature = 19C
    mass of ribbon = 0.5g
    volume of HCl = 100mL

    I can't use any known values (i.e. like the ones you'd find in a textbook). I must use the experimental values above. However, i can use specific heat capacity values, c, if i must.
  2. jcsd
  3. [tex]Q=mc\Delta t[/tex]
    where Q is the enthalpy change. You cannot just plug numbers into this formula though; remember that it applies to the entire system, and can be used in conjuction with
    [tex]\mbox{heat gain}=-\mbox{(heat loss)}[/tex]
    How is heat energy transfered in the reaction? (what gains, what loses?)
  4. Did you perform that experiment in a "bomb calorimeter" (or a suitable improvised version thereof)? If not, the temperature change is meaningless, because an unknown amount of heat was dissipated to the surroundings.
  5. Astronuc

    Staff: Mentor

    As Pack_rat2 alluded to, ideally one is doing this in an adiabatic system so that heat is not lost to the environment outside the reaction vessel (presumably test tube). One might have to correct of the mass of the reaction vessel as well if it is heated.

    From the mass of HCl solution and temperature, one can determine the change in enthalpy of the solution.

    Then assuming that all the heat originated from the chemical reaction - one can determine the energy per unit mass or mole of Mg (the other known quantity).
  6. But what will i use the specific heat capacity of HCl? I can't find that in my textbook.
  7. The Specific heat capacity of HCl (s) is 3.93 Jg-1C-1
  8. Borek

    Staff: Mentor

    I suppose reaction was going on in a relatively diluted solution of the acid. If so, use specific heat of water.
  9. Yea I would just use the specific heat capacity of water. My class just did this lab the other day. We used 0.5 M HCl and the teacher did some calculations on the board to show us that only about 0.2%(or somewhere around there) of the solution was HCl and the rest was water.
  10. Borek

    Staff: Mentor

    More like 1.8% (closer to 2 then to 0.2).

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