Chapter 16 ACIDS AND BASES. (Part I) Dr. Al Saadi. Brønsted Acids and Bases

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1 Chapter 16 ACIDS AND BASES (Part I) Dr. Al Saadi 1 Brønsted Acids and Bases A Brønsted acid is a species that donates a proton. (a proton donor). HCl(aq) + H 2 O(l) H 3 O + (aq) + Cl (aq) acid hydronium ion conjugate base 16.1 A Conjugate base is what remains of the acid after the donation of a proton. A Brønsted base is a species that accepts a proton. (a proton acceptor). NH 3 (aq) + H 2 O(l) NH 4+ (aq) + OH (aq) Dr. Al Saadi base conjugate acid A Conjugate acid is a newly formed protonated species. 2 1

2 Brønsted Acids and Bases 16.1 Any reaction, using the Brønsted theory, involves both an acid and a base. Water is an amphoteric species which can act as a Brønsted acid or as a Brønsted base. Dr. Al Saadi 3 Brønsted Acids and Bases 16.1 Dr. Al Saadi 4 2

3 Brønsted Acids and Bases 16.1 Identify acids, conjugate acids, bases and conjugate bases in the reactions below: (a) H 2 SO 4 (aq) + H 2 O(l) HSO 4 (aq) + H 3 O + (aq) acid base conjugate base conjugate acid (b) NH 4+ (aq) + H 2 O(l) H 3 O + (aq) + NH 3 (aq) acid base conjugate acid conjugate base What is the conjugate base of H 2 PO 4? H 2 PO 4 (aq) + H 2 O(l) HPO 2 4 (aq) + H 3 O + (aq) conjugate base Dr. Al Saadi 5 The Acid Base Properties of Water 16.2 Water is a very weak electrolyte. It undergoes ionization to a very small extent. H 2 O (l) H + (aq) + OH (aq) Autoionization of 2H 2 O (l) H 3 O + (aq) + OH (aq) water The equilibrium expression of water autoionization is: K w = [H + ] [OH ] or K w = [H 3 O + ] [OH ] Dr. Al Saadi 6 3

4 Equilibrium Constant of Water Autoionization Reaction K w = [H + ] [OH ] or K w = [H 3 O + ] [OH ] At 25 C for pure water: [H 3 O + ] = [OH ] = M. K w = ( )( ) = One can influence the concentrations of [H 3 O + ] or [OH ] by adding an acid or base to pure water. However, at 25 C, the product of the hydronium ion and hydroxide ion concentrations is always constant. For example, if you add an acid to pure water, [H 3 O + ] increases and [OH ] decreases (a new equilibrium position), yet K w remains constant. Dr. Al Saadi Equilibrium Constant in Aqueous Solutions 16.2 For any aqueous solution: K w = [H 3 O + ] [OH ] = at 25 C Remember that K w is always constant at a given temperature. However, there can be infinite possibilities of the equilibriumpositions for aqueous solution. o When [H 3 O + ] = [OH ] ; the solution is neutral. o When [H 3 O + ] > [OH ] ; the solution is acidic. o When [H 3 O + ] < [OH ] ; the solution is basic. Dr. Al Saadi 8 4

5 Equilibrium Constant in Aqueous Solutions 16.2 Dr. Al Saadi 9 Equilibrium Constant in Aqueous Solutions 16.2 Calculate l [OH ] in a solution in which hthe concentration of protons is M at 25 C. Is the solution acidic, basic or neutral? At 25 C, K w is always equal to K w = [H 3 O + ] [OH ] w = (0.0012) [OH ] [OH ] = / = M The solution is acidic. Dr. Al Saadi 10 5

6 Equilibrium Constant in Aqueous Solutions 16.2 At 60 C, the value of K w is (a) Predict whether the reaction: 2H 2 O (l) H 3 O + (aq) + OH (aq) is exothermic or endothermic. Since K w increases with temperature, the reaction must be endothermic. (b) Calculate [H + ] and [OH ] in a neutral solution at 60 C. At 60 C, K w = [H + ][OH ] = ; and [H + ] = [OH ] = [H + ] [H + ] [H + ] = ( ) ½ = Dr. Al Saadi 11 The ph Scale 16.3 The ph scale is a convenient way to express the acidity and basicity of a solution (the concentration of H + 3 O ions). The ph of a solution is defined as: ph = log [H 3 O + ] or ph = log [H + ] Example: Two solutions with hydronium ion concentrations of M and M. Find their ph values. 2 sig. fig. The first solution; ph = log ( ) = 4.00 The second solution; ph = log ( ) = decimal places The number of sig. fig in the log is equal to Dr. Al Saadi the number of decimal places in the answer 12 6

7 The ph Scale 16.3 Find the ph of a neutral solution. A neutral solution has [H 3 O + ] = [OH ]. Since K w = [H 3 O + ] [OH ] = then [H 3 O + ] = ph = log ( ) = Lower ph values ( < 7.00 ) Neutral Higher ph values ( > 7.00 ) Higher [H 3 O + ] Lower [H 3 O + ] Acidic solution Basic solution Dr. Al Saadi 13 The ph Values vs. Hydronium Ion Concentrations 16.3 Dr. Al Saadi 14 7

8 The ph Meter 16.3 A ph meter is commonly used inlaboratories to determine the ph of solutions. The ph meter is an electrical device with a probe that measures the proton concentration in a solution and displays the ph value. Dr. Al Saadi 15 The ph Scale 16.3 Since ph = log [H 3 O + ] then [H 3 O + ] = 10 ph A measured ph is a convenient way to determine the concentration of hydronium ions in solutions Dr. Al Saadi 16 8

9 The ph Scale What is the ph of a solution that has a hydronium ion concentration of 6.5 x 10 5 M? ph log[h3o ] 5 ph log[6.54 x 10 ] ph 4.19 What is the hydronium ion concentration of a solution with ph 3.65? ph [ HO ] 10 3 [ HO ] [H3O ] 2. 2x10 Dr. Al Saadi 17 4 M 16.3 The poh Scale 16.3 A poh scale is analogous to the ph scale. It is defined as: poh = log [OH ] Also, [OH ] = 10 poh Dr. Al Saadi 18 9

10 The poh Scale Recall that at 25 C: K w = = [H 3 O + ] [OH ] log ([H 3 O + ] [OH ]) = log ( ) log [H 3 O + ] log [OH ] = ph + poh = ph Dr. Al Saadi 19 The poh Scale What is the poh of a solution that has a hydroxide ion concentration of 4.3 x M? poh log[oh poh log[4.3 x 10 poh What is the hydroxide ion concentration of a solution with poh 8.35? poh [ OH ] 10 [ OH ] 10 Dr. Al Saadi 20 ] 835. [OH ] 4. 5x ]

11 The ph and poh Scale 16.3 What is the hydroxide ion concentration and the poh of a solution that has ph 9.45 at 25 C? Dr. Al Saadi 21 Strong Acids and Bases 16.4 Strong acids and strong bases are completely ionized when dissolvedinin water. Dr. Al Saadi 22 11

12 Strong Acids 16.4 Because strong acids are completely ionized in aqueous solutions, it is easy to determine the ph of that solution. [H 3 O + ] eq = [strong acid] initial Calculate the concentration of HNO 3 and the ph of an aqueous solution that is M in HNO 3 at 25 C. Dr. Al Saadi 23 Strong Bases 16.4 Strong bases are also completely ionized in aqueous solutions. Theyare: Group 1A Group 2A o the hydroxides of the alkaline metals (Li, Na, K, Rb and Cs), and LiOH (aq) Li + (aq) + OH (aq) o the hydroxides of the heaviest earth alkaline metals (Ca, Sr and Ba). Ca(OH) 2 (aq) Ca 2+ (aq) + 2OH (aq) [OH ] eq = [t [strong base] initial The ph of solutions containing a strong base can be easily calculated. Dr. Al Saadi 24 12

13 ph of Strong Bases 16.4 A L 00 solution contains 072mol 0.72 of NaOH at 25 C. Find its ph? Dr. Al Saadi 25 ph of Strong Bases 16.4 At 25 C, what is the ph of a solution that is M in Ca(OH) 2? Ca(OH) 2 completely ionizes, and the [OH ] is equal to twice the initial concentration of Ca(OH) 2. Ca(OH) 2 (aq) Ca 2+ (aq) + 2OH (aq) [OH 2 mol OH ] = M Ca(OH) 2 = M 1 mol Ca(OH) 2 poh = log (0.068) = 1.17 At 25 C ; ph + poh = ph = = Dr. Al Saadi 26 13

14 Most of the acids are weak acids. Weak acids are NOT completely ionized in water. They are ionized in water only to a limited extent. The degree of dissociation of a weak acid depends on: o acid concentration. o equilibrium constant. Weak Acids Molar concentrations Strong acids Weak acids 16.5 Dr. Al Saadi 27 The Acid Ionization Constant For a weak monoprotic acid HA: HA(aq) + H 2 O (l) H 3 O + (aq) + A (aq) or: HA(aq) H + (aq) + A (aq) the equilibrium expression is: K a = [H 3O + ] [A ] [HA] or K a = [H + ] [A ] [HA] K a is called the acid ionization constant. Its magnitude indicates how strong a weak acid is. o Large K a values indicates a stronger acid. o Smaller K a values indicates a weaker acid. Dr. Al Saadi

15 The Acid Ionization Constant 16.5 The lar rger the K a value, the str onger the acid is. Dr. Al Saadi Calculating ph from K a Calculate the ph at 25 C of a 0.18 M solution of a weak acid that has K 92 6 a = 9.2 x K a = [H + ] [A ] [HA] = 9.2 x 10 6 We use the same method we studied in the previous chapter x +x +x 0.18 x x x Continue on the Dr. Al Saadi next slide 30 15

16 K a = [H+ ] [A ] (x) (x) x = 9.2 x 10 6 = = [HA] 0.18 x 0.18 x Since K a is very small compared to the initial acid concentration, we can make use of a useful approximation instead of solving the problem using a quadratic equation. The 5% rule x 2 x 0.18 x 0.18 = 9.2 x 10 6 Solving for x: x = 1.3 x 10 3 M ph = log [H + ] = log (1.3 x 10 3 ) The above approximation is valid only when x is significantly smaller than the [HA] initial. ( x must be less than 5% of [HA] initial in order for the above approximation to be valid). 1.3 x 10 Check the validity => 3 X 100% = 0.72% 0.18 = Dr. Al Saadi < 5% OK 16.5 Calculating ph from K a Try the previous example again with a M solution at 25 C. Dr. Al Saadi 32 16

17 Calculating K a from ph An aqueous solution that is 0.25 M in a monoprotic weak acid has a ph of 3.47 at 25 C. Find its K a. [HA] initial = 0.25 M [H + ] eq = = 3.39 x 10 4 M 16.5 K a = [H+ ] [A ] [HA] x x x x x 10 4 = (3.39 x 10 4 ) = 4.6 x 10 7 Dr. Al Saadi 33 Weak Bases Most of the bases are weak bases. Weak bases are not ionized completely when dissolved in water weak base conjugate acid conjugate base The equilibrium expression for the ionization of a weak base is: K b = [HB+ ] [OH ] [B] K b is called the base ionization constant. Its magnitude indicates how strong a weak base is. o Large K b values indicates a stronger base. o Smaller K b values indicates a weaker base. Dr. Al Saadi 34 17

18 Weak Bases 16.6 The larg ger the K b value, the stron nger the base is. Dr. Al Saadi 35 Weak Bases 16.6 What does make these weak bases able to accept H +? The lone pair of electrons on the N atom enables these weak bases to grab H + ions from Brønsted acids. Dr. Al Saadi 36 18

19 Solving Problems Involving Weak Bases 16.6 Solving problems involving weak bases requires thesame approach used for weak acids. Make sure you notice that solving for x in a weak base problem gives the hydroxide ion concentration rather than the hydronium ion concentration. Dr. Al Saadi 37 Solving Problems Involving Weak Bases 16.6 Determine the K b of a weak base if a 0.50 M solution of the base has a ph of 9.59 at 25 C. At 25 C: poh = ph = = 4.41 [OH ] = = M. B(aq) + H 2 O(l) BH + (aq) + OH (aq) Continue on the Dr. Al Saadi next slide 38 19

20 Solving Problems Involving Weak Bases 16.6 Exercise (Continue): Determine the K b of a weak base if a 0.50 M solution of the base has a ph of 9.59 at 25 C. K b = [BH+ ][OH ] [B] = ( ) K b = Dr. Al Saadi 39 20