221-chapter-6 - Chemical Equilibrium Introduction 1 2...

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First, system reaches equilibrium Chemical Equilibrium Introduction 1.) Equilibria govern diverse phenomena Protein folding, acid rain action on minerals to aqueous reactions 2.) Chemical equilibrium applies to reactions that can occur in both directions: reactants are constantly forming products and vice-versa At the beginning of the reaction, the rate that the reactants are changing into the products is higher than the rate that the products are changing into the reactants. When the net change of the products and reactants is zero the reaction has reached equilibrium. At equilibrium the amount of reactants and products are constant, but not necessarily equal Then, system continually exchanges products and reactants, while maintaining equilibrium distribution. Reactants Product
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Chemical Equilibrium Equilibrium Constant 1.) The relative concentration of products and reactants at equilibrium is a constant. 2.) Equilibrium constant ( K ): For a general chemical reaction Equilibrium constant: Where: - small superscript letters are the stoichiometry coefficients - [A] concentration chemical species A relative to standard state b a d c ] [ ] [ ] [ ] [ B A D C K =
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Chemical Equilibrium Equilibrium Constant 2.) Equilibrium constant ( K ): A reaction is favored when K > 1 K has no units, dimensionless - Concentration of solutes should be expressed as moles per liter (M). - Concentrations of gases should be expressed in bars. express gas as P , emphasize pressure instead of concentration ] [ ] ][ [ HA A H K 1 - + = 1 ' 1 K / 1 A H HA K = = - + ] ][ [ ] [ Consider the following reaction: Reversing the reaction results in a reciprocal equilibrium reaction:
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K 1 Chemical Equilibrium Equilibrium Constant 3.) Manipulating Equilibrium Constants K 2 K 3 If two reactions are added, the new K is the product of the two individual K values: ] ][ [ ] H ][ [ ] ][ [ ] [C ] [ ] ][ [ C HA C A C H H HA A H K K K 2 1 3 + - + + - + = = = ] [ ] ][ [ HA A H K 1 - + = ] ][ [ ] [ C H CH K 2 + + = ] ][ [ ] H ][ [ C HA C A K 3 + - =
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Chemical Equilibrium Equilibrium Constant 3.) Manipulating Equilibrium Constants Example: K w = 1.0 x 10 -14 Given the reactions and equilibrium constants: K NH 3 = 1.8 x 10 -5 Find the equilibrium constant for the reaction: Solution: K 1 = K w K 2 =1/K NH 3 K 3 =K w *1/K NH 3 =5.6x10 -10
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Chemical Equilibrium Equilibrium and Thermodynamics 1.) Equilibrium constant derived from the thermodynamics of a chemical reaction. deals with the relationships and conversions between heat and other forms of energy 2.) Enthalpy H – is the heat absorbed or released when the reaction takes place under constant applied pressure H = H products – H reactants Standard enthalpy change ( H o ) – all reactants and products are in their standard state. H o – negative heat released - Exothermic - Solution gets hot H o – positive heat absorbed - Endothermic - Solution gets cold
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Chemical Equilibrium Equilibrium and Thermodynamics 3.) Entropy Measure of a substances “disorder” Greater disorder Greater Entropy - Relative disorder: Gas > Liquid > solid S = S products – S reactants S o – change in entropy when all species are in standard state.
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