# 15.S: Chemical Equilibrium (Summary)

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, $$K_p$$

$\displaystyle K_p=\frac{(P_P)^p(P_Q)^q}{(P_A)^a(P_B)^b}$

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

## 15.4: Heterogeneous Equilibria

• homogeneous equilibria – substances in the same phase
• heterogeneous equilibria – substances in different phases
• concentration of pure liquid or solid
• $\displaystyle \frac{\textit{density}}{M} = \frac{\textit{mol}}{cm^3}$
• density of pure liquid or solid is constant at any temperature
• if pure solid or liquid is involved in a reaction, its concentration is excluded from equilibrium expression
• pure solids must be present for equilibrium to be reached even through they are excluded from equilibrium expression

## 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

$PV = nRT$

$P = (n/V)RT = MRT$

$PA = [A](RT)$

$K_p=K_c(RT)D^n$

• D n = change in moles from reactants to products

## 15.6: Applications of Equilibrium Constants

• equilibrium constant:
1. product direction reaction mixture will proceed
2. 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