q and Enthalpy

Moderators: Chem_Mod, Chem_Admin

Orly Termeie 3I
Posts: 94
Joined: Fri Sep 25, 2015 3:00 am

q and Enthalpy

Postby Orly Termeie 3I » Sat Jan 09, 2016 5:12 pm

At constant pressure why does q equal Enthalpy? Thanks!

Sandeep Gurram 2E
Posts: 42
Joined: Fri Sep 25, 2015 3:00 am

Re: q and Enthalpy

Postby Sandeep Gurram 2E » Sat Jan 09, 2016 5:30 pm

I'll take a mathematical perspective to show why q = Enthalpy at constant pressure:

Change in Enthalpy as a state function can be represented by the equation: ΔH = ΔU + PΔV. Where H is enthalpy, U is internal energy, P is pressure, and V is volume. To show that q = enthalpy at constant pressure we can use basic mathematical manipulation. Firstly, we can insert the equation ΔU = q + w into our original equation. Now, we have ΔH = q + w + PΔV.

Next, we can assume that the system can only do expansion work and is incapable of reversible expansion. Because of this, we can plug yet another equation in: w = -PexΔV. Now we have ΔH = q - PexΔV + PΔV. Because the system is open to the atmosphere, internal pressure and external pressure are equivalent - therefore Pex = P. These terms cancel each other out leaving us with our desired equation, ΔH = q. Basically, it comes down to mathematical manipulation of equations.

This process is outlined clearly on page 278-279 in the green box if you need further clarification. Hopefully that helped!

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 2 guests