Thermo Chem?

[destain]

In some questions that i've been doing, it seems to take the coefficient of the reactant as the number of mols? Is there a reason for this?
E.g

If each mole of A releases 400kJ when it reacts with B according to : 5A + 3B ----> 11C
Write a thermochemical equation for the given reaction.

And the answer would have deltaH = -2000kJ/mol

So, I assume they took the coefficient as the no. of mol? Why?

[nisha]

In some questions that i've been doing, it seems to take the coefficient of the reactant as the number of mols? Is there a reason for this?
E.g

If each mole of A releases 400kJ when it reacts with B according to : 5A + 3B ----> 11C
Write a thermochemical equation for the given reaction.

And the answer would have deltaH = -2000kJ/mol

So, I assume they took the coefficient as the no. of mol? Why?

As per my understanding, the energy provided would be 400kJ/mol. Therefore, because we want the change in H over the whole reaction, we must multiply this by five to make the ratios the same, as in the reaction.
Normally, I just use the ratio:
Energy to no.moles needed to provide this energy
change in H to coefficient of limiting reactant/reactant in question.

Therefore:
400kJ->1 mole
change in H->5

Evaluating the ratio gives the answer.

[charmanderp]

If it supplies a delta H value, that value is for the coefficient's worth of moles for any substance.

[destain]

yeah but what i don't understand is how it's assumed that there's 5 mol of A, because it just says that 400 refers to 1 mol of A, the coefficient of A is 5 but that doesn't mean that there's 5 mol of A does it

[Hancock]

Te enthalpy change, dH isn't sort of stupid, because it's units of kJ / mol isn't really the amount of energy released by 1 mole of the reactant.

The dH is dependant on the equation. I don't really know why, but people are correct in saying that it should be 2000 kJ / mol for this reaction, since the coefficient of the first term is 5.

[kenhung123]

Enthalpy values by convention should refer to the coefficient.
You are right in saying that there isn't necessarily 5 moles of A in a given reaction.

[destain]

so what's the final verdict? whenever there's no definite value of mole given, you assume the coefficient is the no. of mol?

[kenhung123]

Enthalpy values always refer to the coefficient in the reaction
Heat of combustion always refers to 1 mole of reactant

But when doing calculations, you use these values to calculate the energy of your given/required amount of reactant by ratios.

[destain]

ANOTHER THERMO CHEM QUESTION

.............
there are so many equations, i get a little confused,
so..thing is for Energy released - DeltaH/coefficient x n

Energy change/released - CF x DeltaT

are these the same thing? as in..it works out the same thing?