Acids and Bases. Basic Definitions & Concepts

Transcription

1 Acids and Bases CHEM 102! T. Hughbanks! Basic Definitions & Concepts Most basic concepts are given clearly in your text - these notes will only list these as topics discussed, so there will be less detail.! Brønsted acids, bases (and distinction with Arrhenius and Lewis definitions)! Strong acids and bases!!(eg., HCl, H 2 SO 4 ; NaOH, KNH 2 )!

2 Acids & Bases: A Review In Chapter 3, there are two definitions of acids and bases: the Arrhenius and the Brønsted Lowry definition.! Arrhenius acid: Any substance that when dissolved in water increases the concentration of hydrogen ions, H +.! Arrhenius base: Any substance that increases the concentration of hydroxide ions, OH, when dissolved in water.! A Brønsted Lowry acid is a proton (H + ) donor.! A Brønsted Lowry base is a proton acceptor.! Strong & Weak Acids/Bases

3 Strong & Weak Acids/Bases HNO 3, HCl, H 2 SO 4 and HClO 4 are classified as strong acids.! HCl(aq) + H 2 O(l) H 3 O + (aq) + Cl (aq)! Strong & Weak Acids/Bases Other common strong bases include KOH and Ca(OH) 2.! CaO (lime) + H 2 O! Ca(OH) 2 (slaked lime)!

4 Acidic and Basic Oxides! The oxides that one uses to form acids and bases in aqueous solution often have reactivity that reflects their acidic or basic character.! Examples: Li 2 O, CaO, and BaO react with water to form basic solutions and can react with acids directly to form salts. Likewise, SO 3, CO 2, and N 2 O 5 form acidic aqueous solutions and can react directly with bases to give salts.! Basic Definitions & Concepts Weak acids and bases!!(eg., HCN, HF, CH 3 COOH; NH 3, pyridine, CH 3 NH 2 )! Conjugate acids and Bases, eg.,! conjugate base acid HCN(aq) + H 2 O H 3 O + (aq) + CN (aq)! acid conjugate base

5 Water Autoionization (Autoprotolysis)! Water self-dissociates, even in the absence! of added acids or bases:!!h 2 O + H 2 O H 3 O + (aq) + OH (aq)! K eq = [H 3 O + ][OH ] = = K w [H 2 O] is constant, so a H 2O = 1.! In pure water, what are [H 3 O + ] & [OH ]?! What is G for this reaction?! ph Scale H 3 O + concentration is conveniently measured! using the logarithmic ph scale:!!!!ph = -log[h 3 O + ]! In pure H 2 O,!!![H 3 O + ] = 10 7, ph = -log 10 7 = 7! poh is defined similarly: poh = -log[oh - ]!

6 ph and poh ph = -log [H 3 O + ]! poh = -log [OH - ]! In aqueous solutions, [H 3 O + ][OH - ] = 10 14! This is true even with added acids or bases.! -log {[H 3 O + ][OH - ]} = -log {10 14 }! -log [H 3 O + ] - log [OH - ] =14! ph + poh = 14! The acid-base Seesaw in Water - the ph Scale! 2 H 2 O H 3 O + (aq) + OH (aq) K eq = [H 3 O + ][OH ] = = K w The autoionization equilibrium always operates in aqueous solution, even when other reactions are occurring.! It is convenient to work with -log 10 (ph) scale:! log K w = -log[h 3 O + ] log[oh ] = 14 pk w = ph + poh = 14

7 ph vs poh -log {[H 3 O + ][OH ]} =! -log {10 14 }! -log [H 3 O + ] - log [OH ] =14! ph + poh = 14! ph for Strong Acids and Bases! There are only three cases where calculations differ:! (a) Very high concentrations - activities differ from concs. - we won t worry about treating this case quantitatively! (b) Low to moderate concs. - very easy to handle (eg., ph of a 0.05 M strong acid or base)! (c) Very dilute - e.g., M LiOH!!

8 Acids - Definition of K a! HA(aq) + H 2 O H 3 O + (aq) + A (aq) K eq = [H 3O + ][A ] [HA] = K a Once again, the water term is omitted in tabulating the equilibrium constants of acids.! An acid, say HA, produces H 3 O + in water. K eq for an acid is called K a.! pk a is defined like ph: pk a log K a! Definition of K b Example:! NH 3 (aq) + H 2 O NH + 4 (aq) + OH (aq) Again, [H 2 O] is not included.! pk b log K b! K b = [NH + 4 ][OH ] [NH 3 ]

9 ph for Weak Acids and Bases! Success in calculations depends on skill in using approximations! What to remember:! (a) Except for very dilute solutions, [OH ] can be initially neglected in acidic solutions; [H 3 O + ] can be initially neglected in basic solutions! (b) At moderate to high concentrations, the percent dissociation is low!! ph for Weak Acids and Bases! Even when approximations fail, remember what you have to use:! (a) Autoionization: K w! (b) Equilibrium constants: K a or K b! (b) charge balance! (d) mass balance!

10 Example 1! CH 3 COOH(aq) + H 2 O!!!!H 3 O + (aq) + CH 3 COO (aq)! K a = [H 3O + ][CH 3 COO ] [CH 3 COOH] = For acetic acid, pk a = What is the ph of a 0.1 M solution?! Example 2! NH 3 (aq) + H 2 O NH 4 + (aq) + OH (aq)! K b = [NH 4 + ][OH ] [NH 3 ] = For ammonia, pk b = What is the ph of a 0.1 M solution? What % of NH 3 is converted to NH 4 +?!

11 Example 3! HF(aq) + H 2 O H 3 O + (aq) + F (aq)! For hydrofluoric acid, pk a = What is the ph of a 10-3 M solution?! ph - case of high dilution! M LiOH - two sources of OH! LiOH Li + (aq) + OH (aq)! 2 H 2 O H 3 O + (aq) + OH (aq)! The set-up (what we know):! [Li + ] = ;![H 3 O + ][OH ] = ! Charge balance: [OH ] = [Li + ] + [H 3 O + ]! Solve, [OH ] = poh = 6.89! So, [H 3 O + ] = [OH ] [Li + ] = ! ph = 7.11 (check: ph + poh = 14)!

12 Conjugate Acids and Bases HCN(aq) + H 2 O H 3 O + (aq) + CN (aq)! acid conjugate base acid conjugate base HCN is a rather weak acid, K a = (pk a = 9.31). What is the general relationship between the strengths of conjugate acidbase pairs?! What is K b for the conjugate base, CN -?! Conjugate acid-base pairs! HClO 2 (aq) + H 2 O H 3 O + (aq) + ClO 2 (aq)! HCO 2 H(aq) + H 2 O H 3 O + (aq) + HCO 2 (aq)! HClO(aq) + H 2 O H 3 O + (aq) + ClO (aq)! NH + 4 (aq) + H 2 O H 3 O + (aq) + NH 3 (aq)! CH 3 NH + 3 (aq) + H 2 O H 3 O + (aq) + CH 3 NH 2 (aq)!

13 Ionization Constants for Acids/Bases Increase strength! Increase strength! Equilibrium Constants for Acids & Bases (Guide to Table 17.3) The strongest acids are at the upper left. They have the largest K a values.! K a values become smaller on descending the chart as the acid strength declines.! The strongest bases are at the lower right. They have the largest K b values.! K b values become larger on descending the chart as base strength increases.!