[neon]

All the substances involved in the reaction given below can undergo combustion reaction and their combustion enthalpies at 300K are given below,
2A(g)+B(g)  3C(g) + D(g)
Given: ΔH A = -1000cal/mol ΔH B= -2000cal/mol ΔH C=-500 cal/mol ΔH D = -100 cal/mol
If the reaction occurs at constant volume at 300K and the reaction obeys first order with respect to A and if A reduces to half its initial amount in 69.3 sec, then calculate

1) heat released when 2*10^-3 mole of A has reacted at 300K(in cal)

2) rate at which heat is released initially, if initially 1mol of A is present.

ΔH of rxn = 2400 cal/mol
ΔH = ΔU + Δn_g RT
Since the reaction occurs at constant volume ΔQ = ΔU
ΔQ = ΔH - Δn_g RT

Substituting appropriate values , I get ΔQ = 1800 cal/mol. But this is not the correct answer.

[mjc123]

ΔH = ΔU + PΔV
Constant volume, so ΔH = ΔU
At constant P and T, PΔV = Δn_gRT - but that is not the case here.
And remember, question 1 does not ask for the heat per mole.

[neon]

My book states that ΔH = -2400cal/mol. But as far what I know, ΔH = ΔH_prod - ΔH_react. My book seems to follow the reverse of this.

[mjc123]

What you say is true if the ΔHs are the heats of formation of the compounds. But here you have heats of combustion. The formation of A is a reaction in which A is a product. The combustion of A is a reaction in which A is a reactant. So the signs are opposite. Try drawing a Hess's Law cycle and see how it works. (Assume the combustion products of (2A + B) are the same as the combustion products of (3C + D).)