Chapter 11 - Chapter 11 Chemical Equilibrium 11.1 The...

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Chapter 11 Chemical Equilibrium 11.1 The Equilibrium Condition 11.2 The Equilibrium Constant 11.3 Equilibrium Expressions Involving Pressures 11.4 The Concept of Activity 11.5 Heterogeneous Equilibria 11.6 Applications of the Equilibrium Constant 11.7 Solving Equilibrium Problems 11.8 Le Chatelier's Principle 11.9 Equilibria Involving Real Gases
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The Equilibrium Condition (General) Thermal equilibrium indicates two systems in thermal contact with each do not exchange energy by heat. If two bricks are in thermal equilibrium their temperatures are the same. Chemical equilibrium indicates no unbalanced potentials (or driving force). A system in equilibrium experiences no change over time, even infinite time. The opposite of equilibrium systems are non-equilibrium systems that are off balance and change with time. Example 1 atm O 2 + 2 atm H 2 at 298K
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aA + bB cC + dD The same equilibrium state is achieved whether starting with pure reactants or pure products. The equilibrium state can change with temperature. The Equilibrium Condition (Chem Rxn)
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The Equilibrium State (Chem Rxn) H 2 O (g) + CO (g) H 2 (g) + CO 2 (g) Change [CO] to P CO [H 2 O] to P H2O etc
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As the equilibrium state is approached, the forward and backward rates of reaction approach equality. At equilibrium the rates are equal, and no further net change occurs in the partial pressures of reactants or products. 1. No macroscopic evidence of change. 2. Reached through spontaneous processes. 3. Show a dynamic balance of forward and backward processes. 4. Same regardless of the direction from which they are approached. Fundamental characteristics of equilibrium states: Chemical Reactions and Equilibrium 5. No change over time.
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Use this in an equilibrium expression. Use this to indicate resonance. Arrows: Chemical Symbolism
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Chemical Reactions and Equilibrium The equilibrium condition for every reaction can be described in a single equation in which a number, the equilibrium constant (K) of the reaction, equals an equilibrium expression , a function of properties of the reactants and products. H 2 O( l ) H 2 O( g ) @ 25 o C Temperature ( o C) Vapor Pressure (atm) 15.0 0.01683 17.0 0.01912 19.0 0.02168 21.0 0.02454 23.0 0.02772 25.0 0.03126 30.0 0.04187 50.0 0.1217 H 2 O( l ) H 2 O( g ) @ 30 o C K = 0.03126 K = 0.04187
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Partial pressures and concentrations of products appear in the numerator and those of the reactants in the denominator. Each is raised to a power equal to its coefficient in the balanced chemical equation. aA + bB cC + dD if gases P C ( ) c P D ( ) d P A ( ) a P B ( ) b = K if concentrations C [ ] c D [ ] d A [ ] a B [ ] b = K Law of Mass Action (1)
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1. Gases enter equilibrium expressions as partial pressures, in atmospheres. E.g., P CO2 2. Dissolved species enter as concentrations, in molarity (M) moles per liter . E.g., [Na + ] 3. Pure solids and pure liquids are represented in equilibrium expressions by the number 1 (unity); a solvent taking part in a chemical reaction is represented by unity, provided that the solution is dilute. E.g., I 2 ( s ) ↔ I 2 ( aq ) [I 2 ( aq ) ] = K )] ( [ 1 )] ( [ )] ( [ )] ( [ ) ( ) ( 2 2 2 2 2 2 aq I aq I s I aq I K aq I s I = = = Law of Mass Action (2)
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P H 2 O P ref = K P ref is numerically equal to 1 The convention is to express all pressures in atmospheres
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