ACIDS AND BASES Chapter 18

Properties of Acids taste SOUR acids change litmus RED their aqueous (water) solutions CONTAIN HYDRONIUM (H 3 O + ) IONS react with BASES TO FORM WATER AND A SALT evolve hydrogen gas (H 2 ) upon reaction WITH METALS pH of LESS THAN 7

Properties of Bases Taste BITTER feel SLIPPERY (THEY BREAK DOWN OILS) bases don't change the color of litmus (LITMUS STAYS BLUE); they can turn PHENOLPTHALEIN PINK their aqueous (water) solutions CONTAIN HYDROXIDE (OH - ) IONS. react with ACIDS TO FORM WATER AND A SALT pH of MORE THAN 7

Examples of Common Strong Acids and Bases Formula Name Formula Name HI hydroiodic acid NaOH sodium hydroxide HBr hydrobromic acid LiOH lithium hydroxide HCl hydrochloric acid KOH potassium hydroxide HClO 4 perchloric acid RbOH rubidium hydroxide H 2 SO 4 sulfuric acid Sr(OH) 2 strontium hydroxide HSCN thiocyanic acid Ba(OH) 2 barium hydroxide HNO 3 nitric acid H 2 CrO 4 chromic acid

Arrhenius Concept of Acids and Bases This guy named Arrhenius concocted the first successful concept of acids and bases. He did this by defining acids and bases according to the effect these substances have on water. The Arrhenius concept of acids and bases is as follows: Acids are substances which produce hydrogen ions in solution. Bases are substances which produce hydroxide ions in solution. The hydrogen ion, is not just a bare proton, it is a proton bonded to a water molecule, H 2 O. This results in a hydronium ion, H 3 O + In Arrhenius's thoery, something that is a strong acid is a substance that completely ionizes in aqueous solution to give a hydronium ion, H 3 O +, and an anion. An example of a strong acid is perchloric acid: HClO 4  H 3 O + + ClO 4 -

Arrhenius Concept of Acids and Bases This guy named Arrhenius concocted the first successful concept of acids and bases. He did this by defining acids and bases according to the effect these substances have on water. The Arrhenius concept of acids and bases is as follows: Acids are substances which produce hydrogen ions in solution. Bases are substances which produce hydroxide ions in solution. The hydrogen ion, is not just a bare proton, it is a proton bonded to a water molecule, H 2 O. This results in a hydronium ion, H 3 O + In Arrhenius's thoery, something that is a strong acid is a substance that completely ionizes in aqueous solution to give a hydronium ion, H 3 O +, and an anion. An example of a strong acid is perchloric acid: HClO 4  H 3 O + + ClO 4 -

Bronsted-Lowry Acids and Bases The Bronsted-Lowry concept of acids and bases is that acid-base reactions can be seen as proton-transfer reactions. This results in acids and bases being able to be defined in terms of this proton (H + ) transfer. According to the Bronsted-Lowery concept: An acid is a Proton (Hydrogen ion) Donor A Base is a Proton (Hydrogen Ion) Acceptor As an example, lets look at the reaction of hydrochloric acid with ammonia. When we write it as an ionic equation we get: H 3 O + (aq) + Cl - (aq) + NH 3 (aq) -> H 2 O(l) + NH 4 + (aq) + Cl - (aq) which reduces to: H 3 O + (aq) + NH 3 (aq) -> H 2 O(l) + NH 4 + (aq) ACID BASE ACID

Bronsted-Lowry Acids and Bases Some things can act as either an acid or a base. These are called _AMPHIPROTIC or AMPHOTERIC_, they can either lose or gain a proton, depending on the other reactant. An example of an amphiprotic species would be HCO 3 -. In the presence of OH -, it acts as an acid. In the presence of HF it acts as a base. Water is also amphiprotic, as are most anions with ionizable hydrogens and certain solvents. Water as an amphiprotic species is very important to the acid-base reactions.

We have found that… 1. A base is a species that accepts protons, while an acid is a species that donates protons. 2. Acids and bases can be ions as well as molecular substances. 3. Some species can act as either acids or bases, depending on what the other reactant is.

Conjugate Acid- Base Pairs Both the forward and reverse reactions involve proton transfer reactions: HA(aq) + H 2 O(l) A - (aq) + H 3 O + (aq) acid + base base + acid In the forward reaction, HA donates a proton to H 2 O HA is the Brønstead-Lowry acid H 2 O is the Brønstead-Lowry base In the reverse reaction, H 3 O + donates a proton to A - H 3 O + is the Brønstead-Lowry acid A - is the Brønstead-Lowry base

Conjugate Acid- Base Pairs When HA behaves as an acid and donates a proton, what remains is A -, which behaves like a base When H 3 O + behaves as an acid and donates a proton, what remains is H 2 O, which (in this reaction) behaves like a base (although H 2 O is amphoteric) HA and A - are a Bronsted Lowry conjugate acid-base pair: A - is the conjugate base of HA H 3 O + and H 2 O are a conjugate acid-base pair: H 3 O + is the conjugate acid of H 2 O

Conjugate Acid- Base Pairs In any acid-base proton transfer reaction there will be conjugate acid-base pairs

Relative Strengths of Acids and Bases A strong acid is a molecule that has a strong preference to donate a proton. Thus, its CONJUGATE BASE will have a weak tendency to accept a proton A strong base is a molecule that has a strong preference to accept a proton Thus, its _CONJUGATE ACID will have a weak tendency to donate a proton There is an inverse relationship between the strength of an acid and its conjugate base (likewise a strong base and its conjugate acid)

pH and pOH pHpOH pH is a measure of the hydrogen ion concentration, [H + ] pOH is a measure of the hydroxide ion concentration, [OH - ] pH is calculated using the following formula: pH = -log 10 [H + ] pOH is calculated using the following formula: pOH = -log 10 [OH - ] pH = 14 – pOHpOH = 14 - pH Ionization Constant for Water (K W ) K W = [H+][OH-] = (1.0 x 10-7)(1.0 x 10-7) = 1.0 x 10-14

pH and pOH Problems 1. Calculate [H+] of a solution whose [OH-] = 1 x 10-5 M 2. Calculate [OH-] of a solution whose [H+] = 1 x 10-2 M 1 x x

pH and pOH Problems 1. Find the pH of a solution of sodium hydroxide that has a pOH of 2 2. Find the pOH of a solution of hydrochloric acid that has a pH of 3.4 pH = 12 pOH = 1o.6

pH and pOH Problems 1. Find the [H + ] in a solution of sodium hydroxide that has a pOH of 1 2. Find the [OH - ] of a sulfuric acid solution with a pH of 3 pH = 1 pOH = 11 [H + ] = 10 -pH [H + ] = = M [OH - ] = 10 -pOH [OH - ] = = M