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Chemical Equilibria Chemical Equilibrium A system is in an equilibrium state if its macroscopic properties do not change spontaneously even over a prolonged period of time. A system at equilibrium appears static (on an atomic/molecular level is dynamic, though). At equilibrium, any change and the exact reverse of that change take place at the same rate. Rate forward: Rate reverse: At equilibrium: Rate forward = Rate reverse Chemical Change & Equilibrium H2(g) + I2(g) <=====> 2HI(g) Expt # Initial Composition Composition at Equilibrium K [H2] [I2] [HI] [H2] [I2] [HI] 1 0.638 0.570 0.000 0.165 0.0978 0.945 2 0.610 0.686 0.000 0.103 0.179 1.013 3 0.000 0.000 0.612 0.0644 0.0654 0.482 4 0.000 0.000 0.256 0.0270 0.0274 0.202 Note: Equilibrium does NOT mean equal amounts of products and reactants! [HI]2 Define new expression K = [H2][I2] Note: The value of K does not change significantly with changes in equilibrium compositions. The exact composition of the equilibrium state is NOT determined by the direction from which equilibrium is approached. Determination of the Equilibrium Constant, K for a system at a given Temp, K is fixed. 1. Write a balanced equation. 2. Form of K depends on how reaction is written: K1 K2 i.e A + B <====> C + D vs C + D <=====> A + B K1 = 1/K2 3. Expression for K written as “Products over Reactants”. 4. Include terms for gases and substances in solution. P(NO2) = pressure in atmospheres; [Br2] = molarity 5. Terms for pure liquids and solids NOT included. 6. Raise concentration to the power of the coefficient. [HI]2 H2(g) + I2(g) <=====> 2HI(g) K = [H2][I2] Examples: Determine the equilibrium expression for the following: 1. NH3(g) + HCN(l) <====> NH4+ (aq) + CN-(aq) 2. 5Fe2+(aq) + MnO4-(aq) + 8H+(aq) <====> 5Fe3+(aq) + Mn2+(aq) + 4H2O(l) Reaction Quotient, Q same form as K, but used when a system is not at equilibrium. value of Q will change until equilibrium is reached. e.g. N2O4(g) <=====> 2NO2(g) K = P(NO2)2/P(N2O4) = 80 atm form of Q = P(NO2)2/P(N2O4) By comparing Q and K, can predict direction of spontaneous change for system not at equilibrium: Q > K reaction goes to left (towards reactants) Q < K reaction goes to right (towards products) Q = K reaction is at equilibrium Reaction Quotient Example N2O4(g) <=====> 2NO2(g) K = P(NO2)2/P(N2O4) = 80 atm suppose: P(NO2) = 0.2atm P(N2O4) = 0.004atm P(NO2) = 1.0atm P(N2O4) = .002atm P(NO2) = 0.4atm P(N2O4) = 0.002atm Meaning of Equilibrium Constants K >> 1; mostly products in solution K<< 1; mostly reactants in solution K ~ 1; significant amounts of both present in solution Determining Equilibrium Concentrations 1. Express [equilibrium] of all species in terms of a single unknown, y. All concentrations are related to y and consistent with the coefficients of the balanced equation. 2. Substitute the terms for each concentration into the expression for K. This will yield an algebraic equation in terms of y. 3. Solve for y. May have to use the quadratic equation. -b ± b2 - 4ac y = 2a 4. Calculate equilibrium concentration using value for y. Example #1 0.1 mole HI is placed in a 5.0-L flask. What concentrations of H2 and I2 will be in the flask at equilibrium? [HI] at start = 0.10mole/5.0L = 0.02M [HI]2 H2(g) + I2(g) <=====> 2HI(g) K = = 50.53 [H2][I2] Example #2 1 mole each of PCl3, Cl2 and PCl5 gases are placed in a 1.0-L vessel at 230oC. What are their equilibrium concentrations? PCl3 + Cl2 <=====> PCl5 K = 49 Factors Affecting Equilibrium LeChatelier’s Principle: “When a system in an equilibrium state is subjected to an external stress, the system will establish a new equilibrium state, when possible, to minimize the external stress.” A. Adding/Removing Reactant/Product. e.g. 3H2(g) + N2(g) <=====> 2NH3(g) P(NH3)2 P(NH3)2 K = Q = P(N2) P(H2 )3 P(N2) P(H2)3 suppose we stress the system by adding N2 at constant V. Q < K; therefore system will go to right to minimize stress. add NH3? remove NH3? Factors Affecting Equilibrium 3H2(g) + N2(g) <=====> 2NH3(g) B. Change in Volume (for gases) 1. decrease volume and reaction will proceed in the direction which produces the smaller number of moles of gas. why? because decreasing the volume increases the pressure of each component by a constant factor. denominator will increase P(NH3) 2 more than numerator Q = P(N2) P(H2)3 Q < K reaction shifts to right 2. increase in volume will result in shifting the reaction towards the side which produces the greater moles of gas. Factors Affecting Equilibrium C. Change in temperature. 3H2(g) + N2(g) <=====> 2NH3(g) H = -92.4kJ reaction is exothermic (heat is a product) 3H2(g) + N2(g) <=====> 2NH3(g) + heat add heat; shift to left remove heat; shift to right D. Catalyst a catalyst has NO effect on an equilibrium will bring the system to equilibrium faster Quantitative Effect of Change in Equilibrium 2HI(g) <=====> H2(g) + I2(g) is at equilibrium when [HI] = 0.0800M and [H2] = [I2] = 0.0100M suppose enough HI is added to raise its concentration to 0.0960M; when equilibrium is re-established, what will the new concentrations be?

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