CHEMICAL EQUILIBRIUM
A. Dynamic -- opposing reactions never cease (continue at same rate)
B. Equilibrium -- the concentration's do not change
II. Reaction Reversibility
A. For a given overall system composition, we always reach the same equilibrium concentrations whether equilibrium is approached from the forward or reverse direction
III. the Equilibrium Law for a Reaction
A. Mass action expression
1. ratio of product concentrations raised to their "mole" power versus reaction concentrations raised to their "mole" power
2. Reaction quotient (Q) -- numerical value of mass action expression
3. Equilibrium Law -- relationship between concentrations for the system
4. Equilibrium constant (Kc) -- constant value of mass action expression at a given temperature
5. For chemical equilibrium to exist in a reaction mixture, the reaction quotient Q must equal the equilibrium constant, Kc
B. Predicting the Equilibrium Law
1. The exponents in the mass action expression are the same as the stoichiometric coefficients in the balanced equation
2. The molar concentrations of the products are always placed in the numerator and those of the reatants appear in the denominator
C. Manipulating Equations for Chemical Equilibrium
1. Changing the direction of an Equilibrium
a. When the direction of an equation is reversed, the new equilibrium constant is the reciprocal of the original
2. Multiplying the Coefficients by a Factor
a. When the coefficients in an equation are multiplied by a factor, the equilibrium constant is raised to a power equal to that factor
3. Adding Chemical Equilibria
a. When chemical equilibria are added, their equilibrium constants are multiplied
IV. Equilibrium Laws for Gaseous Reactions
A. Molar concentration of gases is proportional to their partial pressures
1. P = (molar conc.) x RT
2. Mass action expressions can be written using concentrtions or partial pressures
a. Kp when partial pressures are used
V. The Significance of the Magnitude of K
A. When K is very large
1. The reaction proceeds far toward completion. the position of equilibrium lies far toward the products
B. When K = 1
1. The concentrations of reactants and products are nearly the same at equilibrium. the position of equilibrium lies approximately midway between reactants and products
C. When K is very small
1. Extremely small amounts of products are formed. The position of equilibrium lies far toward the reactants
VI. The Relationship between Kp and Kc
A. Kp = Kc(RT)^Delta(ng)
1. Delta ng = change in number of moles of gas in the reaction
a. Delta ng = (moles of gaseous products) - (moles of gaseous reactants)
B. When Delta ng = 0, Kp = Kc, since RT to power of 0 = 1
VII. Heterogeneous Equilibria
A. Homogeneous reaction -- all reactants and products are in the same phase
B. Heterogeneous reaction -- two or more phases exist
1. For any pure liquid or solid, hte ratio of amount of substance to volume of substance is a constant
2. The equilibrium law for a heterogeneous reaction is written without concentration terms for pure solids or liquids
VIII. Le Chatelier's Principle and Chemical Equilibria
A. Adding or Removing a Reactant or Product
1. An equilibrium will shift in a direction that will partially consume a reactant or product that is added, or partially replace a reactant or product that is removed
a. For the system to return to equilibrium, the concentrations muxt change in a way that causes Q to equal Kc again
B. Changing the Volume in Gaseous Reactions
1. reducing the volume of a gaseous reaction mixture shifts the equilibrium in whichever direction will, if possible, decrease the number of molecules of gas
a. If we reduce the volume of the reaction mixture, we expect the pressure to increase
C. Changing Temperature
1. Increasing the temperature shifts an equilibrium in a direction that produces an endothermic change
a. The concentrations change even though the volume stays the same and no chemical substances have been added or removed
2. If the forward reaction in an equilibrium is exothermic, raising the temperature causes the equilibrium constant to become smaller
D. Agents that do not Affect the Position of Equilibrium
1. adding a Catalyst
a. A catalyst only makes the system go to equilibrium faster
2. Adding an Inert Gas at Constant Volume
a. If the gas can not react with the gases already present, then the concentrations of the reactants and products won't change
IX. Equilibrium Calculations
A. Calculating Kc from Equilibrium Concentrations
1. "ICE" the problem (ex. on page646-647)
a. I = Initial concentrations
b. C = Changes in Concentration
c. E = Equilibrium Concentrations
B. The Concentration Table - A Summary
1. The only values that we can substitute into the mass action expression in the equilibrium law are equilibrium concentrations
2. When we enter initial concentrations into the table, they should be in units of moles per liter
3. The changes in concentrations always occur in the same ratio as the coefficients in the balanced equation
4. In constructing the "change" row, be sure the reactant concentrations all change in the same direction, and that the product concentrations all change in the opposite direction
C. Calculating Equilibrium Concentrations Using Kc
1. (see example on page 649)
D. Calculating Equilibrium Concentrations Using Kc and Initial Concentrations
1. More complex type of problem (need to apply algebraic methods)
2. Most comlex problems will involve the use of the quadratic equation (we will not deal with these are a regular basis!)
a. Rule of Thumb: you can neglect an x when it is added or subtracted, but you can never neglect an x when it is multiplied or divided
Outline based upon:
Brady, J. E., Holum, J. R., Russell, J. W. (2000). Chemistry: The Study of Matter and Its Changes. (3rd ed.). New York: John Wiley & Sons, Inc. pp. 627-657.
