Section 18-1: The Nature of Chemical Equilibrium
Le Châtelier's Principle: When a system at equilibrium is disturbed by the application of a
stress, it attains a new equilibrium position that minimizes the stress
A. Reversible Reactions
I. Definition: A chemical reaction in which the products can react to re-form the
reactants
II. Example: Mercury Oxide Decomposes when heated:
2HgO(s) → 2Hg(l) + O2(g)
Mercury and Oxygen Combine to form Mercury Oxide when heated:
2Hg(l) + O2(g) → 2HgO(s)
As the first reaction ends, the second reaction can begin, which allows the first reaction to
start again
III. Chemical Equilibrium: when the rate of forward reaction equals the rate of its
reverse reaction and the concentrations of its products and reactants remain
unchanged.
Example Equation:
2HgO(s) ↔ 2Hg(l) + O2(g)
B. Equilibrium, a Dynamic State
I. Left-to-Right Favored Reaction: When the “forward” reaction is favored over the
“reverse” reaction, and the concentrations of reactants and products grow
accordingly. (More products than reactants)
HBr(aq) + H O(l) ↔ H O+(aq) + Br-(aq)
a. Example:
2
3
Forming H3O+ and Br- is favored over re-forming HBr and H2O
II. Right-to-Left Favored Reaction: When the “reverse” reaction is favored over the
forward reaction, thus there will be more reactants than products.
a.
Example:
H2CO3(aq) + H2O(l) ↔ H3O+(aq) + HCO3-
III. How can we determine which kind of reaction is happening?
C. The Equilibrium Expression
I. nA + mB ↔ xC + yD
II. As compounds A and B react, they form C and D at a specific rate. Once C and D
are able to react, they form A and B at a specific rate. When both rates become
equal, equilibrium is achieved.
III. Mathematically, this can be measured by: K = [C]x[D]y/[A]n[B]m
(brackets are Molarity)
IV. The Equilibrium Constant “K” must be determined experimentally with actual
concentration values.
a. If K = 1, the products and reactants are formed in equal amounts
b. If K = Large, the reaction is Left-to-Right (forward)
c. If K = Small, the reaction is Right-to-Left (reverse)
V. K = Equilibrium Constant, Equation for K = chemical-equilibrium expression
D. The H2,I2, HI Equilibrium System
I. H2(g) + I2(g) ↔ 2HI(g)
II. Expression for K = ?
III. Value of K = ?
Example Problems:
#1. An equilibrium mixture of N2, O2, and NO gases at
1500K is determined to consist of 6.4x10-3mol/L N2,
1.7x10-3mol/L O2, and 1.1x10-5mol/L NO.
What is the equilibrium constant for the system at this
temperature?(1.1x10-5<---Notice, no units!)
------------------------------------------------------------------------#2 At equilibrium a mixture of N2,H2, and NH3 gases at
500°C is determined to consist of
0.602mol/L N2, 0.420mol/L H2, and 0.113mol/L NH3.
What is the equilibrium constant for the reaction
N2 + 3H2 ↔ 2NH3 at this temperature?(K = 0.286)
------------------------------------------------------------------------#3 At 450°C, the system in #2 has an equilibrium
constant Kc = 6.59x10-3.
If [NH3] = 1.23x10-4M and [H2] = 2.75x10-3M at
equilibrium, determine the value of [N2]. ([N2] = 110)
------------------------------------------------------------------------#4 The reaction AB2C(g) ↔ B2(g) + AC(g) reached
equilibrium at 900K in a 5.00L vessel. At equilibrium
0.084mol of AB2C, 0.035mol B2, and 0.059mol of AC
were detected.
What is the equilibrium constant at this temperature for
this system?(Kc = 4.9x10-3)