- the equilibrium partial pressure of reactant appear in the denominator.
- each partial pressure is raised to a power to equal its coefficient in the balanced equation.
B. Changing the chemical equation.
1. The expression for K depends on the form of the chemical equation written to describe the equilibrium system . Ex:.
N2O4(g) «2NO2(g) K= (PNO2)2 .
PN2O4.
2.This illustrates the general rule defined as the coefficient rule. If the coefficient in the balanced equation are multiplied by a factor n, the equilibrium constant is then raised to the nth power. .
3. The reciprocal rule shows when the numerator and denominator are inverted. .
C. Adding Chemical Equations.
1. Rule of multiple equilibrium: if a reaction can be expressed as the sum of two or more reactions, K for the overall reaction is the product of the equilibrium constant of the individual actions. .
D. Heterogeneous Equilibria.
A. Such systems are considered heterogeneous because more than one phase is present. Ex:.
CO2(g) + H2 (g) « CO(g) + H2O(l).
1.The position of the equilibrium is independent of the amount of a solid or liquid, as long as some is present. .
2.Terms for pure liquids or solids need not appear in the expression for K.
B. Table 12.3 Equilibrium Constant Expressions for CO2(g) + H2 (g) « CO(g) +H2O(l).
Mass H2O (l) 8g 6g 4g 2g.
PH2O(atm) 3'10-2 3'10-2 3'10-2 3'10-2.
Kl 9'10-6 9'10-6 9'10-6 9'10-6.
KII 3'10-4 3'10-4 3'10-4 3'10-4.
.
C. True facts about expressions in the equilibrium:.
1. Gases enter as their partial pressures in the atmosphere.
2.Pure liquids or solids do not appear, neither does the solvent for a reaction in dilute solution.
3.Species in water solution enter as their molar concentration.
IV. Determination of K.
A. Strategy: First write the expression for K, then substitute equilibrium partial pressures into that expression and solve. .
Solution: K= (PNH3)'(P HCL)= (3.0)'(5.0)=15.
B. Ammonium chloride is sometimes used as a flux in soldering because it decomposes on heating:.