CHEM 481 Lecture Notes _Engle 2nd - Chapter 4_

CHEM 481 Lecture Notes _Engle 2nd - Chapter 4_ - Chapter 2-...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Last printed 2/16/2010 4:43 PM 2-1 Chapter 2- Thermochemistry Use calorimetry to measure the heat (q) produced or absorbed by a chemical or physical process. We can then equate q with either U if process is carried out at constant V H if process is carried out at constant P Conversely, if we know U or H we can predict the amount heat produced (or required). We will develop the methodology for H, because most of chemistry is carried out in open vessels (i.e. constant pressure). Standard Enthalpy Change ( H o ): Change in Enthalpy when the initial and final substances are in their standard states (i.e. pure form at 1 bar and specified T) A. Enthalpy of Physical Change Change in enthalpy associated with the change in physical state of a substance 0 vap H Δ heat of vaporization l g 0 fus H Δ heat of fusion s l What about s g (sublimation)? Since H is a state function, Δ H is independent of the path. Therefore, Step #1 s l 0 fus H Δ Step #2 l g 0 vap H Δ Overall s g 000 vap sub fus HHH Δ= Δ+ Δ Although in the sublimation case, the system never passes thru the liquid state, we are able to express 0 sub H Δ as a sum because it is a state function. This is a simple example of “Hess’ Law” Hess’ Law: The standard enthalpy of an overall reaction is the sum of the standard enthalpy changes of the individual reactions into which the overall reaction can be divided. Note: Individual reactions may be hypothetical, that is the system may not pass thru that state. s l g 0 fus H Δ 0 vap H Δ 0 sub H Δ
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Last printed 2/16/2010 4:43 PM 2-2 B. Enthalpy of Chemical Change Consider a general chemical reaction: A 2B As a chemical reaction progresses the amounts of each species change. Since H is an extensive property, then dH (or H) can be written as: N N m,A A m,B B Change Enthalpy in amt per mole of A dH H dn H dn =+ (at constant T and P) o
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 03/14/2011 for the course CHEM 481 taught by Professor Berkowitz during the Spring '09 term at UNC.

Page1 / 6

CHEM 481 Lecture Notes _Engle 2nd - Chapter 4_ - Chapter 2-...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online