Determination of Solution Enthalpy: An Easy and Simple Method K. K a r u n a k a r a n V.O. Chidambaram College Tuticorin-628003, India The conventional method of determining the solution enemploys a complicated procedure involving the thalpy, Ma, nreoaration and analvsis of saturated solutions a t different . . temperatures. The main drawback of this method lies in accurately obtaining and analyzing samples of the solutions. In our method, a system of known weight of the liquid solvent and solid solute taken in the freezing point determination set-up is first heated to a temperature of 5-6'C above the complete dissolution point. It is then gradually cooled, and the kmpersture at which the firit solute crystal just sepmites from the solution is noted. Since at this temprrnture the equilibrium between solid solutl. and saturatrd solution is just established, this is the saturation temprrature. 'I:, fim the composition taken. The cumpusition of the system IS then altered by adding a known weight of the solvent imd the saturation mints are found as brti~refor each composition. A plot of In the mole fraction of solute) versus 1/T, is made and AH. is calculated from the slope. This method was successfully employed for a number of solute-solvent systems, such as naphthalene-toluene and
Compact Compacts J a y S. Huebner, Department of Natural Sciences, University of North Florida, Jacksonville, suggests the following demonstration. Many introductory science courses consider that oxidizing hydrocarbons and carbohydrates in combustion and respiration nroduce water as a bv-vroduct. This process can be illustratkd inexpensively by placing a burning candle inside an inverted clear elass iar. Usina a 1-1iar, on a humid day, water droplets f o g g 2 over the insl'de &face in 15 see. he candle
went out in 25 sec. Using dessicant (CaSOd dried air, the water droplets were visible in 20 sec. A second demonstration of v w i h l e interest is to then warm one side of the jar by firmly it with one hand. In 15sec, the water driplets "vapor transport" to cooler parts of the jar, clearing the part of the jar warmed by the hand. A simple model for demonstrating primary, secondary, and rertiary protein structure has heen developed by H. R. Shiflett of Meredith Colleee. - . Raleinh. - . North Carolina. .4nv. larae spring such as those used in physics to demonstrate simple harmonic motion or the common children's tov known as the ''Slinky"TMcan be an excellent model for demonstrating the various structural features of a protein molecule. The nrimaw structure of helical areas is easily demonstrated by using cardboard cutouts of various amino acids with dimensions appropriatr to the diameter of the spring used and noting that there arr ?.fiamino acids per turn. These amino acids can he taped to the spring in such s manner as tu show hotv thr secondary structure arlses due to hydrogen bonding. Finally, the tertiary structure is then demonstrated by having sevrral students srretch the spring and then fold it until the amino acids capable of forming disulfide h n d s are adjacent. Harvey F. Blanck, Austin Peay State {Jniversity, Clarksville.Tennessee. reports that discussion of applications of the firstlaw of thermodynamics to the conservat&n of energy, especially electrical energy, have been well-received locally hv students and civic organizations. The following examole rarelv fails to generate an enthusiastic response. The first law of ~ h e r m o d y ~ a m idictates cs that whatever energy is used to operate electrical appliances must end up as either potential or kinetic energy. Except for the evaporation of water, nearly all electrical energy "consumed" by electrical equipment (refrigerators, televisions, stoves, etc.) ends up in the form of thermal motion ("heat"). Hence, for a house heated hv direct electrical enerm (as opposed to a heat pump) it matteis not in the least whAher thkknergy is suppiied by the ooeration of a T V or an electrical heater. In other words, e1ect;ical equipment costs nothing tooperate in the winter if the building is heated by direct electrical means, and only about one half if a heat pump is used. If air conditioning is used in the summer, electrical equipment operating costs are ahout 50%more than the actual cost of theenergy consumed by the equipment since the "heat" generated must be pumptd out by the Hir conditioner.
Volume 56, Number 6,June 1979 / 389
