Equilibrium Law. Introduction to the Equilibrium law 2H 2 (g) + O 2 (g)  2H 2 O (g) Step 1:Set up the “equilibrium law” equation Kc = Step 2:Product.

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Equilibrium Law

Introduction to the Equilibrium law 2H 2 (g) + O 2 (g)  2H 2 O (g) Step 1:Set up the “equilibrium law” equation Kc = Step 2:Product concentrations go in numerator [H 2 O] 2H 2 (g) + O 2 (g)  2H 2 O (g) [H 2 O] 2 2H 2 (g) + O 2 (g)  2H 2 O (g) Step 3:Concentration in mass action expression is raised to the coefficient of the product 2H 2 (g) + O 2 (g)  2H 2 O (g) [H 2 ] 2 [O 2 ] Step 4:Reactant concentrations go in denominator Step 5:Concentrations in mass action expression are raised to the coefficients of reactants 2H 2 (g) + O 2 (g)  2H 2 O (g) [H 2 ] 2 [O 2 ] Mass action expression Equilibrium constant

Equilibrium law: important points State (g, l, s, aq) may or may not be added at this point since we will only be dealing with gasses for this section. Later it will matter. The equilibrium law includes concentrations of products and reactants in mol/L (mol L -1 ) The value of Kc will depend on temperature, thus this is listed along with the Kc value Tabulated values of Kc are unitless By substituting equilibrium concentrations into equilibrium law, we can calculate Kc …

CO(g) + 2H 2 (g)  CH 3 OH(g) [CH 3 OH] Equilibrium law Kc = [CO] [H 2 ] 2 [ ], Kc = [0.105] [0.250] 2 =0.398 C 2 H 4 (g) + H 2 O(g)  C 2 H 5 OH(g) [C 2 H 5 OH] [C 2 H 4 ] [H 2 O] [0.150], Kc = [0.0222] [0.0225] Kc = = 300 Concentrations at equilibrium

[CH 3 OH] Equilibrium law CO(g) + 2H 2 (g)  CH 3 OH(g) Kc = [CO] [H 2 ] 2 [x], Kc = [0.210] [0.100] 2 = N 2 (g) + 3H 2 (g)  2NH 3 (g) [NH 3 ] 2 [N 2 ] [H 2 ] 3 [0.280] 2, Kc = [ ] [x] 3 Kc = = 64 x = [0.210] [0.100] 2 (0.500) = mol L -1 x 3 = [0.280] 2 / [ ] (64) =0.146 x 3 = 0.146x = 0.53 mol L -1

When Kc  mass action expression We can use the equilibrium law to determine if an equation is at equilibrium or not If mass action expression equals equilibrium constant then equilibrium exists Q - consider: C 2 H 4 (g) + H 2 O(g)  C 2 H 5 OH(g) If Kc = 300, []s = M, M, M which direction will the reaction need to shift? [C 2 H 5 OH] [C 2 H 4 ] [H 2 O] [0.175], Kc = [0.0197] [0.0200] Kc = =  must be reduced to 300. Therefore, the top must decrease and the bottom must increase. A shift to left is required to establish equilibrium.

More equilibrium law problems [PCl 5 ] [PCl 3 ] [Cl 2 ] [ ], Kc = [0.0520] [0.0140] Kc = =  0.18 Top must , bottom must  - shift to left is needed SO 2 (g) + NO 2 (g)  NO(g) +SO 3 (g) [NO] [SO 3 ] [SO 2 ] [NO 2 ] [0.0100][0.0400], Kc = [ ] [ ] Kc = =  85 Top must , bottom must  - shift to left is needed PCl 3 (g) + Cl 2 (g)  PCl 5 (g)