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Chemistry 1011

Chemistry 1011. TOPIC Gaseous Chemical Equilibrium TEXT REFERENCE Masterton and Hurley Chapter 12. 12.1 The N 2 O 4 - NO 2 Equilibrium. YOU ARE EXPECTED TO BE ABLE TO: Distinguish equilibrium from steady state situations.

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Chemistry 1011

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  1. Chemistry 1011 TOPIC Gaseous Chemical Equilibrium TEXT REFERENCE Masterton and Hurley Chapter 12 Chemistry 1011 Slot 5

  2. 12.1 The N2O4- NO2 Equilibrium YOU ARE EXPECTED TO BE ABLE TO: • Distinguish equilibrium from steady state situations. • Recognize chemical equilibrium as a dynamic process taking place in a closed system. • Identify the changes taking place at the molecular level in a chemical equilibrium process. • Identify the equilibrium constant as a temperature dependent constant related to the equilibrium partial pressures of reactants and products. Chemistry 1011 Slot 5

  3. Review - Partial Pressure • Total gas pressure is proportional to the number of moles present; it is independent of the identity of the material • The partial pressure of a gas in a gas mixture is the fraction of the total pressure that is due to that gas • The partial pressure of gas X2 in a mixture is equal to the mole fraction of X2x Total Pressure Chemistry 1011 Slot 5

  4. Partial Pressure Example • Air is approximately 80% N2 and 20%O2 • Total air pressure is approximately 100kPa PN2+ PO2 = PTOTAL PN2 = 80% x 100kPa = 80kPa PO2 = 20% x 100kPa = 20kPa • Partial pressures can be used to express the concentrations of reactants and products in chemical equilibria Chemistry 1011 Slot 5

  5. The N2O4- NO2 Equilibrium • At any given temperature, a sample of NO2 will exist as an equilibrium mixture of NO2 and N2O4 N2O4(g) 2NO2(g) • The forward and reverse reactions are taking place at the same rate • The concentrations of the species remain constant Chemistry 1011 Slot 5

  6. Chemistry 1011 Slot 5

  7. Getting to Equilibrium • Equilibrium can be approached starting from either “reactants” or “products” • In the N2O4- NO2 example, we can theoretically start from either 100% reactants or 100% products • The equilibrium position will be the same Chemistry 1011 Slot 5

  8. The Equilibrium Constant • Equilibrium Measurements for the N2O4- NO2 System at 100oC Original P (atm) Equilibrium P (atm) Expt 1 N2O4 1.00 0.22 NO2 0.001.56 Expt 2 N2O4 0.000.07 NO2 1.00 0.86 Expt 3 N2O4 1.00 0.42 NO2 1.00 2.16 Chemistry 1011 Slot 5

  9. The Equilibrium Constant • The relationship (PNO2 )2/ PN2O2 is a constant (P is the equilibrium partial pressure of the species) Experiment 1 (PNO2 )2/ PN2O2 = (1.56)2/0.22 = 11 Experiment 2 (PNO2 )2/ PN2O2 = (0.86)2/0.07 = 11 Experiment 3 (PNO2 )2/ PN2O2 = (2.16)2/0.42 = 11 • The constant is called the equilibrium constant K for the system Chemistry 1011 Slot 5

  10. Characteristics of Equilibrium • A dynamic equilibrium can only exist in a closed system – neither reactants nor products can enter or leave the system • At equilibrium, the concentrations of reactants and products remain constant • At equilibrium, the forward and reverse reactions are taking place at equal and opposite rates • Equilibrium can be approached from either side of the reaction equation Chemistry 1011 Slot 5

  11. Steady State vs Equilibrium • When a process takes place in an open system, a steady state may be set up • In a steady state, the concentration of a product may be constant, but this will be because it is being formed, and is leaving the system at the same rate • Consider the evaporation of water in closed and open systems: Chemistry 1011 Slot 5

  12. The Evaporation of Water • Water in an open beaker • Water will evaporate at a rate dependent on its temperature. • The concentration of water vapour in the top of the beaker will be constant, but water molecules are entering the space and leaving it at the same rate • This is an open system – it is a steady state • Eventually, all of the water will evaporate Chemistry 1011 Slot 5

  13. The Evaporation of Water • Water in a closed container • Water will evaporate at a rate dependent on its temperature • As the concentration of water vapour rises, water molecules will condense • Eventually a dynamic equilibrium will be established at that temperature • The rate of evaporation will equal the rate of condensation H2O(l) H2O(g) Chemistry 1011 Slot 5

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