• Chemical Equilibrium:
o A dynamic process in which concentrations of reactants and products
remain constant over time.
o Rate of a reaction in the forward direction matches its rate in the reverse
• When→and ← Reaction Rates are Equal:
o Reaction: 2 NO(g) ⇌ N 2 4g)
o From experimental data:
Ratef= kf[NO 22
Rate = k [N O ]
r r 2 4 2
o At equilibrium: f [NO2] =r [2 4 ]
k [N O ]
f = 2 4 =constant, K
k [NO ]2
• Equilibrium Constant:
o Equilibrium Constant Expression:
Ratio of equilibrium concentrations or partial pressures of products
to reactants, each term raised to a power equal to the coefficient of
that substance in the balanced chemical equation.
2 NO(g) ⇌ N O (g)
K = [N2O 4]
o Equilibrium Constant:
The value of K at a specific temperature.
• Equilibrium Constant Expressions:
o Generic Reaction: aA + bB ⇌ cC + dD
[ ] [ ] D
K c = a b
[ ] [ ] B
Where [X] = concentration units of moles/liter.
K = (P C) ( P D )
p a b
o (P A ) ( PB )
Where P = units of partial pressure.
X • The Value of K:
o Value of K indicates extent of reaction:
2 H 2g) + O 2g) ⇌ 2 H O2g) K = 3 × 10
• Very large K: favors formation of products.
o 2 CO (g) ⇌ 2 CO(g) + O (g) K = 3 × 10 92
Very small K: favors reactants; not much product formed at
o H O2g) + CO(g) ⇌ H (g) +2CO (g) K2= 24
Intermediate value of K: comparable amounts of products and
reactants at equilibrium.
• Types of Equilibria:
o Homogeneous equilibria:
Equilibria involving reactants and products in the same phase.
Mostly gas phase in Ch. 16; aqueous solutions in Ch. 17
o Heterogeneous equilibria:
Equilibria involving reactants and products in more than one
phase, e.g., decomposition of a solid that produces a gas.
• Heterogeneous Equilibria:
o CaCO (s)3⇌ CaO(s) + CO (g) 2
o Concentrations of solids are constant at a given
Density (g/L) = mol = (M)
Molar mass (g/mol) L
• Le Châtelier’s Principle:
o “A system at equilibrium responds to a stress in such a way that it relieves
o Factors that will change the relative rates of forward/reverse reactions, or
change the value of Q compared to K, will cause a shift in the position of
• Concentration Stress:
o H O(g) + CO(g) ⇌ H (g) + CO (g)
2 2 2
o Remove CO (g):2
Rate of reverse reaction decreases; reaction proceeds in forward
direction to establish new equilibrium.
o Q compared to K:
Removing CO , K2> Q; reaction shifts right (toward products).
• Effects of Pressure/Volume:
o 2 NO (g2 ⇌ N O (2) 4
o Changing volumes will change partial pressures, change Q relative to K.