Equilibrium constants and acids & bases

What is the thermodynamic equilibrium constant K?

• uses activities

• has no units

• solids have an activity of 1

What is K_c?

Equilibrium constant using concentrations

assumes a ≈ [x]

What is K_p?

Equilibrium constant using partial pressures

What is Q?

non equilibria

What does it mean if:

• Q = K

• Q > K

• Q < K

• Q = K at equilibrium

• Q > K reaction shifts to left

• Q < K reaction shifts to right

Equation for G at equilibrium and non equilibrium

What is solubility?

The amount of substance that can be dissolved in a given amount of substance

What is the equation for K_sp?

What does large/small K_sp mean?

Large K_sp = high solubility

Small K_sp = low solubility

Acids and bases according to Arrhenius theory?

Acids are substances that dissociate in water to give H⁺ ions

Bases are substances that dissociate in water to give OH^- ions

Brønsted-Lowry acids and bases

Acid = H⁺ donor

Base = H⁺ acceptor

Lewis acids and bases

Acids are substances that accept a pair of electrons

Bases are substances that accept protons

How to calculate pH?

What are the names of the H₃O⁺ ion?

oxonium / hydronium ion

Dissociation of base equation

A^- (aq) + H₂O (l) ⇌ HA (aq) + OH^- (aq)

pOH equation

Dissociation of acid equation

HA + H₂O ⇌ H₃O⁺ + A^-

How to calculate K_a?

How to calculate K_b?

How does stability of A^- ion affect POE?

The more stable the ion, the more the equilibrium will lie to the RHS

• value of K_a will be larger

How are anions stabilised?

How do electronegative atoms/groups affect anions?

How else can electron density spread?

When electron density is spread over more atoms

• electronegative species draw the negative charge away from the COO

• electron density can be spread through delocalised p orbitals

What does polyphonic mean?

Acids/bases that are capable of donating or accepting multiple protons

What does amphoteric mean?

Can behave as an acid or a base

How does pKa change with Ka?

When Ka increases, pKa decreases

How does pKb change with pKa?

Small pKb = large pKa and vice versa

What is the fraction of dissociation (α_HA) a measure of?

What is the equation?

A measure of acid strength; the fraction of acid molecules that are dissociated

• F_HA is the formal acid concentration

What is F_HA for a weak acid?

F_HA ≈ [HA]

What is pKa (in terms of pH)?

The pH at which half an acid is protonated, and half is deprotonated

What does it mean when

• pH < pKa

• pH > pKa

pH < pKa = lots of H₃O⁺ ions and equilibrium is far to left (pH is low when acidic)

pH > pKa = not as many H₃O⁺ ions, equilibrium lies to right

What is the fraction of association, α_HB?
What is the equation?

The equivalent for a base

What is a buffer?

A mixture of a weak acid and the salt of its conjugate base. It resists changes in pH when acids/bases are added or if it’s diluted.

What is the Henderson-Hasselbalch equation?

(involves pH and pKa and buffers)

What are the assumptions?

Assumes almost all HA molecules are unchanged

All A^- ions come from salt

What happens when an acid is added to a buffer solution?

Equilibrium shifts to the left to reduce the concentration of H₃O⁺

What happens when a base is added to a buffer solution?

Equilibrium shifts to the right to reduce the concentration of OH^- by reacting with H₃O⁺ ions

Equation involving K_w, K_a and K_b

K_w = K_{a x} K_b