# Acid/Base equilibria -4

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• Created by: Shannon
• Created on: 02-04-15 15:46
• Acid/base equilibria
• Defintions
• Lewis:
• Acid is an electron acceptor
• Base is an electron donor
• Bronsted-Lowry:
• Acid is a proton donor
• Base is a proton acceptor
• The extent of dissociation
• Strong acids dissociate completely in aqueous solution
• Weak acids only partially dissociate in aqueous solution. The majority of molecules remain undissociated
• Acid dissociation constant, Ka
• Ka = [H3O+] [A-] / [HA]
• HA = intial concentration of weak acid
• Assumptions
• 1) Concentration of acid at equilibrium is same as initial concentraion
• 2) All the H3O+ comes from the acid
• Conjugate pairs
• In a reaction if an acid loses a H+, the resulting species is referred to as a conjugate base
• The species resulting from a base gaining a H+ is called a conjugate acid
• An amphoteric species can act as an acid and a base e.g H2O
• pH
• pH = -log[H3O+]
• You can calculate [H30+] if you have been given pH by using the following - 10^-pH
• In alkaline solution pH is calculated by - pOH = -log[OH-]
• pH = 14 - pOH
• Calculating pH of strong bases
• pKa = pH @ half equivalence point
• Known as buffer region
• pH = pKa + log[A-]/[HA]
• Henderson-Hasselbach equation
• The dissociation of water - Kw
• Kw = [H3O+] [OH-]
• Pure liquids do not appear in the equilibrium expression
• Titration curves
• Strong acid v strong base
• pH @ equivalence point = 7
• Strong acid v weak base
• pH @ equivalence point = 5 - (due to H3O+)
• Weak acid v weak base
• pH @ equivalence point = 7
• Weak acid v strong base
• pH @ equivalence point = 9 (due to OH-)
• The end point is where the indicator changes colour
• Buffer solutions
• A buffer solution resists changes in pH upon small additions of acid or base
• A buffer is made up of a weak acid and its conjugate base or a weak base and its conjugate acid