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March 26, 2017

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Zinc dissolves in acid according to the balanced reaction:

Zn(s) + 2H^+(aq) -> Zn^2+(aq) + H2(g)

A sample of zinc is placed in the ice calorimeter. If 0.0657g of zinc causes a decrease of 0.109mL in the ice/water volume of the calorimeter, what is the enthalpy change, per mole of zinc, for the above reaction per mole of zinc.

Extra info:
Given that the enthalpy change for the process H2O(s) -> H2O(l) is +6.01 kJ/mol or 333 J/g, it can be determined that 3.68 kJ are released per mL change in the volume of the ice/water mixture.

This is what I have so far..

N(zinc)= m(zinc)/M(zinc) = 0.001 mol Zn

q(calorimeter)= -3.68 kJ/mL x 0.109mL
= -0.401 kJ

-q(rxn) = q(calorimeter)
q(rxn)= 0.401 kJ

(delta)H = 0.401 kJ/0.001 mol
= 401 kJ/mol

The answer is right, it's just that I do not understand why we would use 3.68 kJ/mL and multiply it with 0.109 mL?

Please help, much appreciated. Thank you!

  • Chemistry - ,

    3.68kj of energy are released per ml so it is actually 3.68kj/ml you need to cancel out the ml by multiplying to get the heat lost by ice water for this specific amount of ice water

  • Chemistry - ,

    Thanks!

  • Chemistry - ,

    Because we need to Cancel the mL and keep kj our main unit and the heat is released as a form of energy which is measured in Kj so basically u are multiplying these 2 numbers just for the sake of getting a pure q(calorimeter) in its energy form i.e Kj because ML is not an energy unit its used to measure volume and when u use volume ml is not used often it is mostly converted into Liters (L). I wish this helped!

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