*In every acid-base reaction, equilibrium favors transfer of a proton from the stronger acid to the stronger base.

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1 16.2 Bronsted-Lowry Acids and Bases An acid is a substance that can transfer a proton to another substance. A base is a substance that can accept a proton. A proton is a hydrogen ion, H +. Proton transfer is hydrogen ion transfer. Acid-base reactions: HCl + H 2 O H 3 O + + Cl - Acid-base conjugate pairs: HCl + NH 3 NH Cl - Relative strengths of acids and bases: see and know Figure 16.4 Strong acids Weak acids *In every acid-base reaction, equilibrium favors transfer of a proton from the stronger acid to the stronger base Water auto-ionizes to give equal, small concentrations of H + and OH -. H 2 O(l) H + (aq) + OH - (aq) 2 H 2 O(l) H 3 O + (aq) + OH - (aq) What is H 3 O +? Water has both a conjugate acid and a conjugate base: Conjugate acid conjugate base H 2 O H 2 O Acid ionization in water: strong acid: HCl(aq) + H 2 O(l) H 3 O + (aq) + Cl - (aq) Weak acid: HF(aq) + H 2 O(l) How to write an equilibrium expression: What does the value of the equilibrium constant tell us about the reaction? Write the equilibrium expression and constant for water: 2 H 2 O(l) H 3 O + (aq) + OH - (aq) K w = Possible solutions: neutral acidic basic In terms of [H + ] and [OH ] The ph scale is a way to measure hydronium ion concentration: ph = poh = The ph is a measure of acidity. When ph = 7, the solution is neutral. When ph > 7, the solution is basic. When ph < 7, the solution is acidic. 1

2 A solution is a homogeneous mixture of solute and solvent. The amount of solute in a given volume is called the concentration of that solution. Molarity is a concentration; it tells us the number of moles of solute in 1 liter of solution. The symbols [ ] mean molarity. Thus, [HCl] = M means we have a solution of HCl that is M in concentration. ph relationships: ph = log [H 3 O + ] poh = log [OH ] ph + poh = 14 [H3O + ] = 10 ph [OH - ]. = 10 poh [H3O + ][OH - ]. = Strong Acids and Bases. Strong acids and bases ionize completely. If [HCl] = M, then {H 3 O + ] = M. If [NaOH] = M, then [OH ] = M. If [Ba(OH) 2 ] = M, then [OH ] = M = M 1) A solution is M Ba(OH) 2. Calculate the [H3O + ] and [OH - ]. 2) What is the poh of a 0.25 M solution of HCl. 3) A solution is M HCl. Calculate the ph, poh, [H3O + ] and [OH - ]. 4) A solution is M NaOH. Calculate the ph and poh. 5) How many moles of OH are there in a mixture of 25.0 ml of a M NaOH solution together with 35.0 ml of M KOH solution? Determine poh of the resulting mixture. 6) What is the molarity of an NaOH solution formed by adding ml water to ml of M NaOH? 7) 30.0 ml of a M HCl is added to 50.0 ml of M HCl and enough water is added to make ml solution. What is the ph of HCl in the final solution? 2

3 Ionization of a weak acid Equilibrium expression: HA(aq) + H 2 O(l) H 3 O + (aq) + A -(aq) K a = Given the Ka and the molarity of HA, we can always solve for [H 3 O + ]; if we have [H 3 O + ], we can find ph. Weak Acid Equilibria 1. Write the equation and the equilibrium expression for the ionization of carbonic acid in water. 2. Write the equation and the equilibrium expression for the ionization of hypochlorous acid in water. 3. What is the ph of a M solution of acetic acid, CH 3 COOH? CH 3 COOH(aq) + H 2 O(l) H 3 O + (aq) + CH 3 COO (aq) Ka = 4. What is the ph of a M solution of arsenic (3) acid. 5. Find the ph of a 0.65 M solution of propionic acid. What is the fraction ionized at equilibrium? 6. What is the conjugate base of NH 4 +? 7. What is the conjugate base of hydrazoic acid? 3

4 Ionization of the conjugate base For any general weak acid, HA, HA(aq) + H 2 O(l) H 3 O + (aq)+ A (aq) and Ka = [ A ][ H3O ] a) What is the conjugate base of HA? b) Complete the equation for the ionization of the conjugate base in water: A (aq) + H 2 O(l) - [ HA] + c) Write the equilibrium expression for this reaction. Is this the same as the Ka expression? What is different? d) Ka values are ionization constants for weak acids in water. We need a value for Kb when we have a conjugate base in water. Ka Kb = Kw, so Kb = Examples: Identify whether the following are conjugate acids or bases as listed on the Ka table. Write the equation for its reaction with water. Write the value of Ka or Kb depending on whether it is an acid or a base. 3 IO 3 H 2 CO 3 N 3 AsO 4 1) Write the equation and the equilibrium expression for the reaction of HCOO and water. Find K b. b) Calculate the ph of a M solution of HCOO. What do you need to know to answer this? 2) Find the ph of a M solution of F. 3) Find the ph of a M solution of NH3. 4

5 4) Find the ph of a M solution of AsO 4 3 Salts that contain conjugate bases of weak acids will form basic solutions. 5) Compute the ph of a M solution of NaF in water. The K a of HF is 6.6 x ) Calculate the ph of a M solution of KNO Hydrolysis of a salt: Hydrolysis is the process of reacting with water to produce H + or OH ions. Many salts will form an acidic or basic solution when dissolved in water. Why? To decide this, we must look at the acid and the base that produced the salt. General Summary for hydrolysis of salts in water: Both the cation and the anion will be hydrolyzed so you have to consider the effect of both ions in the solution. All cations except Group I metals, Ca 2+, Sr 2+, Ba 2+, and Ag + act as weak acids in aqueous solution. Salts of a strong acid + strong base neutral solution Salts of a strong acid + weak base acid solution Salts of a strong base + weak acid basic solution Salts of a weak acid + weak base? If K a (cation ) > K b (anion), acid solution. If not, basic solution Examples: KC 2 H 3 O 2 2 acidic, basic, or neutral? KCl 2 acidic, basic, or neutral? Zn(NO 3 ) 2 2 acidic, basic, or neutral? NH 4 CN 2 acidic, basic, or neutral? 5