Kp Kp The equilibrium constant in terms of partial pressures Mole fraction Partial pressures Partial pressure, p The contribution of a gas towards the total pressure Partial pressure = mole fraction x Total pressure A gas mixture with a total pressure of 320 kPa contains mol of N2(g) and mol of O2(g) Mole fractions Partial pressures x(N2) = = 0.4 p(N ) = x(N2)P p(N ) + p(O ) = 128 + 192 = 320 kPa = 0.4 x 320 = 128 kPa Sum of partial pressures = Total pressure 2 x(O2) = = 0.6 p(O2) = x(O2)P = 0.6 x 320 = 192 kPa What is Kp Similar to Kc but partial pressures used in place of concentration Equilibrium: 2SO2(g) + O2(g) ⇌ 2SO3(g) Kp = Units: p(SO3 )2 p(SO2 )2 p(O2 ) Kp = (kPa )2 ( kPa )2 ( kPa ) = kPa −1 Calculating Kp Equilibrium: 2SO2(g) + O2(g) ⇌ 2SO3(g) Partial pressures: SO2(g), 74 kPa; O2(g), 23 kPa; Kp = SO3(g), 142 kPa p(SO3 )2 p(SO2 )2 x p(O2 ) 142 Kp = 74 x 23 = 0.160 kPa–1 Heterogeneous equilibria Equilibrium contains different phases Equilibrium: CaCO3(s) ⇌ CaO(s) + CO2(g) Kp expression contains only gaseous species Kp = p(CO2) Solid species are omitted (solids have no gas pressure)