[sega18]

Hello everyone,

I have a couple of thermochemical problems that I am having trouble with:

Consider the following reaction, which can be used in a fuel cell (a type of battery):
O2(g) + 4 H+(aq) + 2 Ni(s) → 2 Ni2+(aq) + 2 H2O(l) DeltaE = –682.1 kJ
a) Calculate DeltaH for this reaction at 25ºC.

To solve this problem, I need to subtract the enthalpies of formation of the products from that of the reactants, but I can't find a value for the enthalpy of formation of Ni2+. Is there an alternate route that I'm missing?

Chemist A burns 10.000 grams of C2H6 in a constant-volume container at 25ºC. Chemist B
burns an unknown mass of C2H6 in a constant-pressure container at 25ºC. The chemists observe
equal amounts of heat. Assuming no useful work is done in either case, calculate the mass of
C2H6 that was used by chemist B. (Refer to problem 4 for useful information.)

I assume that for this problem I need to set the two reactions equal to one another, because the heats measured are equal. Unfortunately I am not clear on which formula to use.

Thanks in advance,

Sega

[Caustikola]

Hint:
enthalpy of formation of H+ &Ni2+ are their ionization energies

[Zeppos10]

I do not believe ionisation-energies will get you anywhere.
the following might work (D=delta):
1. guess that DE= -681.2 is the change in internal energy, normally indicated as U.
2. the relation between U and H: DH = DU + pDV.
3. DV is determined here by the reaction of 2 mol O2(g)
4. ideal gas at constant p and T: pDV=RTDn = 8.341*298*(-2)=-4.95kJ
5. DH=-681.2-4.95=-687 kJ
you can look up the enthalpies of all participating substances:
my data tell that DH of the given reaction =-692kJ: close enough.