Chemical Equilibrium

Dynamic equilibrium

the forward and reverse reactions occur at the same rate, resulting in no observable change in the system

Equilibrium Constant

Keq = k_f/k_r = [products]^x/[reactants]^x

• excludes concentrations of pure liquids and solids

• only looks at aqueous solutions (aq) and gases (g)

• K_eq = K_c for aqueous solutions

• K_eq = K_p for gaseous solutions

If K_eq < 1, then…

there is greater concentration of reactants that products at equilibrium

If K_eq = 1, then…

the ratio of products to reactants at equilibrium is equal

If K_eq > 1, then…

there is a greater concentration of products than reactants at equilibrium

K_eq to Gibb’s Free Energy Equation

G^o = -RTln(K_eq)

If K_eq < 1, ln(K_eq) < 0, and G^o is positive

reactants are favored at equilibrium

If K_eq = 1, ln(K_eq) = 0, G^o is zero

ratio of reactants and products are equal

If K_eq > 1, ln(K_eq) > 0, G^o is negative

products are favored at equilibrium

Reaction quotient

uses concentrations of the reactants/products at any point in the reaction when concentrations are still changing

difference between reactant quotient and equilibrium constant

equilibrium is for at equilibrium whereas quotient is for any time during a given reaction

If Q < K_eq, there is a….

higher concentration of reactants than there is at equilibrium, and the reaction proceeds in the forward direction_eq

If Q = K_eq, the reaction…

is in dynamic equilibrium

If Q > K_eq, there is a ….

higher concentration of products than there is at equilibrium, and the reaction proceeds in the reverse direction

Le Chatelier’s Principle

shift by a system that occurs to restore the equilibrium state when its disturbed

Change in Concentration: reactants are added

Q < Keq, so reaction shifts towards forward reaction until Q = Keq

reactants are removed:

Q > Keq, so reaction shifts towards reverse reaction until Q = Keq

products are added

Q > Keq, so reaction shifts towards reverse reaction until Keq = Q

products are removed

Q < Keq, so reaction shifts towards forward reaction until Q = Keq

Pressure increases, volume decreases…

reaction shifts towards side with fewer moles of gas

Pressure decreases and volume increases:

reaction shifts towards side with more moles of gas

Endothermic reaction, requires an input of energy → heat can be considered a reactant, so lower temperature →

shifts towards left, higher temperature → shifts towards right

Exothermic reaction, releases energy → heat can be considered a product, so, lower temperature →

shifts reaction to right, higher temperature → shifts reaction to the left

precipitation reactions

ions in solution react to form an insoluble solid

K_sp: Solubility Product Constant

describes the extent to which an ionic compound dissolves in water

• solid + liquids not included in equation

Q_sp = Solubility product quotient

describes the current state of solution

Unsaturated

can dissolve more solute - Qsp < Ksp

Saturated

cannot dissolve more solute - equilibrium has been reached - Qsp = Ksp

supersaturated

cannot dissolve more solute - formation of precipitate - Qsp > Ksp

Molar Solubility

number of moles that can be dissolved per liter of solution until the solution becomes saturated

Common-ion Effect

describes the decrease in solubility of an ionic precipitate when a soluble compound is added to the solution that shares an ion common to the precipitate

Amphoteric Species

can act as an acid (proton donor) or base (proton acceptor)

Autoionization of Water

proton transferred from one water molecule to another water molecule

Strong Acids

• HCl

• HI

• HClO₄

• H₂SO₄

• HClO₃

• HNO₃

• HBr

Strong Bases

• LiOH

• KOH

• CsOH

• Sr(OH)₂

• NaOH

• RbOH

• Ca(OH)₂

• Ba(OH)₂

Weak acids and bases only

partially ionize in solution

K_{w =}

(K_a)(K_b) - only applies to conjugate acids and bases

Strong Acid added

equilibrium shifts left

Strong Base added

equilibrium shifts right

[HA] increased

equilibrium shifts right

[A^-] decreased

equilibrium shifts right

pH increased

lower hydronium ion concentration → equilibrium shifts right