Acids, Bases, and Salts CHM 1010 PGCC Barbara A. Gage

AcidBase Litmus color Phenolphthalein color pH range Reaction with active metal (like Mg) Taste Formula component Characteristics of Acids and Bases CHM 1010 PGCC Barbara A. Gage

Acids Strong hydrochloric acid, HCl hydrobromic acid, HBr hydroiodic acid, HI nitric acid, HNO 3 sulfuric acid, H 2 SO 4 perchloric acid, HClO 4 Weak hydrofluoric acid, HF phosphoric acid, H 3 PO 4 acetic acid, CH 3 COOH (or HC 2 H 3 O 2 ) ionizes completely in waterionizes partially in water carbonic acid, H 2 CO 3 CHM 1010 PGCC Barbara A. Gage

Strong acid: HA( g or l ) + H 2 O( l ) H 3 O + ( aq ) + A - ( aq ) The extent of dissociation for strong acids. H + and H 2 O  H 3 O + (hydronium ion) CHM 1010 PGCC Barbara A. Gage

The extent of dissociation for weak acids. Weak acid: HA( aq ) + H 2 O( l ) H 3 O + ( aq ) + A - ( aq ) CHM 1010 PGCC Barbara A. Gage

Bases (or alkalis) Strong Weak sodium hydroxide, NaOH calcium hydroxide, Ca(OH) 2 potassium hydroxide, KOH strontium hydroxide, Sr(OH) 2 barium hydroxide, Ba(OH) 2 ammonia, NH 3 (NH 4 OH) Moderate Dissociates completelyDissociates completely but is not very soluble aluminum hydroxide, Al(OH) 3 magnesium hydroxide, Mg(OH) 2 Dissociates partially carbonates, CO 3 2- bicarbonates, HCO 3 1- CHM 1010 PGCC Barbara A. Gage

An aqueous strong acid-strong base reaction on the atomic scale. MX is a “salt” – an electrolyte that is not an acid or base CHM 1010 PGCC Barbara A. Gage

Acid and Base Definitions Arrhenius Acid = compound that forms hydrogen (H + ) ions in water Base = compound that forms hydroxide (OH - ) ions in water CHM 1010 PGCC Barbara A. Gage

Acid and Base Definitions Bronsted-Lowry Acid = proton donor (H + is a proton) Base = proton acceptor An acid-base reaction can now be viewed from the standpoint of the reactants AND the products. An acid reactant will produce a base product and the two will constitute an acid-base conjugate pair. CHM 1010 PGCC Barbara A. Gage

Proton transfer as the essential feature of a Brønsted- Lowry acid-base reaction. (acid, H + donor)(base, H + acceptor) HClH2OH2O + Cl - H3O+H3O+ + Lone pair binds H + (base, H + acceptor)(acid, H + donor) NH 3 H2OH2O + NH 4 + OH - + Lone pair binds H + CHM 1010 PGCC Barbara A. Gage

The Conjugate Pairs in Some Acid-Base Reactions BaseAcid+ Base+ Conjugate Pair Reaction 4H 2 PO 4 - OH - + Reaction 5H 2 SO 4 N2H5+N2H5+ + Reaction 6HPO 4 2- SO Reaction 1HFH2OH2O+F-F- H3O+H3O+ + Reaction 3NH 4 + CO Reaction 2HCOOHCN - +HCOO - HCN+ NH 3 HCO HPO 4 2- H2OH2O+ HSO 4 - N 2 H PO 4 3- HSO CHM 1010 PGCC Barbara A. Gage

SAMPLE PROBLEMIdentifying Conjugate Acid-Base Pairs PROBLEM:The following reactions are important environmental processes. Identify the conjugate acid-base pairs. (a) H 2 PO 4 - ( aq ) + CO 3 2- ( aq ) HPO 4 2- ( aq ) + HCO 3 - ( aq ) (b) H 2 O( l ) + SO 3 2- ( aq ) OH - ( aq ) + HSO 3 - ( aq ) SOLUTION: Identify proton donors (acids) and proton acceptors (bases). (a) H 2 PO 4 - ( aq ) + CO 3 2- ( aq ) HPO 4 2- ( aq ) + HCO 3 - ( aq ) proton donor proton acceptor proton donor conjugate pair 1 conjugate pair 2 (b) H 2 O( l ) + SO 3 2- ( aq ) OH - ( aq ) + HSO 3 - ( aq ) conjugate pair 2 conjugate pair 1 proton donor proton acceptor proton donor CHM 1010 PGCC Barbara A. Gage

Molecules as Lewis Acids acidbaseadduct An acid is an electron-pair acceptor. A base is an electron-pair donor. M 2+ H 2 O( l ) M(H 2 O) 4 2+ ( aq ) adduct CHM 1010 PGCC Barbara A. Gage

SAMPLE PROBLEMIdentifying Lewis Acids and Bases PROBLEM:Identify the Lewis acids and Lewis bases in the following reactions: (a) H + + OH - H 2 O (b) Cl - + BCl 3 BCl 4 - (c) K + + 6H 2 O K(H 2 O) 6 + SOLUTION: PLAN:Look for electron pair acceptors (acids) and donors (bases). (a) H + + OH - H 2 O acceptor donor (b) Cl - + BCl 3 BCl 4 - donor acceptor (c) K + + 6H 2 O K(H 2 O) 6 + acceptor donor CHM 1010 PGCC Barbara A. Gage

Acid Anhydrides Non-metal oxides react with water to form acidic solutions CO 2 (g) + H 2 O (l)  H 2 CO 3 (aq) N 2 O 5 (s) + H 2 O (l)  2 HNO 3 (aq) SO 3 (g) + H 2 O (l)  H 2 SO 4 (aq) Dissolved non-metal oxides cause acid rain. CHM 1010 PGCC Barbara A. Gage

Basic Anhydrides Metal oxides react with water to form alkaline solutions Na 2 O (s) + H 2 O (l)  2 NaOH (aq) CaO (s) + H 2 O (l)  Ca(OH) 2 (aq) Al 2 O 3 (s) + 3 H 2 O (l)  2 Al(OH) 3 (aq) Lime (CaO) is used on lawns and is converted to Ca(OH) 2 when it rains. CaO is less hazardous to handle. CHM 1010 PGCC Barbara A. Gage

An acid-base titration. Start of titration Excess of acid Point of neutralization Slight excess of base CHM 1010 PGCC Barbara A. Gage

Sample ProblemFinding the Concentration of Acid from an Acid-Base Titration PROBLEM: You perform an acid-base titration to standardize an HCl solution by placing mL of HCl in a flask with a few drops of indicator solution. You put M NaOH into the buret, and the initial reading is 0.55 mL. At the end point, the buret reading is mL. What is the concentration of the HCl solution? SOLUTION: NaOH( aq ) + HCl( aq ) NaCl( aq ) + H 2 O( l ) ( ) mL x 1L 10 3 mL = L LX M= 5.078x10 -3 mol NaOH Molar ratio is 1: x10 -3 mol HCl L = M HCl CHM 1010 PGCC Barbara A. Gage

K c = [H 3 O + ][OH - ] [H 2 O] 2 K c [H 2 O] 2 =[H 3 O + ][OH - ] The Ion-Product Constant for Water K w = A change in [H 3 O + ] causes an inverse change in [OH - ]. = 1.0 x at 25 0 C H 2 O( l ) + H 2 O( l ) H 3 O + ( aq ) + OH - ( aq ) In an acidic solution, [H 3 O + ] > [OH - ] In a basic solution, [H 3 O + ] < [OH - ] In a neutral solution, [H 3 O + ] = [OH - ] CHM 1010 PGCC Barbara A. Gage

The relationship between [H 3 O + ] and [OH - ] and the relative acidity of solutions. [H 3 O + ][OH - ] Divide into K w ACIDIC SOLUTION BASIC SOLUTION [H 3 O + ] > [OH - ] [H 3 O + ] = [OH - ][H 3 O + ] < [OH - ] NEUTRAL SOLUTION CHM 1010 PGCC Barbara A. Gage

SAMPLE PROBLEMCalculating [H 3 O + ] and [OH - ] in an Aqueous Solution PROBLEM:A research chemist adds a measured amount of HCl gas to pure water at 25 0 C and obtains a solution with [H 3 O + ] = 3.0x10 -4 M. Calculate [OH - ]. Is the solution neutral, acidic, or basic? SOLUTION: Use the K w at 25 0 C and the [H 3 O + ] to find the corresponding [OH - ]. K w = 1.0x = [H 3 O + ] [OH - ] so [OH - ] = K w / [H 3 O + ] = 1.0x /3.0x10 -4 = [H 3 O + ] is > [OH - ] and the solution is acidic. 3.3x M CHM 1010 PGCC Barbara A. Gage

The pH values of some familiar aqueous solutions. pH = -log [H 3 O + ] CHM 1010 PGCC Barbara A. Gage pOH = -log [OH - ] pH + pOH = 14

The relations among [H 3 O + ], pH, [OH - ], and pOH. CHM 1010 PGCC Barbara A. Gage

SAMPLE PROBLEMCalculating [H 3 O + ], pH, [OH - ], and pOH PROBLEM:In an art restoration project, a conservator prepares copper-plate etching solutions by diluting concentrated HNO 3 to 2.0M, 0.30M, and M HNO 3. Calculate [H 3 O + ], pH, [OH - ], and pOH of the three solutions at 25 0 C. SOLUTION: PLAN:HNO 3 is a strong acid so [H 3 O + ] = [HNO 3 ]. Use K w to find the [OH - ] and then convert to pH and pOH. For 2.0M HNO 3, [H 3 O + ] = 2.0M and -log [H 3 O + ] = = pH [OH - ] = K w / [H 3 O + ] = 1.0x /2.0 = 5.0x M; pOH = [OH - ] = K w / [H 3 O + ] = 1.0x /0.30 = 3.3x M; pOH = For 0.3M HNO 3, [H 3 O + ] = 0.30M and -log [H 3 O + ] = 0.52 = pH [OH - ] = K w / [H 3 O + ] = 1.0x /6.3x10 -3 = 1.6x M; pOH = For M HNO 3, [H 3 O + ] = M and -log [H 3 O + ] = 2.20 = pH CHM 1010 PGCC Barbara A. Gage

Buffers Solutions that resist change in pH Can maintain any pH value between 0 and 14 (not just neutral pH 7) Composed of a weak acid and a salt made from the weak acid or weak base and salt made from the weak base Examples: HC 2 H 3 O 2 and NaC 2 H 3 O 2 NH 4 OH and NH 4 Cl CHM 1010 PGCC Barbara A. Gage

Buffers Reaction with acid: HC 2 H 3 O 2 + C 2 H 3 O H +  HC 2 H 3 O 2 + HC 2 H 3 O 2 Reaction with base: HC 2 H 3 O 2 + C 2 H 3 O OH -  C 2 H 3 O C 2 H 3 O HOH A buffer regenerates it’s own components. The pH it maintains depends on the ratio of salt to acid (or base) and the nature of the acid (or base). CHM 1010 PGCC Barbara A. Gage