How do changes in pressure affect equilibria? You will recall that if the pressure of a gas is reduced, its volume will increase; pressure and volume are inversely proportional. With this in mind, suppose that the reaction 2 NO2(g) → N2O4(g) is in equilibrium at some arbitrary temperature and pressure, and that we double the pressure, perhaps by compressing the mixture to a smaller volume. From the Le Châtelier principle we know that the equilibrium state will change to one that tends to counteract the increase in pressure. This can occur if some of the NO2 reacts to form more of the dinitrogen tetroxide, since two moles of gas are being removed from the system for every mole of N2O4 formed, thereby decreasing the total volume of the system. Thus increasing the pressure will shift this equilibrium to the right. It is important to understand that changing the pressure will have a significant effect only on reactions in which there is a change in the number of moles of gas. For the above reaction, this change Δng = (nproducts – nreactants) = 1 – 2 = –1. In the case of the nitrogen oxidation reaction N2 + O2 → 2 NO, Δng = 0 and pressure will have no effect. The volumes of solids and liquids are hardly affected by the pressure at all, so for reactions that do not involve gaseous substances, the effects of pressure changes are ordinarily negligible. Exceptions arise under conditions of very high pressure such as exist in the interior of the Earth or near the bottom of the ocean. A good example is the dissolution of calcium carbonate CaCO3(s) → Ca2+ + CO32–. There is a slight decrease in the volume when this reaction takes place, so an increase in the pressure will shift the equilibrium to the right, with the results that calcium carbonate becomes more soluble at higher pressures. The skeletons of several varieties of microscopic organisms that inhabit the top of the ocean are made of CaCO3, so there is a continual rain of this substance toward the bottom of the ocean as these organisms die. As a consequence, the floor of the Atlantic ocean is covered with a blanket of calcium carbonate. This is not true for the Pacific ocean, which is deeper; once the skeletons fall below a certain depth, the higher pressure causes them to dissolve. Some of the seamounts (undersea mountains) in the Pacific extend above the solubility boundary so that their upper parts are covered with CaCO3 sediments. More on marine sediments The effect of pressure on a reaction involving substances whose boiling points fall within the range of commonly encountered temperature will be sensitive to the states of these substances at the temperature of interest. For reactions involving gases, only changes in the partial pressures of those gases directly involved in the reaction are important; the presence of other gases has no effect. Problem example 1 The commercial production of hydrogen is carried out by treating natural gas with steam at high temperatures and in the presence of a catalyst (“steam reforming of methane”): CH4 + H2O → CH3OH + H2 Given the following boiling points: CH4 (methane) = –161°C, H2O = 100°C, CH3OH = 65°, H2 = –253°C, predict the effects of an increase in the total pressure on this equilibrium at 50°, 75° and 120°C. Solution: Calculate the change in the moles of gas for each process: temp equation Δng shift 50° CH4(g) + H2O(l) → CH3OH(l) + H2(g) 0 none 75° CH4(g) + H2O(l) → CH3OH(g) + H2(g) +1 to left 120° CH4(g) + H2O(g) → CH3OH(g) + H2(g)
Posted on: Wed, 19 Nov 2014 18:54:18 +0000
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