Chemistry 1011 TOPIC Gaseous Chemical Equilibrium TEXT REFERENCE

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

12. 1 The N 2 O 4 - NO 2 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 2

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 X 2 in a mixture is equal to the mole fraction of X 2 x Total Pressure Chemistry 1011 Slot 5 3

Partial Pressure Example • Air is approximately 80% N 2 and 20%O 2 • Total air pressure is approximately 100 k. Pa PN 2 + PO 2 = PTOTAL PN 2 = 80% x 100 k. Pa = 80 k. Pa PO 2 = 20% x 100 k. Pa = 20 k. Pa • Partial pressures can be used to express the concentrations of reactants and products in chemical equilibria Chemistry 1011 Slot 5 4

The N 2 O 4 - NO 2 Equilibrium • At any given temperature, a sample of NO 2 will exist as an equilibrium mixture of NO 2 and N 2 O 4(g) 2 NO 2(g) • The forward and reverse reactions are taking place at the same rate • The concentrations of the species remain constant Chemistry 1011 Slot 5 5

Chemistry 1011 Slot 5 6

Getting to Equilibrium • Equilibrium can be approached starting from either “reactants” or “products” • In the N 2 O 4 - NO 2 example, we can theoretically start from either 100% reactants or 100% products • The equilibrium position will be the same Chemistry 1011 Slot 5 7

The Equilibrium Constant • Equilibrium Measurements for the N 2 O 4 - NO 2 System at 100 o. C Original P (atm) Equilibrium P (atm) Expt 1 N 2 O 4 NO 2 Expt 2 N 2 O 4 NO 2 Expt 3 N 2 O 4 NO 2 1. 00 0. 00 1. 00 Chemistry 1011 Slot 5 0. 22 1. 56 0. 07 0. 86 0. 42 2. 16 8

The Equilibrium Constant • The relationship (PNO 2 )2/ PN 2 O 2 is a constant (P is the equilibrium partial pressure of the species) Experiment 1 (PNO 2 )2/ PN 2 O 2 = (1. 56)2/0. 22 = 11 Experiment 2 (PNO 2 )2/ PN 2 O 2 = (0. 86)2/0. 07 = 11 Experiment 3 (PNO 2 )2/ PN 2 O 2 = (2. 16)2/0. 42 = 11 • The constant is called the equilibrium constant K for the system Chemistry 1011 Slot 5 9

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 10

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 11

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 12

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 H 2 O(l) H 2 O(g) Chemistry 1011 Slot 5 13