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15.S: Chemical Equilibrium (Summary)

Last updated

Sep 8, 2020
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- 15.E: Exercises
- 16: Acid–Base Equilibria

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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]

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 reactions and products at equilibrium
- If :
- (equilibrium expression)
- 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

15.2.2 The Magnitude of Equilibrium Constants

- k>>1; equilibrium lies to the right; products favored
- k<<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
concentration of pure liquid or solid
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
- 1) tabulate known initial and equilibrium concentrations

- 2) calculate change in concentration that occurs as system reaches equilibrium

- 3) use stoichiometry to determine change in concentration of unknown species
- 4) from initial concentrations and changes in concentrations, calculate equilibrium concentrations

15.4.1 Relating k_c and k_p

- PV = nRT; P = (n/V)RT = MRT
- P_A = [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

15.5.1 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

15.5.2 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

15.6.1 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

15.6.2 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

15.6.3 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

15.6.4 The Effect of Catalysts

- catalysts increase rate at which equilibrium is obtained
- does not change composition of equilibrium mixture