Altitude analogy

Note that arbitrarily assigning zero AHf for each element in its most stable form at the standard state does not affect our calculations in any way. Remember, in thermochemistry we are interested only in enthalpy changes, because they can be determined experimentally whereas the absolute enthalpy values cannot. The choice of a zero reference level for enthalpy makes calculations easier to handle. Again referring to the terrestrial altitude analogy, we find that Mt. Everest is 8708 ft higher than Mt. McKinley. This difference in altitude is unaffected by the decision to set sea level at 0 ft or at 1000 ft. 

To understand why this is so, recall that cell potentials are analogous to altitude differences for water. Whether 10,000 or 20,000 L of water flows down a spillway, the altitude difference between the top and bottom of the spillway is the same. In the same way, multiplying a reaction by some integer changes the total number of moles of electrons transferred, but it does not change the potential difference through which the electrons are transferred. We return to this point in Section 19-1. 

The water in the saturated zone settles with a smooth horizontal upper face, called the water face, or water table. By analogy, water poured into a vessel settles with a smooth horizontal upper face—a miniature water table. The depth of the water table, or the water level, is expressed either in relation to the land surface or as the altitude above sea level. The depth of the water table varies from zero, in marshy areas, to several hundred meters, in dry mountainous terrains. A depth of a few tens of meters is common. 

If instead of the reflection method mentioned above wc use a method analogous to Laue s, the possibilities of error in the determination of Eq are very much greater, as the diffraction maximum is then determined by three quantities, the azimuth, the altitude, and the electron velocity. 

The choice of an arbitrary reference for measuring electrode potential is analogous to choosing the surface of the ocean as the reference for altitude, calling it zero meters, and then referring to any terrestrial altitude as being a certain number of meters above or below sea level. 

This equation is analogous to the equation for the change in barometric pressure B with altitude h under gravitational acceleration g ... 

Let us assume that the vertical concentration distribution of A can be represented generally as cA = cA exp(—//a ) by analogy to the exponential decrease of pressure with altitude, p = poexp(-Hz). Show that the tropospheric lifetime of A over the tropospheric height Hj- is given by... 

C. Clinical Uses The major application of carbonic anhydrase inhibitors is in the treatment of glaucoma. Acetazolamide must be administered orally, but topical analogs are now available (dorzolamide, brinzolamide) for use in the eye. Carbonic anhydrase inhibitors are also used to prevent acute mountain (high-altitude) sickness. These agents are used for their diuretic effect only if edema is accompanied by significant metabolic alkalosis. 

Just as the average sea level serves as the zero level for geographical altitude readings, the state of matter where the substances are decomposed into their elements at standard conditions represents the zero level of the potential scale. Analogously, Celsius temperature readings usually used in everyday life can replace those of absolute temperature if melting ice is chosen as reference state. 

An analogy may help you understand the difference between quantities that are state functions and those that are not. Suppose you drive from Chicago, which is 596 ft above sea level, to Denver, which is 5280 ft above sea level. No matter which route you take, the altitude change is 4684 ft. The distance you travel, however, depends on your route. Altitude is analogous to a state function because the change in altitude is independent of the path taken. Distance traveled is not a state function. 

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