Find the heat of formation for a mole of hydrazine (N2H4) given: 2N2H4(l) + 2O2(

Question

Find the heat of formation for a mole of hydrazine (N2H4) given: 2N2H4(l) + 2O2(g) ? 2N2(g) + 4H2O(g) ? Ho = -1068.4 kJ H2(g) + 1/2O2(g) ? H2O(g) ? Ho = -241.8 kJ Hint: In a formation reaction you form one mole of the product and the reactants must all be elements in their most common states. Write out the formation reaction and then use Hess' Law, and the reactions that you are given, to calculate its ?H.

Explanation / Answer

let 2N2H4(l) + 2O2(g) ? 2N2(g) + 4H2O(g) ? Ho = -1068.4 kJ ---> equation1 H2(g) + 1/2O2(g) ? H2O(g) ? Ho = -241.8 kJ --------> equation2 in order to make N2H4 to become the product, we have to shift equation 1 so it will be 2N2(g) + 4H2O(g) ?2N2H4(l) + 2O2(g) ? Ho = +1068.4 kJ and the equation 2 must be multiply with 4 in order to cancel out both of the equation. the new equation will be ; 4H2(g) + 2O2(g) ? 4H2O(g) ? Ho = ( -241.8 kJ x4) so then we can cancel out H2O and O2. the remaining is 2N2 + 4H2 -----> 2N2H4 ? formation = +1068.4kJ + (-967.2kJ) = +101.2kJ here we get 2 moles of N2H4, we divide the above equation by 2, we get N2 + 2H2 -----> N2H4 ? formation = +101.2/2 kJ = +50.6kJ

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