Ch 14 slides - Chemical Equilibrium The Extent of Reactions Chapter 14 dynamic equilibrium K and Q ICE charts Le Chateliers Principle Kinetics vs

Ch 14 slides - Chemical Equilibrium The Extent of Reactions...

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Unformatted text preview: Chemical Equilibrium: The Extent of Reactions Chapter 14 dynamic equilibrium K and Q ICE charts Le Chatelier’s Principle Kinetics vs. Equilibrium Speed vs. Extent of Reaction… Ea vs. ΔE Equilibrium A→B reactions are reversible At equilibrium, concentrations of reactants and products no longer change with time At equilibrium: rateforward = ratereverse dynamic state H2(g) + I2(g) → 2HI(g) Dynamic equilibrium: Example reaction: N2O4(g) → 2NO2(g) ratefwd = kfwd[N2O4] raterev = krev[NO2]2 At equilibrium, ratefwd = kfwd[N2O4]eq = raterev = krev[NO2]eq2 The Equilibrium Constant (Keq or K) N2O4(g) → 2NO2(g) Rearrange rate equations: ratefwd = kfwd[N2O4]eq = raterev = krev[NO2]eq2 kfwd[N2O4] = krev[NO2]2 kfwd = [NO2]2 = Keq krev [N2O4] The Formula for Keq For the reaction aA + bB ↔ cC + dD For the equilibrium concentrations of reactants and products [ ]eq Or Keq = [products]x [reactants]y The exponents in Keq are the stoichiometric coefficients from the balanced reaction Keq is a numerical value, characteristic for a particular What Keq tells you: © For a reaction at dynamic equilibrium aA + bB → cC + D Keq = [C]c[D]d = [products]x [A]a[B]b [reactants]y For Keq, the exponents are the stoichiometric coefficients of the balanced reaction Meaning of Keq Every reaction has its own characteristic Keq At dynamic equilibrium, the ratio of products to reactants must equal Keq If Keq is large, the reaction favors PRODUCTS If Keq is small, the reaction favors REACTANTS vs. Units for Keq K doesn’t have units Units cancel out in ratios K for multistep reactions For overall reaction with elementary steps: Overall reaction = reaction 1 reaction 2 reaction 3… © Koverall = K1 x K2 x K3… K for reverse reactions: K depends on how the reaction is written: Kfwd = 1 Krev © Multiplying by common factor When multiplying an entire reaction by a number n: n(aA + bB ↔ cC + dD) Knew = Kn = ([C]c[D]d/[A]a[B]b)n K and Partial Pressures © For reactions in the gas phase, it may easier to use partial pressures than concentrations Kc vs. Kp (PV = nRT rearranges to n/v = P/RT = M, so P = MRT) Kp = Kc(RT)products-reactant Kp = Kc(RT)Δn where Δn is the difference in number of moles in gas K for Heterogeneous Equilibria © When there are 2 or more physical phases present: [solid] stays constant (the interior of a solid stays the same) [pure liquid] stays constant if it is a separate phase (the interior of the liquid stays the same when it is in a separate phase) Don’t include them in Keq Calculating K from data Using ICE Charts to calculate [ ]’s from K (pg 551) Initial Change Equilibrium A I C E + B ↔ C The Law of Mass Action and the Reaction Quotient (Q) “A chemical system reaches a state in which a particular ratio of reactant and product has a specific value” (at T, this is Keq) It doesn’t matter what you started with! Reaction Quotient (Q): is [ ]’s at ANY given time, Q = [C]c[D]d [A]a[B]b Q is a measure of how much the reaction has progressed. Q moves toward Keq Q equation has the same form as Keq At equilibrium, Q = Keq Reaction Direction To predict which way a reaction will proceed, compare Q and K… Q<K overall direction: reactant → product Q>K Q=K reactant ← product reactant → product right) © (rxn procedes (rxn procedes left) (rxn at equilibrium) Summary of calculations you should be able to carry out: 1. Determine K from equilibrium concentrations 3. Determine K with ICE charts 4. Use K to find [ ]eq’s with ICE charts 5. Use the quadratic equation: ax2 + bx + c 6. Use assumptions to simplify equations 7. Determine Q and use it to predict direction of reaction (Q vs. K) Determining [ ]eq’s with ICE charts pg 556-7 A ↔ 2B [A]I = 1.00 [B]I = 0.00 Use stoichiometry to set up the Change term (easiest to make “x” like something with stoichiometry of 1) The key to solving ICE charts: Set up all three lines of the ICE chart correctly There are “x” values in the C and E lines You will be given some information to solve for “ x” You will likely have a numerical value for one of the following the amount that [ ] changes then solve C line, which is x itself The [ ]initial and [ ]eq values for one of the chemicals from E line, solve [ ]initial + x =[ ]eq The value for Keq you will plug the values in the E line into the equation for Keq and solve for x The quadratic equation you will sometimes need to solve ICE charts For the equation ax2 + bx + c = 0 Simplifications to make the algebra easier: Sometimes x is probably a very small value You should suspect this if Keq is very small Mutiplying by a small x is always important Adding or subtracting a small x to a big number can be negligible, so can be left out (it won’t affect sig figs) An example When is x small enough to be left out? Try to simplify by leaving x out of terms where added or subtracted Solve the equation for remaining x See if x is <5% (0.05) of the larger number If Yes, OK If No, go back and re-solve without simplification Le Châtelier’s Principle “when a chemical system at equilibrium is disturbed, it re-attains equilibrium by undergoing a net reaction that reduces the effect of the disturbance” disturbances: Add or remove chemical species (changes concentration) T P (or V) Reaction “shifts” left or right to offset disturbance Le Châtelier’s Principle Changes in Concentration: If a concentration increases, the system will react to consume it If a concentration decreases, the system will react to replenish it (pure liquids and solids-no effect) Because of disturbance, Q will no longer = K, so reaction will shift to bring back Q = K Table 17.3 The Effect of Added Cl2 on the PCl3-Cl2-PCl5 System Concentration (M) PCl3(g) Original equilibrium 0.200 Disturbance New initial Change New equilibrium + Cl2(g) 0.125 PCl5(g) 0.600 +0.075 0.200 0.200 0.600 -x -x +x 0.200 - x 0.200 - x 0.200 + x (0.637)* *Experimentally determined value. Le Châtelier’s Principle Changes in Pressure Pressure changes when: Concentration of gas changes (and vice versa) Volume changes Reaction will shift to either produce or consume gas and restore pressure Negligible effect on solids, liquids Addition of inert gas doesn’t affect concentration © Changes in Temperature © Changes in T actually change Keq itself! Consider heat as a reactant (endothermic reaction) a product (exothermic reaction) Effect of changing T depends on whether process is exothermic or endothermic… Effect of Catalysts on K There is no effect of catalysts on K However, catalysts cause system to reach equilibrium faster ...
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