Ch 6_Part 2_021612 - 2/16/2012 Equilibrium Calculations The...

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2/16/2012 1 TIME The reaction quotient (Q) is defined as the value of the expression for K (known as the “law of mass action”) at any time during the reaction. We can compare Q to K to determine in which direction the reaction is proceeding. Equilibrium Calculations Three cases when we can compare Q to K: o If Q is equal to K, the system is at equilibrium o If Q<K, then the reaction will proceed toward products o If Q>K, then the reaction will proceed toward reactants Equilibrium Calculations (cont’d.) ° = ???±?²³´ ?µ¶²³¶·³´ ¸¹ º»¼½ ¹¾¼½ "¹" ¿ = ???±?²³´ ?µ¶²³¶·³´ ¸¹ ½À?¾Á¾Âþ?¼ Q has same form as K, but the concentrations are the actual concentrations at any time (t) rather than the concentrations after equilibrium is reached.
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2/16/2012 2 Reaction Direction and the Relative Sizes of Q and K “Excess reactants initially” “Excess products initially” For the following reaction K p = 3.81 at 773 K. If P(NH 3 )=0.063 atm, P(N 2 )=0.0043 atm, and P(H 2 )=0.035 atm, is the reaction at equilibrium? If not, in which direction is the reaction occurring in order to establish equilibrium? Equilibrium Calculations (cont’d.) N 2 (g) + 3 H 2 (g) ↔ ± NH ² (g) Q = P nh ³ 1 atm 2 P n ´ 1 atm P h ´ 1 atm ² = 0.063 2 0.0043 0.035 ²
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2/16/2012 3 In general , we want to know how to: − Use K to determine the equilibrium concentrations (or pressures) of reactants and/or products − Compare Q and K to determine the direction in which a reaction will proceed to establish equilibrium Types of Equilibrium Problems We can use what we know about stoichiometry to determine the equilibrium constant given concentration (or pressure) information. Example: consider the following reaction If the initial pressure of PCl 5 was 0.500 atm, and the total pressure at equilibrium is 0.830 atm, what is K p ? Using basic algebra, we know that for every molecule of PCl 5 that decomposes, two gaseous molecules are formed. Equilibrium Calculations (cont’d.) PCl 5 (g) ↔ PCl 3 (g) + Cl 2 (g)
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2/16/2012 4 Expressing this idea in mathematical terms: 0.500 atm – x x x Using the pressure at equilibrium, we can solve for x: 0.830 atm = (0.500 atm – x) + x + x 0.830 atm = 0.500 atm + x x = 0.330 atm
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This note was uploaded on 02/22/2012 for the course CHEM 142 taught by Professor Zoller,williamh during the Winter '07 term at University of Washington.

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Ch 6_Part 2_021612 - 2/16/2012 Equilibrium Calculations The...

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