i need to find Δf H (MgO) (change in heat of formation of MgO)

I've done experiments and got values for:
Δf H (Mg,Mg2+) = 69425.065 ± 2651.8 J
Δf H (H2O) = -285.8 kJ/mol (from a table)



Is this statement true? i don't believe it is, however it is what the lecturer wrote down.

Δf H(MgO) = Δf H(Mg,Mg2+) + Δf H(H2O) – ΔrxnH (MgO,Mg2+)





the Δf H of all the things above are in J/mol, however the ΔrxnH (MgO,Mg2+) which i have recorded experimentally is in Joules for a certain amount of solute/etc/etc.

Help me fix this eqn above, not too sure how to, and it's messing with my mind.

Eq:
H2O + Mg --> MgO + H2

Hess's law is :



in this case, you can make it product (MgO) - reactants (Mg,Mg2+ + H2O) = Rxn

by doing some algebra: product (MgO) = Rxn + reactants

so, for the reaction, it is assuming BCE which is why it is just in joules. for the formation, you just need to multiply by 1 since they already just 1 mol/reaction.

/e hopefully this is right/makes sense, its early in the morning for me

This post was edited by cialda on May 3 2013 06:58am

did the work myself and recieved the answer from your wisdom.




cheers.