L22: acids, bases and buffers

biophysics

what is the water equilibrium equation: Kw? why is this different from the Keq of water?

Kw = [H+][OH-]

[H2O] is constant in almost all aqueous solutions so it can be emitted from the equation

what can Kw of pure water be simplified to? why?

Kw = [H+]²

because in pure water [H+] = [OH-]

what is the definition of an acid?

proton doner - produces H+ ions by dissociation

what is the definition of a base?

substance that releases OH- ions

what is the equation for PH?

PH = -log₁₀ [H+]

what are the 2 methods of preparing a base?

• mixing a (large volume of) WEAK acid with its conjugate base

• mixing a (large volume of) WEAK base with its conjugate acid

what is the acid dissociation constant? equation?

the equilibrium constant for a weak acid

at what PH do buffers stabilise at?

their pKa

what does pKa tell us?

how strong an acid is

pKa equation?

pKa = -log₁₀ Ka

what equation do we use to calculate the PH of a buffer?

henderson-hasselbalch equation:

PH = pKa + log [A-]/[HA]

what happens if we add a strong acid near a buffers pKa?

adding [H+]

soaked up by the conjugate base A- to form more weak acid

what will happen if we add strong base near the pKa?

adding [OH-]

soaked up by the weak acid to form more conjugate base and water

what does a PH of a molecule bigger than, smaller than and the same as its pKa tell us about the state of the molecule?

• PH<pKa = mainly weak acid form

• PH>pKa = mainly conjugate base form

• PH = pKa: half half

amino acids exist as zwitterions at neutral PH, at what PH are they anions/cations?

• anion (-): Ph above pka (high PH) - donate H+

• cation (+): PH below pKa (low PH) - accept H+