Chapter 16 Acids and Bases Arrhenius Definition Acids produce hydrogen ions in aqueous solution. Bases produce hydroxide ions when dissolved in water. Definition is limited to aqueous solutions. Only one kind of base for this definition. NH 3, ammonia could not be an Arrhenius base. Bronsted-Lowry Definitions An acid is a proton (H + ) donor and a base is a proton acceptor. Acids and bases always come in pairs. HCl is an acid. When it dissolves in water it gives its proton to water. HCl + H 2 O(l) H 3 O + + Cl - Water is a base making the hydronium ion A Brnsted-Lowry acid must have a removable (acidic) proton. A Brnsted-Lowry base must have a pair of nonbonding electrons. Types of Acid-Base reactions Recall. Strong acid strong base H + (aq) + OH - (aq) H 2 O Weak acid strong base (1) HA H + + A - (weak acid) (2) H + + OH - H 2 O So, HA + OH - A - + H 2 O Types of acid base reactions Strong acid weak base (1) B + H 2 O BH + + OH - (weak base) (2) H + + OH - H 2 O So, B + H + BH + Common weak bases: ammonia (NH 3 ), methylamine (CH 3 NH 2 ) Acid Base Pairs General equation HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) Acid + Base Conjugate acid +Conjugate base This is an equilibrium. Competition for H + between H 2 O and A - ion If H 2 O is a stronger base it will take the H + Equilibrium moves to right. If it can either add or lose a proton it is amphiprotic. Such as. HCO 3 - HSO 4 - H 2 O What Happens When an Acid Dissolves in Water? Water acts as a Brnsted-Lowry base and attracts a proton (H + ) from the acid. As a result, the conjugate base of the acid and a hydronium ion are formed. Conjugate Acids and Bases Reactions between acids and bases always yield their conjugate bases and acids. Acid and Base Strength Strong acids are completely dissociated in water. Their conjugate bases are quite weak. Weak acids only dissociate partially in water. Their conjugate bases are stronger weak bases. Acid and Base Strength Substances with negligible acidity do not dissociate in water. Their conjugate bases are exceedingly strong. Acid dissociation constant K a HA(aq) H + (aq) + A - (aq) K a = [H + ][A - ] [HA] We can write the expression for any acid. Strong acids dissociate completely. Equilibrium far to right. Conjugate base must be weak. Acid and Base Strength In any acid-base reaction, the equilibrium will favor the reaction that moves the proton to the stronger base. HCl (aq) + H 2 O (l) H 3 O + (aq) + Cl - (aq) H 2 O is a much stronger base than Cl -, so the equilibrium lies so far to the right that K is not measured (K>>1). Acid and Base Strength In any acid-base reaction, the equilibrium will favor the reaction that moves the proton to the stronger base. Acetate is a stronger base than H 2 O, so the equilibrium favors the left side (K