5.S: Fundamental Equilibrium Concepts (Study Guide)
chemical equilibrium – condition where the concentration of products and reactants do not change with time
5.2: Chemical Equilibria
- Chemical equilibrium happens when the rates of forward and reverse reactions are equal, so the concentrations of products and reactants are constant
- Equilibrium is a dynamic process:
the concentrations of reactants and products no longer change with time
reactions do not stop
5.3 The Equilibrium Constant
- Relationship between concentrations of reactants and products at equilibrium
If [latex]aA + bB\rightleftharpoons cC+dD[/latex]
then an equilibrium expression can be constructed
[latex]\displaystyle K_c=\frac{[C]^c[D]^d}{[A]^a[B]^b} \nonumber[/latex]
- 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
- Can also be expressed in terms of pressure, [latex]K_p[/latex]
[latex]\displaystyle K_p=\frac{(P_P)^p(P_Q)^q}{(P_A)^a(P_B)^b} \nonumber[/latex]
- Magnitude of Equilibrium Constants
- [latex]K\gg 1[/latex]; equilibrium lies to the right; products favored
- [latex]K \ll 1[/latex]; equilibrium lies to the left; reactants favored
- equilibrium expression written in one direction is the reciprocal of the one in the other direction
- 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
- Predicting the Direction of Reaction
- determine reaction quotient Q
- at equilibrium Q=K
- Q>K; reaction moves right to left
- Q<K; reaction moves left to right
5.4 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
- Relating Kc and Kp
[latex]PV = nRT \nonumber[/latex]
[latex]P = (n/V)RT = MRT \nonumber[/latex]
[latex]PA = [A](RT) \nonumber[/latex]
[latex]K_p=K_c(RT)D^n \nonumber[/latex]
- D n = change in moles from reactants to products
If system at equilibrium is disturbed by change in temperature, pressure or concentration then system will shift equilibrium position
5.5 Shifting Equilibria – Le Chatelier’s Principle
- 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
- 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
- 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
- The Effect of Catalysts
- catalysts increase rate at which equilibrium is obtained
- does not change composition of equilibrium mixture