## Effect of Pressure on Gas Phase Chemical Equilibria

Instead of partial pressures, mixture compositions are often more conveniently expressed in terms of the mole fractions of the species present, as in the previous example. The following formulation illustrates how total pressure affects the equilibrium composition. Using Dalton's Rule (Eq (7.3), pi = where xi is the mole fraction of species i and p is the total pressure, Eq (9.21) becomes:

K — p^reactants products ) ^ k — Products (9 24)

reactants

Although KP is a function of temperature only, the equilibrium constant in terms of mole fractions, K, is also total-pressure dependent. The pressure term multiplying KP demonstrates Le Chatelier's principle: increasing the total pressure in a gas phase reaction favors the .side of the reaction with the fewest number of moles. With respect to Eq (9.24), the pressure effect is determined by the exponent of p; if the sum of the stoichiometric coefficients of the reactants is greater than the sum of the product coefficients is positive, increasing pressure increases K, which drives the equilibrium composition to the product side.

Examples: the equilibrium composition of the reaction of Eq (9.1) is independent of pressure because the sum of the balancing numbers is the same on both sides of the equation. According to Eq (9.24), K = KP. The total pressure effect on the gas-phase reaction: O2 + 2H2 = 2H2O is p3-2 = p, or K = pKP ## Getting Started With Solar

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