Chapter 14-1 (pt 2) Properties of Acids and Bases St. Augustine Preparatory School April 19, 2017

Arrhenius Acids and Bases Named after a Swedish chemist - Svante Arrhenius (1859 – 1927) Arrenhius acid: a chemical compound that increases the concentration of hydrogen ions, H+(aq) , in solution Arrenhius base: a chemical compound that increases the concentration of hydroxide ions, OH-(aq) , in solution

Arrhenius Acids

Modified Arrhenius Modified Arrhenius definition: - The hydrogen ion reacts with water to form hydronium. - This solved the problem as to why molecules “fell apart” (HCl -> H+ + Cl-) in solution. Modified def’n for acids: Acids are substances that react with water to produce hydronium ions

Modified Arrhenius Acids

Acid Strength A strong acid is one that ionizes completely in aqueous solutions. Examples: HCl(aq), H2SO4(aq), HBr(aq), HI(aq) A weak acid is one that ionizes only partially in aqueous solutions Examples: HF(aq), CH3COOH(aq), HCN(aq), H3PO4(aq)

Modified Arrhenius Bases Most bases are ionic compounds containing metal cations and the hydroxide ion (OH-), such as NaOH, sodium hydroxide. Not all bases are ionic compounds though Take ammonia for example: NH3(aq) + H2O(l) -> NH4+(aq) + OH-(aq)

Strength of Bases The strength of bases depends on how well the bases dissociates. Strong Bases: Ca(OH)2, Sr(OH)2, NaOH, KOH, RbOH (all ionic compounds) Weak Bases: NH3, C6H5NH2 (analine) (molecular)

End of Section 14-1 You should be able to answer the following questions: Are acids and bases electrolytes? What makes a compound/molecule an acid? What makes one a base? What determines the strength of an acid or a base? What are some characteristics of acids? What are some characteristics of bases?

Summary of Arrhenius Original: Modified Arrhenius Acids – H+ ions from a dissociation result in a solution being acidic Bases – OH- ions from a dissociation result in a solution being basic Modified Arrhenius Acids – Acids react with water to produce hydronium ions Bases – Bases react with water to produce hydroxide ions

Monoprotic and Polyprotic Acids/Bases Monoprotic – Reacts with water once to produce a hydroxide ion (bases) or a hydronium ion (acids) Polyprotic – Reacts more than once with water. The product of the original reaction is able to react with water as well to form a hydroxide or hydronium ion

Percentages of Reactions If it is a strong acid or base, the reaction will go >99%, which should be indicated above the arrow in the reaction For polyprotic acids and bases (if they are WA or WB), it is <50%, then <1%, and if a third step is possible, it is ~0%

Examples H3PO4 is a polyprotic (triprotic) weak acid. Show the dissociation equations for H3PO4. Mg(OH)2 is a strong diprotic base. Show the dissociation equations for Mg(OH)2.

Chapter 14-2 Acid-Base Theories Generally speaking, Arrhenius’ definition of acids and bases is adequate. However, his definition requires that the substances be aqueous, which isn’t always the case.

1923 – J.N. Bronsted and T.M. Lowry Brønsted (Danish) and Lowry (English) independently expanded the Arrhenius acid definition. A Brønsted-Lowry acid is a molecule or ion that is a proton donor. A Brønsted-Lowry base is a molecule or ion that accepts protons. Unfortunately, this theory still was unable to explain everything.

Question What is the difference between an Arrhenius acid/base and a Bronsted-Lowry Acid/Base?

Phosphoric acid: Below, provide all relevant dissociations of the weak acid phosphoric acid, which is a triprotic acid when in solution

Acid-Base Video https://www.youtube.com/watch?v=mAXEmlGrgzE