Thursday
July 31, 2014

Homework Help: Chemistry

Posted by Kjd on Monday, October 15, 2012 at 10:33pm.

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!

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