CHE 141 1st Edition Lecture 7Manipulating Equilibrium Constant Expressions - We can manipulate equilibrium constant expressions to calculate K values of related chemical reactions such as:o K for reverse reactionso K for a reaction multiplied by a numbero Calculating overall K for combined reactionsK for Reverse Reactions- Consider this hypothetical combination reaction with Kc=100: A+B reversible 2C- What would be the Kc value for the reverse reaction? 2C reversible A+Bo Firstly write the Kc expressions for both reactions: Kc=[C]^2/[A][B]=100 K reverse=[A][B]/[C]^2 K reverse is the reciprocal of Kc K reverse=1/100=0.01o Thus for any reversible chemical reaction: Kforward=1/Kreverse- Consider a generic reversible reaction in which the equilibrium favors the product: A reversible B, K>>1- The reverse of this reaction therefore favors the reactant: B reversible A K<<1- This makes mathematical sense-the reciprocal of a large number is a small number and vice versa- Consider this reaction with Kp=7.3: 2NO2 reversible N2O4o The Kp value for the reverse reaction N2O4 reversible 2NO2 is: Kr=1/Kf=1/7.3=0.14o The forward raction has K>>1 so it favors formation of product (equilibrium lies to the right)o The reverse reaction has K<<1 so it favors reactant (equilibrium lies to the left)- What if instead of A+B reversible 2C were only preparing 1 mole of C? 1/2A+1/2B reversible Co For A+B reversible 2C: Kc=[C]^2/[A][B]o For 1/2A+1/2B reversible C: K1/2=[C]/[A]^1/2[B]^1/2o Therefore if the balanced equation of a reaction is multiplied by some factor n, then the value of K is raised to the nth power (K^n)K Values of Combined ReactionsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- Consider the sum of two reactions:oo 1. N2+O2 reversible 2NOo 2. 2NO+O2reversible 2NO2o Overall: N2+2O2 reversible 2NO2o The equilibrium constant expression for the overall reaction is: Kc=[NO2]^2/[N2][O2]^2o The equilibrium constant expression for reaction 1 and 2 are: K1=[NO]^2/[N2][O2] and K2=[NO2]^2/[NO]^2[O2]- If we multiply K1 by K2: K1(K2) we get [NO2]^2/[N2][O2]^2- K1xK2=Kc=Koverall- This is true for all series of reactions, thus the general rule for combining K value is: Koverall=K1xK2xK3xK4…xKn- The overall K for a sum of two or more reactions is the product of the Ks of the individual reactionsRules Summary- Reverse reactions: Kr=1/Kf- Multiplying chemical equation 1 by a number n to give chemical equation 2: K2=K1^n- Combining chemical reaction 1,2,3,4…and n: Koverall=K1xK2xK3xK4…xKnReaction Quotient- The equilibrium constant expression applies only to chemical reactions that are at equilibrium - A mass action expression can be applied to concentrations (or partial pressure) or productand reaction mixtures that have not yet reached equilibrium- We write a mass action expression the same way as an equilibrium constant expression- For the generic reaction: aA+bB reversible cC+dD- The mass action expression is: Qc=[C]^c[D]^d/[A]^a[B]^b- Reaction quotient(Q) is the numerical value of the mass action expression for any values of the concentrations (or partial pressure) of reactants and products. At equilibrium, Q=K- Comparing reaction quotient Q with equilibrium constant K allows us to:o State whether a reaction is at equilibriumo Predict which way a reaction is proceeding to reach equilibriumo Q<K Reaction as written proceeds in forward direction()o Q=K Reaction is at equilibrium (reversible)o Q>K Reaction as written proceeds in reverse direction ()o
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