Enthalpy of a Reaction

[kenhung123]

Can we say that the enthalpy of 1 mole HCl in the reaction HCl+NaOH=>H2O+NaCl is 1321J? Or is the enthalpy of 1 mole HCl reacting with 1 mole NaOH is 1321J?

Because quite a lot of the time we use calorimeter to find the energy released by a known mole of reactant, then we use this to find the specific energy released for the mole of the reactant in the equation. This assumes that the enthalpy of reactant=enthalpy of reaction...

[happyhappyland]

thats a good question... I think the one which is limiting.. say HCL was limiting then you say 1 mol of HCL. beacuse for combustion reactions you see CH4 + 02 you tend to say 1mole of CH4 releases x amount of energy even though o2 is in excess

[kenhung123]

Hmm I see but that means if HCl is excess NaOH releases no energy at all

[masonnnn]

We went over something just like this today,
it is the limiting reactant
ie. if the concentrations were 1.0M HCL and 1.5M NaOH then you'd say 1.0 mole HCL produces/releases 1321J of energy.
once you've figured that out you change the answer to the enthalpy symbol(AFTER, as enthalpy is a different thing) and it will be the opposite (negative if positive)
ie, enthalpy = -1321J because the reactants 'produced' or lost 1321J.
i'm no chemistry scholar though, someone feel free to correct me.

[kenhung123]

Yea I understand you use limiting reagent to calculate the energy released/absorbed but like I am saying does the enthalpy of a given reactant refer to individual reactants or all reactants in the case above: Is 1312J released by 1 mole of HCl or 1 mole of HCl and 1 mole of NaOH (2moles of reactants)