What is the change in reaction enthalpy ( ) for the following
2 NO (g) + 2 O2 (g) + 2 N2O (g) → 2 N2O4 (g) + N2 (g) = ?
having carried out the experiments listed below
2 N2 (g)+ O2 (g)→ 2 N2O (g) = +164.1 kJ
N2 (g)+ O2 (g)→ 2 NO (g) = +180.5 kJ
N2 (g)+ 2 O2 (g)→ N2O4 (g) = +9.2 kJ
2006-10-20 08:06:16 · 2 answers · asked by hockeyislife21 1 in Science & Mathematics ➔ Chemistry
You've got to add the equations to get the one you want the delta H for. Hess's law says no matter what ridiculous reactions and paths you take to get to a result, it'll give you the same H (enthalpy) values as going straight there.
email me at calcu_lust@yahoo.com, and I will then send you a neatly typed solution, which I really can't make too good in this box
2006-10-20 08:18:25 · answer #1 · answered by calcu_lust 3 · 0⤊ 0⤋
Suppose a reaction is carried out in several steps. We have already seen that the overall reaction is the algebraic sum of the individual steps. Because energy/enthalpy is an additive quantity,
the overall enthalpy change for the overall reaction, obtained by adding the individual chemical equations, will be the sum of the enthalpy changes associated with each of the individual steps/chemical equations.
The above statement is known as Hess's Law.
Hess's Law can be used to calculate the enthalpy of reactions that cannot be carried out straightforwardly in the lab. As an example, consider the reaction of solid carbon, in the form of graphite, with oxygen gas to produce carbon monoxide:
This reaction cannot be carried out directly experimentally. Rather, a two-step process is used, in which solid carbon is first converted to carbon dioxide via the reaction
whose enthalpy can be easily measured at 25 C. Then the decomposition of CO to give oxygen gas and carbon monoxide can be added to this reaction according to:
2006-10-20 08:59:31 · answer #2 · answered by SAM M 4 · 0⤊ 0⤋