Appropriate equation for standard enthalpy of formation

Moderators: Chem_Mod, Chem_Admin

Casey Collet 1I
Posts: 23
Joined: Fri Sep 26, 2014 2:02 pm

Appropriate equation for standard enthalpy of formation

Postby Casey Collet 1I » Sat Jan 24, 2015 4:43 pm

When a question asks for the standard enthalpy of formation of a compound, how do you know what equation that you want to analyze for the reaction? For example, in homework problem 7.71, the solutions manual says that we want the equation N2+(5/2)O2->N2O5, but that is not a given reaction. By looking at the problem, how are we supposed to know that we want this equation?

Niharika Reddy 1D
Posts: 127
Joined: Fri Sep 26, 2014 2:02 pm

Re: Appropriate equation for standard enthalpy of formation

Postby Niharika Reddy 1D » Sat Jan 24, 2015 5:07 pm

We need to find the standard enthalpy of formation of dinitrogen pentoxide. The compound should be formed from its elements in their most stable states. For N2O5, the elements are nitrogen and oxygen, both of which are present as diatomic gas molecules in their most stable form. Hence, the reaction we want will be the balanced form of the formation of 1 mol of the compound at hand from its elements in their most stable form.

For this particular problem, that formation reaction isn't taken into account when finding the enthalpy of formation of dinitrogen pentoxide, but in a problem like question 75, where we are asked to find the enthalpies of formation of different compounds, the chemical equations are written in this way, and the balanced equation is used to find the standard enthalpy of formation.


Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

Who is online

Users browsing this forum: No registered users and 1 guest