chapter 20 acids and bases

• They are described by their role in proton (H+) transfer • acids are proton donors • bases are proton acceptors

• a conjugate acid-base pair contains 2 species that can be intrerconverted by proton transfer

• hydroxonium ions are a part of a conjugate acid-base pair with water

acids are defined by the number of H+ ions that are released * monobasic - one H+ e.g HCl * dibasic - 2 H+ e.g H2SO4 * tribasic - 3 H+ e.g H3PO4

The ionic equation for the reaction of magnesium with Dilute HCl and Dilute H2SO4 is the same

• Metals: Redox reactions = salt + hydrogenn • carbonates: Netralisation = salt + water + CO2 • Bases/alkalis: Neutralisation = salt + water

• Since strong acids completely dissociate, this means [HA] = [H+] e.g

• since weak acids partially discociate in solution, they form an equilibrium which can be represented by Ka • the larger the Ka the more the equilibria liies to the right therefore the lower the pH units: mol dm-3

• Values for Ka are very small so it easier to give the negative log instead as this makes pKa easier to compare • small pKa values mean stronger acids*

• HA discociates into equal [H+] and [A-] (small [H+] from H2O) • the change in [HA] is negligible so [HA]eqm = [HA]start

• prepare a standard solution for weak acid of known conc • measure pH of standard solution using pH meter

• at pH values > 6 water dissociation is significant, this means it doesn’t work for vert weak acids or very dilute solutions • if [H+] is significant there will be difference between [HA]eqm = [HA]start, this means doesn’t work with stronger weak acids or very dilute solutions

• as the dissociation of water is small [H2O] remains constant • this leads to: Ka x [H2O] = [H+][OH-]

• as [H2O] and Ka are constant they form the constant Kw