chemical principle1 revifited
--. .- - MURIEL BOYD BISHOP Clemson University
Clemson. SC 29831
Teaching Freezing Point Lowering Jenelle Ball Chico Senior High School, Chico, CA 95926 Ron C. Cooke and Gmver Willk California State University-Chico, Chico. CA 95929 Should we not teach the principles of chemistry in as unified a manner as possible? In particular, should we not teach freezing point lowering in the same manner that we do all chemical equilibria? For example, consider the equilibrium between ice and pure water a t 0 OC (273.15 K) as seen in eq 1.
The symbols X stand for the mole fraction units, which are the concentration units customarily used for a solid or solvent when discussine eauilibria. Both values are unitv initially because the ice is pure and the water is pure. v
.
Le Chatelier's Prlnclple If we now wish to explain qualitatively the lowering of the freezing temperature of our system caused by dissolving some automobile antifreeze, ethylene glycol, in the water (the ice remains pure), we may use Le Chatelier's principle. This principle states that, if, to a system originally a t equilibrium, a "stress" is applied, either a forward or reverse reaction will begin, whichever one will counteract the stress. A stress may be a change in an intensive property of the system, such as the temperature, the pressure, or the concentration of any reactant or product. In our example the initial stress is the lowering of the concentration (mole fraction) of the water by the addition of ethylene glycol. The forward reaction, the one that produces more water in order to resist the lowering of its concentration, begins. However, the forward reaction absorbs heat, causing the mixture t o cool. This lowering of the tempera-
676
Journal of Chemical Education
ture constitutes an opposing stress causing the exothermic backward reaction to occur, counteracting the lowering of the temperature. As the melting continues, the stress favoring the forward reaction diminishes as the concentration of the water increases, and the stress favoring the reverse reaction increases as the temperature decreases, until they both exactly balance a t a new, lower temperature equilibrium. The van't Hofl Equation For the quantitative treatment of the temperature dependence of any chemical equilibrium one most often uses eq 2, the approximate form of the van? Hoff equation.
Tz refers to a higher (Kelvin) temperature where the equilibrium constant is K2. TI and K1 are the values for a lower temperature. AH,.di,, is the heat (joules. mole-') absorbed by the reaction (melting), and R is the gas constant, 8.314 joules per mole degree. For our case we let the subscript 2 refer t o the ice-pure water system a t its freezing temperature, so that T2= 273.15 and Kz = XHZ~(y/XHZo,sj = 111= 1.Subscript 1refers to the t the water is a t molesolution a t its reezme- ~. o i n where fraction X, T I = some temperature T, K , = XI1 (or X), and AH,is the molar heat of fusion of ice. The van? Hoff relationship (eq 3) for our system then reads:
