Acid/Base Theories The common characteristics of acids

Transcription

1 Acid/Base Theories The common characteristics of acids describe them as: Acids aving a sour taste Being electrolytes (some weak) Reacting with metals to produce gas (usually 2 ) Reacting with bases to produce a salt and water Bases The common characteristics of bases describe them as: aving a bitter taste Being electrolytes (some weak) aving a slippery feel (soaps) Reacting with acids to produce a salt and water Binary Acids If the anion in the acid ends in -ide, change the ending to -ic acid and add the prefix hydro- : Cl: hydrochloric acid Br: hydrobromic acid I: hydroiodic acid Oxyacids If the anion in the acid ends in -ate, change the ending to -ic acid: ClO 3 : chloric acid ClO 4 : perchloric acid Oxyanionic Acids If the anion in the acid ends in -ite, change the ending to -ous acid: ClO: hypochlorous acid ClO 2 : chlorous acid 1

2 Acid/Base Theories There are three modern definitions used to describe acids and bases. These include: Arrhenius theory Bonsted-Lowry theory Lewis Theory When we think of acids, we typically think of the Arrhenius definition. Svante Arrhenius ( ) Arrhenius Acid = Any compound that increases the hydronium ion ( 3 O + ) concentration in aqueous solution Arrhenius Base = Any compound that increases the hydroxide ion concentration in aqueous solution MOVIE Acid Strength or any Acid: Acids that completely ionize in solution are strong electrolytes; Therefore, are strong acids. Acids that only partially ionize in solution are weak electrolytes, or weak acids. There are only seven strong acids: ydrochloric (Cl) ydrobromic (Br) ydroiodic (I) Nitric (NO 3 ) Sulfuric ( 2 SO 4 ) Chloric (ClO 3 ) Perchloric (ClO 4 ) A + 2 O 3 O + (aq) + A - (aq) Acid + Water ydronium + Acid anion The hydrogen of the acid is essentially a single proton that can bond through a coordination bond to water forming 3 O +, called the hydronium ion. Cl (g) + 2 O (l) 3 O + (aq) + Cl - (aq) C 3 CO O 3 O + (aq) + C 3 CO 2 - (aq) Weak acid Polyprotic Acids Polyprotic acids have more than one ionizable hydrogen proton. Acids such as Cl and NO 3 that have only one available proton are called monoprotic acids. Acids such as 2 SO 4 and 2 CO 3 that have two available protons are called diprotic acids. Phosphoric acid, 3 PO 4, is a triprotic acid since it has 3 available protons. Base Strength Bases that completely dissociate or ionize in solution are strong electrolytes; Therefore, are strong bases. bases that only partially dissociate or ionize in solution are weak electrolytes, or weak bases. Strong bases include: Group I hydroxides Sr and Ba hydroxides 2

3 or any Base: MO + 2 O M + + O O Water does not always form a complex for Arrhenius bases as it does with Arrhenius acids 2 O NaO Na + (aq) + O - (aq) N O N 4 + (aq) + O - (aq) Weak base Our understanding of acids and bases are currently restricted to the Arrhenius definition which is limited to the presence of + and O - ions. According to Arrhenius, as [ + ] increases, [O - ] must decrease This is true; however, this definition does not account for compounds that act as acids and bases in non-aqueous solutions. Johannes Bronsted Thomas Lowry (1923) Independently expanded the definition of acids and bases Bronsted-Lowry Acid = any chemical species that donates one or more protons Bronsted-Lowry Base = any chemical species that accepts one or more protons A Brønsted Lowry acid must have a removable (acidic) proton. A Brønsted Lowry base must have a pair of nonbonding electrons. MOVIE Examine the formation of Cl (aq) from Cl (g) + Cl + O O + Cl - We know Cl is an Arrhenius acid by the production of + Cl is also a Bronsted-Lowry (B-L) acid because it gives up a proton. Notice, water gains a proton (B-L Base) 1. ydrogen Chloride and Ammonia 2. Dihydrogen sulfate and water 3. Ammonia and water * Notice, from rxn 2 and 3 that water can act as both an acid and a base. Water is said to be amphoteric. 3

4 Strong acids and bases react only in the forward direction but, weak acids and bases react in both the forward and reverse direction. i.e. acetic acid Conjugate Acids and Bases: rom the Latin word conjugare, meaning to join together. Reactions between acids and bases always yield their conjugate bases and acids. C 3 CO O 3 O + (aq) + C 3 CO 2 - (aq) Exists in equilibrium where the forward and reverse reaction are occurring at the same rate C 3 CO O C 3 CO O + B-L B-L Conj Conj acid base base acid Notice, in the reverse reaction, the hydronium ion acts as an acid by donating a proton to the acetate ion, and the acetate ion acts as a base by accepting the proton Also, the acid of the forward reaction becomes the conjugate base for the reverse reaction and visa versa for the base. Example: Identify the acid, base and their conjugates in the following reaction. + 2 O O + 1. Write the reaction for the following weak acid or base dissociations and identify all acid-base conjugate pairs. a. Ammonia in water b.phosphoric acid dissociating c. Dihydrogen phosphate in water (tricky) 4

5 G.N. Lewis (1920 s) Observe: Noticed that all bases contained an unshared electron pair and coordination bonds always form between this electron pair and the protons. Lewis acid Electron pair acceptor Lewis base Electron pair donor B 3 + N 3 B 3 N 3 Notice, no protons are transferred and + does not increase B N Lewis Acids Lewis Bases Lewis acids are defined as electron-pair acceptors. Atoms with an empty valence orbital can be Lewis acids. Lewis bases are defined as electron-pair donors. Anything that could be a Brønsted Lowry base is a Lewis base. Lewis bases can interact with things other than protons, however. Your turn: 5