Class notes for Chapter 15: Chemical Equilibrium in the class General Chemistry: Macroscopic Investigations and Reaction Principles (CHEM 130) at the University of Michigan. Topics covered include dynamic equilibrium, the equilibrium constant, calculating K, and equilibrium concentrations
Fetal Hemoglobin and Equilibrium
●When we breathe:
○Inhale air into lungs
○Air diffuses into capillaries
■Comes into contact w blood
○Hemoglobin (Hb) reacts w oxygen within red blood cells
■Hb + O2 ⇌ HbO2
●Double arrows in chem equation → reaction can occur in either direction
○Can reach chemical equilibrium
●Concentrations of reactants+products in a reaction at equilibrium described by equilibrium constant (K)
○Large value of K = reaction lies far to the right at equilibrium
■High concentration of products, low concentration of reactants
○Small value of K = reaction lies far to the left
■High concentration of reactants, low concentration of products
○Value of K is a measure of how far a reaction proceeds ■Larger value of K → reaction proceeds more towards products
●Any system at equilibrium responds to changes in ways that maintain equilibrium
○If concentrations of reactants/products shift, reaction shifts to counteract it
●Hemoglobin-oxygen system is in equilibrium
○Hemoglobin binds oxygen when surrounding oxygen concentration is high, but releases oxygen when the surrounding oxygen concentration is low
Dynamic Equilibrium
●Reaction rates generally increase w increasing concentration of reactants, unless reaction order is 0
●Reversible Reaction: describes the ability for a reaction to proceed in either the forward
or reverse direction
○EX: H2 + I2 ⇌ 2HI
■H2 and I2 can react to form 2 HI molecules, or 2 HI molecules can react to form H2 and I2
●Dynamic Equilibrium: point at which the rate of the reverse reaction equals the rate of the forward reaction
○Forward and reverse reactions still occur, just at the same rate
○Concentrations of reactants and products stay the same as long as T is constant
■Concentrations of each do not change, but concentrations of reactants and products at equilibrium are not necessarily equal to each other
○(in example box) Suppose H 2 and I2 are in a closed container
■Initially they react to form HI ■As H2 and I2 react, their concentrations decrease → decreased rate of forward reaction, HI forms
■Concentration of HI increases → reverse reaction occurs at a faster rate
■Eventually, rate of reverse reaction = rate of forward reaction
The Equilibrium Constant
●Concentrations of reactants and products become constant at equilibrium
●Equilibrium Constant ( K): ratio, at equilibrium, of concentrations of products raised to their coefficients to the concentrations of reactants raised to their coefficients
●Take the generic chemical equation aA + bB ⇌ cC + dD
○A and B = reactants
○C and D = products
○a, b, c, and c = stoichiometric coefficients
○
■[X] represents the molar concentration of X
●Law of Mass Action: relationship between the balanced chemical equation and the expression of the equilibrium constant
●To express equilibrium constant, examine balanced chemical equation and apply law of mass action ●EX: 2N2O5 ⇌ 4NO2 + O2
○K=¿¿
Significance of the Equilibrium Constant
●Large value of K (K >> 1) → numerator is larger than denominator → forward reaction is favored
○High concentration of products, low concentration of reactants
●Small value of K (K << 1) → denominator larger than numerator → reverse reaction is favored
○High concentration of reactants, low concentration of products ●K ≈ 1 → neither direction is favored
○Reaction proceeds about halfway, similar concentration of reactants and products
Relationships Between Equilibrium Constant and Chemical Equation
●If chemical equation is modified, then K for the equation changes because of the modification
●Reverse the equation → invert equilibrium constant
○A + 2B ⇌ 3C
■K(forward)=[C]3
[A][B]2
○Reversed equation: 3C ⇌ A + 2B
■K(reverse)=[A][B]2
[C]3=1
K(forward)
●Multiply coefficients in the equation by X → raise K to the X power
○A + 2B ⇌ 3C
■K=[C]3
[A][B]2
○Multiply by n: nA + 2nB ⇌ 3nC
■K=[C]3
[A][B]2=¿
●Adding two individual chemical equations to get an overall reaction → multiply corresponding Ks together
○A ⇌ 2B; K1=¿¿
○2B ⇌ 3C; K2=¿¿¿
○Sum the two equations
■
●According to law of mass action, K overall = ¿¿
●Notice that it is the product of K1 and K2
○
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