15.S: Chemical Equilibrium (Summary)
- Page ID
- 70555
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chemical equilibrium – condition where the concentration of products and reactants do not change with time
15.1: The Concept of Equilibrium
at equilibrium
\[k_f[A] = k_r[B] \nonumber \]
there for the ratio
\[\displaystyle\frac{[B]}{[A]} = \frac{k_f}{k_r} = \textit{constant} \nonumber \]
15.2: The Equilibrium Constant
equilibrium condition can be reached from either forward or reverse direction
Cato Maximillian Galdberg (1836-1902), and Peter Wauge (1833-1900)
- Law of mass action – relationship between concentrations of reactants and products at equilibrium
If \(aA + bB\rightleftharpoons pP+qQ\)
then an equilibrium expression can be constructed
\[\displaystyle K_c=\frac{[P]^p[Q]^q}{[A]^a[B]^b} \nonumber \]
- equilibrium expression depends only on stoichiometry of reaction and not mechanisms
- equilibrium constant:
- does not depend on initial concentrations
- does not matter if other substances present as long as they do not react with reactants or products
- varies with temperatures
- no units
15.2.1 Expressing Equilibrium Constants in Terms of Pressure, \(K_p\)
\[\displaystyle K_p=\frac{(P_P)^p(P_Q)^q}{(P_A)^a(P_B)^b} \nonumber \]
15.2.2 The Magnitude of Equilibrium Constants
- \(K\gg 1\); equilibrium lies to the right; products favored
- \(K \ll 1\); equilibrium lies to the left; reactants favored
15.2.3 The Direction of the Chemical Equation and \(K\)
- equilibrium expression written in one direction is the reciprocal of the one in the other direction
15.3: Interpreting & Working with Equilibrium Constants
15.4: Heterogeneous Equilibria
- homogeneous equilibria – substances in the same phase
- heterogeneous equilibria – substances in different phases
- a pure solid, a pure liquid, and a solvent in dilute solutions all appear in equilibrium laws, but they are all assigned activities that are equal to 1
- by convention the actvities of the pure solid, pure liquid, or solute are not explicitly written as part of the equilibrium law
15.5: Calculating Equilibrium Constants
determining unknown equilibrium concentrations
- tabulate known initial and equilibrium concentrations
- calculate change in concentration that occurs as system reaches equilibrium
- use stoichiometry to determine change in concentration of unknown species
- from initial concentrations and changes in concentrations, calculate equilibrium concentrations
15.5.1 Relating Kc and Kp
\[PV = nRT \nonumber \]
\[P = (n/V)RT = MRT \nonumber \]
\[PA = [A](RT) \nonumber \]
\[K_p=K_c(RT)D^n \nonumber \]
- D n = change in moles from reactants to products
15.6: Applications of Equilibrium Constants
- equilibrium constant:
- product direction reaction mixture will proceed
- calculate concentrations of reactants and products once equilibrium is reached
15.6.1 Predicting the Direction of Reaction
- reaction quotient
- at equilibrium Q=K
- Q>K; reaction moves right to left
- Q<K; reaction moves left to right
15.6.2 Calculating of Equilibrium Concentrations
15.7: Le Chatelier's Principle
If system at equilibrium is disturbed by change in temperature, pressure or concentration then system will shift equilibrium position
15.7.1 Change in Reactant or Product Concentration
- addition of substance will result in consummation of part of added substance
- if substance removed, reaction will move to produce more of the substance
15.7.2 Effects of Volume and Pressure Changes
- reducing volume, reaction shifts to reduce number of gas molecules
- increase volume, reaction shifts to produce more gas molecules
- increase pressure, decrease volume reduces total number of moles
- pressure volume changes do not affect K as long as temperature is constant
- changes concentrations of gaseous substances
15.7.3 Effect on Temperature Change
- endothermic: reactants + heat « products
- exothermic: reactants « products + heat
- increase temperature, equilibrium shifts in direction that absorbs heat
- endothermic: increase T, increase K
- exothermic: increase T, decrease K
- cooling shifts equilibrium to produce heat
15.7.4 The Effect of Catalysts
- catalysts increase rate at which equilibrium is obtained
- does not change composition of equilibrium mixture