## Lecture Notes:

### Equilibrium Part II

• The Equilibrium Constant
• Reversible reactions reach an equilibrium where macroscopic change ceases and the concentration of both the products and the reactants ceases to change
• This idea can be expressed mathematically as an equilibrium constant according to the Law of Mass Action:
• “For a reversible reaction at equilibrium and a constant temperature, a certain ratio of reactant and product concentrations has a constant value, K (the equilibrium constant)”
• The general form of an equilibrium constant expression is:
aA + bB ↔ cC + dD
```      [C]c[D]d
Keq = ————————
[A]a[B]b
```
• A, B, C, and D represent the formulas of reactants and products. The small letters are the coefficients in the balanced chemical equation
• The square brackets around each formula mean that the number used in calculations is a concentration in mol/L (M)
• It is important to note that the concentrations of pure solids and liquids (such as H2O) are not included in the equilibrium expression because they are considered to be constant relative to the much smaller concentrations of other reactants/products
• The units of Keq are not important; just remember that the concentrations of the reactants/products are not constants: they can have different values depending on the initial conditions
• Keq is sometimes written Kc; the ‘c’ stands for concentration in mol/L
• The specific value of the constant depends both temperature and on the balanced equation that is used
• Take the following reaction: which can just as easily be written:
(1) N2O4 ↔ 2NO2 (2) 2NO2 ↔ N2O4
Keq 1 = [NO2]2/[N2O4] Keq 2 = [N2O4]/[NO2]2
And Keq 1 = 1/Keq 2 if both are considered at the same temperature
• If Keq is equal to 1.00 at a given temperature then no reaction will occur at that temperature; the concentrations of the reactants and products will stay as they are initially
• If Keq is greater than 1.00 at a given temperature then products are favored in the reaction as written; the concentrations of products will increase and the concentrations of reactants will decrease
• If Keq is less than 1.00 at a given temperature then reactants are favored in the reaction as written; the concentrations of reactants will increase and the concentrations of products will decrease
• The Reaction Quotient This section will not print out, you are not responsible for these notes
• The equilibrium constant Keq is determined using the equilibrium concentrations of the reactants/products; the reaction quotient (Q) is calculated using the equilibrium constant expression and the initial concentrations (rather than the equilibrium concentrations)
• If the reaction quotient is greater than the value of Keq (Q > Keq) then the initial concentration of products is too large and to reach equilibrium products must be converted to reactants
• If the reaction quotient is equal to the value of Keq (Q = Keq) then the system is already at equilibrium
• If the reaction quotient is smaller than the value of Keq (Q < Keq) then the initial concentration of reactants is too large and to reach equilibrium reactants must be converted to products
• These rules follow logically from the form of the equilibrium expression
• The Solubility Product
• The solubility product (Ksp) is essentially the same thing as the equilibrium constant but is used exclusively for reactions of the form:
aA(s) ↔ bB(aq) + cC(aq)
• In other words, the solubility product is used to describe the solubility of substances that are only slightly soluble
• The concentration of the solid salt is not included in the solubility product expression because it is essentially constant; hence the name
• The expression for Ksp for the above reaction is
```Ksp = [B]b[C]c
```
• The solubility product can be used to find the concentration of the dissociated ions in a solution of a slightly soluble salt; it can also be used to find the solubility of the salt at a given temperature in mol/L
• To find the solubility of the salt, simply use the solubility product expression and the value of Ksp to find the concentration of the dissolved ions; since there is a 1:1 relationship between the solid salt and the ion’s concentration, then this is the molar solubility
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Note: some notes regarding the reaction quotient (Q) are included on this page in an unprintable section, you are not responsible for these notes.