October 7, 2011
CH-111
Chapter 5, Section 3
Thermochemistry
Here's where chem is about to get super-duper fun. Section 5.3 is all about the concept of Enthalpy.
Enthalpy: the internal energy plus the product of the pressure and volume of the system.
Basically, enthalpy is describing the heat flow in processes that occur under constant, or nearly constant, pressure. The properties we use to describe these systems are pressure (P) and volume (V), note capitals. A system consisting of gas confined to a container can be characterized by P and V. P and V are state funcitons because their values are dependent only on their current state and not on the path taken to that state (exothermic or endothermic processes). Enthalpy is denoted H, and therefore:
H=E+PV
The above is the equation describing what we've already said in our previous definiton of enthalpy: internal energy (E) plus the product of pressure (P) and volume (V) is equal to enthalpy (H).
The work involved in the expansion of compression of gases is called pressure-volume work (or P-V work). When pressure is constant in a process, the sign and magnitude of the pressure-volume work are given by:
w = -PΔV
work is equal to pressure multiplied by change in volume (volume-final minus volume-initial).
Note the negative sign. This is necessary in conforming to the sign conventions previously discussed for q and w. To be redundant and annoying, I'll reiterate them here:
The following are the sign conventions for q, w, and ΔE:
For q: + means gain of heat, - means loss of heat
For w: + means gain of heat, - means loss of heat
For ΔE: + means neet gain of energy by system, - means net loss of energy by system
Golly, I do enjoy banging a point home. But really, when it concerns sign conventions, who doesn't?
A little further explanation, particularly because I find the concept a little foreign and confusing. The pressure P is always either a positive number or zero. If the volume of the system expands, then ΔV is positive as well. Because the expanding system does work on the surroundings, w is negative (remember, -w means LOSS of heat, therefore if w is working on its surroundings, energy is flowing from the system to the surroundings, not in the system from, and therefore when an expanding system does work to its surroundings, w MUST be negative). Note that if the gas is compressed, ΔV is negative. Gas expanding, rather than compressing must have the opposite effect and therefore, work is being done ON the system BY the surroundings and w must be........... POSITIVE! Yes!
Back to enthalpy.
ΔH = Δ(E+PV)
Getting heavy with the equations, eh?
Learn to love them.
The equation shown above represents the change in enthalpy resulting from a change occurring at a constant pressure.
ΔH = Δ(E+PV) = ΔE + PΔV (constant pressure)
This equation representions the change in enthalpy which is given by the change in internal energy (ΔE) plus the product of the constant pressure (P, remember P is constant and therefore no Δ is before it), and the change in volume (ΔV).
At this point, it's important to remember that ΔE=q+w and taht the work involved in the expansion or compression of a gas is w=-PΔV. Why is this important you might ask?
Because: ΔH= ΔE + PΔV = (qp+w)-w=qp
I swear if this editor continues to muck up my text fomatting, I'm going to have to destroy it. However, that is the equation with all the necessary values plugged in. Get it? No, neither do I. What is with that tiny p??
What's with that tiny p is that it's a subscript on q. Well, what on God's Earth (or sometimes, on the Flying Spaghetti Monster's Earth) is it doing there?
The p subscript on the q value in the equaton hovering around up there is to indicate that the process occurs at a constant pressure. And thus, we bring it all home:
The change in enthalpy equals the heat qp gained or lost at a constant pressure.
When ΔH is positive (or qp is positive), the system has gained heat from the surroundings, which means the process is endothermic.
When ΔH is negative (or qp is negative, reduuundant), the system has lost heat to the surroundings, which means the process is exothermic.