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Chapter 8 section 4

Chapter 8 section 4. By Bridgette Olivares & Kirsten Vidal. Partial pressure. Partial pressure is the pressure exerted by any one gas in the mixture The concentration of a gas is related to its pressure by the ideal gas equation pV = n RT

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Chapter 8 section 4

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  1. Chapter 8 section 4 By Bridgette Olivares & Kirsten Vidal

  2. Partial pressure • Partial pressure is the pressure exerted by any one gas in the mixture • The concentration of a gas is related to its pressure by the ideal gas equation pV=nRT • n/v is the concentration, since R is a constant, and if the temperature is constant, we see that the concentration of gas is proportional to the pressure of the gas.

  3. Each gas in the mixture contributes to the pressure in proportion to the number of moles present. -The total pressure of a gas equals the sum of the partial pressures of the individual gases: Ptotal = PA + PB + PC …

  4. The Concept of Equilibrium • As the reaction progresses: - A decreases to a constant -B increases from zero to a constant -When A and B are constant, equilibrium is achieved AB

  5. Equilibrium expressions involving partial pressures • p is used for partial pressure • Reactants and products are written as subscripts after p • Square brackets not used • The number of moles of reactants or products are written as a power after the p • The symbol for the equilibrium constant is Kp (in terms of partial pressures)

  6. Units of Kp • Kp is based on molarities of reactants and products at equilibrium • The units of Kp , pressure, are Pascals (Pa) • The units of Kpdepend on the form of the equilibrium expression

  7. Worked example: • For the reaction: N2O4 (g) 2N02 (g) The equilibrium expression is : Kp = P 2NO2 The units are: PN2O4 Pa x Pa = Pa Pa

  8. Calculations using partial pressures • In the reaction : 2SO2 (g) + O2 (g) 2SO3 The equilibrium at partial pressures at constant temperature are SO2 = 1.0 x 106 Pa O2=7.0 x 106Pa So3=8.0 x 106 Pa Calculate the value of Kp for this reaction.

  9. Continued.. -Step 1: Write the equilibrium expression for the reaction in terms of partial pressures KP= P 2 SO3 P 2 SO2x P O3 -Step 2: Substitute the equilibrium concentrations into the expression. Kp= (8.0 x 106)2 (1.0 x 106)2 x 7.0 x 106 = 9.1 x 10-6

  10. Continued… Step3: Add the correct units The units are Pa x Pa = 1 = Pa-1 Pa x Pa x Pa Pa Kp= 9.1 x 10 -6 Pa-1

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