acids and bases

• Arrhenius acids


  Substances that, when added to water, ionize to increase the hydronium ion (H3O+)
• Arrhenius bases


  Substances that, when added to water, dissociate to increase the hydroxide ion (OH-)
• Drawbacks of Arrhenius Definition: Arrhenius base is defined as a substance that increases the concentration of OH- in aqueous solution and also contains at least one unit of OH- in the chemical formula. This can get a little confusing with compounds such as methylamine, CH3NH2
• When methylamine is added to water, the following reaction occurs: Based on the first definition, methylamine would be an Arrhenius base since the OH- ion concentration increases in the solution. By the second definition, however, it would not count as an Arrhenius base since the chemical formula does not include hydroxide.
• Because of this inaccuracy in the definition, and because Arrhenius theory only describe acid-base chemistry in aqueous solutions, where similar reactions can also occur in non-aqueous solvents, however, as well as between molecules in the gas phase, modern chemists usually prefer the Brønsted-Lowry theory, which is useful in a broader range of chemical reactions.
• Brønsted-Lowry Acids


  Substances that can donate H+ ions (so it must contain a hydrogen that can dissociate as H+)
• Brønsted-Lowry Bases


  Substances that can accept H+ ions (so it must have at least one lone pair of electrons to form a new bond with a proton)
• According to the Arrhenius theory, the reaction where HCl and NH3 form NH4Cl would not be an acid-base reaction because neither species is forming H+ or OH- in water. However, the chemistry involved a proton transfer from HCl to NH3 to form NH4Cl is very similar to what would occur in the aqueous phase.
• Hydronium Ion H3O+


  Water can pick up a H+ ion to form a hydronium ion
• With acids, water is a Bronsted-Lowry base. Example: HCl + H2O → H3O+ + Cl-
• With bases, water is a Bronsted-Lowry acid. Example: NH3 + H2O → NH4+ + OH-
• Amphoteric


  Compounds that can act as either a proton donor or acceptor
• Conjugate base


  The remainder of the original acid, after it donates its Hydrogen ion (Conjugate base = acid - H+)
• Conjugate acid


  The particle formed when the original base gains a hydrogen ion (Conjugate acid = Base + H+)
• Identifying conjugate acid-base pairs

  Guided Practice: Identify the acid, base and their conjugates in the following reactions
• Identifying conjugate acid-base pairs


  Check Point: Identify the two Conjugate acid-base pairs in each of the reactions below