Thermochemistry and Hess' Law

The first lecture of Week 5 found us tackling thermochemistry, that subset of thermodynamics describing heat transfer that accompanies chemical reactions. In constant-volume calorimeters (often closed vessels), the heat transfer q we measure is ∆_rxnU, whereas in constant-pressure calorimeters (open vessels), q will be ∆_rxnH. (On Wednesday, we will see a simple method to interconvert them). Since reactions are typically performed at constant temperature and pressure, any results give us important information about the energy stored in chemical bonds.

Using the properties of state functions, we can predict the heat transfer under these two conditions using Hess' Law and, since constant-pressure conditions are more common in chemical systems, we tend to focus on ∆_rxnH rather than ∆_rxnU. The norm is to cast all reactions as simple sums of formation reactions, each of which represents the formation of 1 mole of a substance from constituent elements in their standard states/phases. Hess' Law is particularly powerful in thermochemistry because it applies equally well for any extensive state property. Note that this is our second usage of the ∆ symbol (the first being the familiar ∆Y = Y_final - Y_initial). Whenever the subscript appears on the ∆ itself, as in ∆_combY, we are calculating the sum of the products minus the sum of the reactants, each multiplied by appropriate stoichiometric coefficients. Hopefully this operation is still familiar from general chemistry.

Next lecture, we will finish thermochemistry and begin to tackle entropy, one of the most important and poorly understood concepts in all of science. If your intrepid instructor has the backbone to trudge through the quiz 2 carnage, he may be able to return them on Wednesday. I will admit that the uncharacteristic dearth of questions/comments/emails/office visits so far this quarter (quickly approaching the 50%-done mark) had lulled me to mistakenly believe that this class was further along the thermodynamic path than it actually was. Hopefully quiz 2 will be a valuable learning experience for many as we lumber towards Thursday (and remember, no class on Friday).