Acid-Base Theories Arrhenius Acids and Bases (1884) Acids and Bases An acid is a substance that, when dissolved in water, increases the concentration of hydrogen ions. A base is a substance that, when dissolved in water, increases the concentration of hydroxide ions. H 3 O + and an anion. H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH - (aq) A strong base is a substance that completely ionizes in aqueous solutions to give a cation and a hydroxide ion. NaOH (aq) H2O Na + (aq) + OH - (aq) Bronsted Lowry Acid and Bases (1923) defines an acid and a base according to its function in a proton transfer reaction. Acid-base reactions can be seen as a proton transfer reaction. An acid is a species donating a proton in a proton transfer reaction. A base is a proton acceptor in a proton transfer reaction. H + (aq) + Cl - (aq) + NH 3(aq) H 2 O (l) + NH 4 + (aq) + Cl - (aq) Amphiprotic species is a species that can act as either an acid or a base depending on the other reactant. NH 3(aq) + H 2 O (l) NH 4 + (aq) + OH - (aq) CH 3 COOH (aq) + H 2 O (l) CH 3 COO - (aq) + H 3 O (aq)
Lewis Acids and Bases An electron pair theory of covalent bonding for acid and bases. Lewis acid is a species that can form a covalent bond by accepting an electron pair from another species. Lewis base is a species that can form a covalent bond by donating an electron pair to another species. NH 3(aq) + H 2 O (l) NH 4 + (aq) + OH - (aq) Relative Strength of Acids and Bases An acid base reaction normally goes in the direction of the weaker acid The strongest acids have the weakest conjugate bases, and the strongest bases have the weakest conjugate acids. SO 4-2 (aq) + HCN (aq) HSO 4 - (aq) + CN - (aq)
Molecular Structure and Acid Strength The strength of an acid depends on how easily the hydrogen ion is lost or removed. Two factors are important in determining this: 1. The polarity of the bond 2. The strength of the bond The more polarized the bond is, the more easily the proton is removed. The size of the atom is the dominant factor in determining the acid strength. Following a column in the periodic table - as you go down the column of elements, the size of atom X increases, the H - X bond strength decreases, and the strength of the binary increases. HF < HCl < HBr < HI High electron density is in red and low electron density is in blue. The tighter the proton is held, the greater the strength of the bond. The larger the atom in the bond, the weaker the bond and the greater the acid strength. Going across a row of elements of the periodic table, the electronegativity increases, the H - X bond polarity increases, and the acid strength increases. Oxoacids have the structure H -O-Y. The acidic hydrogen is always attached to an O atom, which in turn, is attached to an atom Y. Bond polarity appears to be the dominant factor determining the relative strengths of the oxoacids For a series of oxoacids of the same structure, differing only in the atom Y, the acid strength increases with the electronegativity of Y.
Acid strength of oxoacids Relative strength of polyprotic acids is dependent on the strength of the polyprotic acid and its corresponding acid anions. The strength of the acids and its anion decreases with increasing negative charge. Self Ionization of water, or autoionization is a reaction in which two water molecules react to give ions. H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH - (aq) Ion product constant for water, K w, is the equilibrium value for the ion product K w = [H 3 O + ] [OH - ] = 1.0 x 10-14 at 25ºC Suppose you dissolve 0.10 mol HCl in 1.0 L of aqueous solution. You would like to know the H 3 O + concentration in this solution. HCl (aq) + H 2 O (l) H 3 O + (aq) + Cl - (aq) Example: Calculate the concentrations hydronium ion and hydroxide ion at 25C in: a. 0.15 M HNO 3, b. 0.010 M Ca(OH) 2
In an acidic solution, [H 3 O + ] > 1.0 x 10-7 M In a neutral solution, [H 3 O + ] = 1.0 x 10-7 M In a basic solution, [H 3 O + ] < 1.0 x 10-7 M Because these concentrations are very small, it is more convenient to give the acidity in terms of ph ph is the negative of the logarithm of the molar hydronium ion concentration ph = - log [H 3 O + ] Example: What is the ph of a sample of gastric acid whose hydronium concentration is 0.045 M? A saturated solution of calcium hydroxide has a hydroxide ion concentration of 0.025 M. What is the ph of the solution?