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# lec13 - Last Time Reaction quotient Qp and Qc Mathematical...

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1 Last Time Reaction quotient – Q p and Q c – Mathematical Relation to K c and K p – Q < K forward : Q > K reverse – Omit solids in heterogeneous systems Equilibrium from initial conditions – Molar proportions – Equilibrium tables Equilibrium Tables aA bB A B Initial concentration [A] [B] Change in conc. –ax +bx Equilibrium conc. [A] – ax [B] + bx Use the definition for K c and solve for x Sign of x determined by Q and K Example – Using “x” K c = 54.6 at 700 K for the reaction of hydrogen with iodine gas to form HI(g). Suppose that 38.4 g of HI, 5.0 g of H 2 , and 26.8 g of I 2 are placed together in a 2 L container at this temperature. What are the concentrations of each substance at equilibrium? Example At 294.8 K, K c is 1.2x10 –4 for the decomposition of NH 4 HS(s). Calculate the equilibrium concentrations of NH 3 (g) and H 2 S(g) when a solid sample of NH 4 HS(s) is allowed to decompose to equilibrium at 294.8 K.

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2 Le Chatelier’s Principle Equilibrium adjusts to release stress Explains changes in reactions due to – n – number of moles used
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