unit 7: equilibrium

qualifications for a reversible reaction

1. both forward and reverse rates are naturally observed

2. the rate of the reverse reaction is significant

common reversible processes

1. weak acids and bases

2. phase changes

3. gas absorption

4. salt precipitation

5. acid-base reactions

6. redox reactions

equilibrium qualities

1. forward and reverse reactions are occurring at the same rate

2. there is no observable net change in the amount of products or reactants

dynamic equilibrium

constant movement back and forth in a reaction (in equal but opposite directions)

Q (reaction quotient)

describes the state of a reversible reaction. chemical reactions that are pure solids & liquids are excluded from the reaction quotient

Q at equilibrium

called the equilibrium constant, Keq (Kc/Kp)

LARGE Keq

indicates that the system is primarily composed of products

very small Keq

indicates that the system is primarily composed of reactants

Keq close to 1

indicates that the system is composed of a significant amount of both products and reactants

manipulating Keq

1. reverse a chemical equation = Keq is reciprocalized

2. stoichiometric coefficients of a reaction are multiplied by a number c, Keq is raised to the same power c

3. if you sum two equations together, multiply the two Keq values together to get the new Keq

equilibrium system shift

system will respond/shift to relieve any stress applied to reach a new equilibrium → increases either products or reactants

types of equilibrium stresses

1. add/remove product/reactant

2. change pressure/volume → only shifts equilibria with an uneven number of moles on each side of the equation

3. change in temperature (affects endothermic/exothermic reactions) → CHANGES KEQ VALUE

4. changing volume of a solvent

5. adding a CATALYST does NOT change the reaction → just makes it go faster both ways

Q < Keq

reaction shifts right (products made)

Q = Keq

reaction at equilibrium

Q > Keq

reaction shifts left (reactants made)

le chatelier’s principle

a system at equilibrium shifts by either increasing products or reactants

Ksp

solubility product constant → equilibrium constant for the dissolution of a salt

• soluble salts ionize ~100% (Ksp > 1)

• insoluble salts ionize <1% (Ksp « 1)

molar solubility

maximum moles of salt per liter solution that will ionize

common ion effect

describes the decrease in solubility of a salt when a soluble ionic compound is added that has an ion in common with the salt

neutral cations

• all alkali metals

• Ca2+, Sr2+, Ba2+

neutral anions

• Cl-, Br-, I-

• NO3-, ClO4-, ClO3-, SO4-* (*only for the first hydrogen)

bases in basic/acidic environments

• basic environment: lots of OH- → equilibrium shifts to reactants, decreasing solubility

• acidic environment: lots of H+ → equilibrium shifts to products, increasing solubility

acids in basic/acidic environments

• basic environment: lots of OH- to react with H+ → reduces H+ concentration, increases solubility

• acidic environment: lots of H+, decreases solubility

gibbs free energy (ΔG)

total energy change during chemical reactions

• ΔG = ΔH - TΔS

• H = enthalpy, T = temperature, S = entropy

• if it is negative, the reaction is spontaneous and requires no energy input to proceed