ACID BASE EQUILIBRIUM

Transcription

1 ACID BASE EQUILIBRIUM Part one: Acid/Base Theories Learning Goals: to identify acids and bases and their conjugates according to Arrhenius and Bronstead Lowry Theories. to be able to identify amphoteric and polyprotic acids and bases. to write an equilibrium equation for a reaction of an acid or base with water. ARRHENIUS THEORY Acids are solutes which produce hydrogen ions, H +, in aqueous solutions eg. HCl H + + Cl Bases are solutes which produce hydroxide ions, OH, in aqueous solutions eg. NaOH Na + + OH This works for most acids and bases, but does not explain all acids and bases (eg. Ammonia, NH 3 ) 1

2 BRONSTEAD LOWRY THEORY An acid is a proton (H + ) donor A base is a proton (H + ) acceptor Both are present in a system An acid will donate a proton to become a conjugate base, A base will accept a proton to become a conjugate acid The conjugate acid base pair Many times proton transfer can occur in both directions, and both forward and reverse reactions involve Bronstead Lowry acid base pairs The pair of substances which differ by a single proton (H+) are called conjugate acidbase pairs 2

3 Try this.. Use the Brønsted Lowry definitions to identify the two conjugate acid base pairs in each of the following acid base reactions. H 2 S aq + H 2 O l > HS aq + H 3 O + aq A B CB CA Competition for protons A strong acid has a very weak attraction for protons A strong base has a very strong attraction for protons The stronger the acid, the weaker it s conjugate base The stronger the base, the weaker it s conjugate acid 3

4 eg. HCl + H 2 O < Cl + H 3 O + HCl is a very strong acid and therefore has a very weak attraction for protons. It s conjugate base is a very weak base and will therefore have a very weak attraction for protons (l) Cl + H 2 O > HCl + OH (l) The six strong acids.. Hydrochloric Hydrobromic Hydroiodic HCl HBr HI Nitric HNO 3 Sulfuric H 2 SO 4 Perchloric HClO 4 all the rest are weak acids 4

5 Let s try the movie showing an acid being a proton donor Let s try the movie showing a base being a proton acceptor Let s try the movie showing a conjugate acid/base pair Amphoteric (amphoteric) substances A substance that may act as a Bronsted Lowry acid in some reactions and as a Bronsted Lowry base in others. NH 3 + H 2 O (l) NH OH 1 Base_ H 2 CO 3 + H 2 O (l) HCO 3 + H 3 O +1 Acid 5

6 Polyprotic Acids Polyprotic acids have more than one ionizable proton H 3 PO 4 + H 2 O (l) H 3 O + + H 2 PO 4-1 H 2 PO H 2 O (l) H 3 O + + HPO 4-2 HPO H 2 O (l) H 3 O + + PO 4-3 Pg. 492 # 1, 2 Pg. 494 #4, 5 6

7 Part two: 1) The autoionization of water, 2) Calculating acidity for strong acids and Bases Learning Goals To understand how water can ionize itself To calculate [H 3 O + ] and [OH ] based on K w To understand what makes a substance acidic, and to calculate acidity To calculate ph for a strong acid and base, and a weak acid and base The autoionization of water Water can ionize itself: H 2 O < > H + + OH or H 2 O + H 2 O < > H 3 O + + OH (H 3 O + hydronium ion) (OH hydroxide ion) This occurs in very little amount (2 molecules in a billion) K w = 1 X

8 I C E H 2 O (l) + H 2 O (l) <==> H 3 O + + OH K w = [H 3 O + ][OH ] 1 X = [H 3 O + ][OH ] 1 X = x 2 (H 2 O is omitted from the equation because it is a pure liquid) Therefore, in neutral water, [H 3 O + ] = [OH ] = 1 X 10 7 Measuring Acidity Acidity is the measure of the hydrogen ion (H + ) or hydronium ion (H 3 O + ) concentration in a solution. In neutral water [H 3 O + ] = 1 X 10 7 mol/l ph is created to express acidity (power of hydrogen) ph = log [H 3 O + ] ph of neutral water is 7 (try it!!) 8

9 Acids increase the [H 3 O + ] concentration in a solution (decrease [OH ] ) Bases increase the [OH ] concentration in a solution (decrease [H 3 O + ]) K w = [H 3 O + ][OH ] [H 3 O + ] K = w [OH ] [OH ] = K w [H 3 O + ] The six strongest acids. Completely dissociate to produce hydronium ions Calculating the ph can be done easily, because the [acid] = [H 3 O + ] HCl + H 2 O Cl + H 3 O + (l) Strong Bases completely dissociate to produce OH 1 Hydroxides of Group 1 and 2 metals are considered strong bases 9

10 Acid/Base Equilibria Calculating ph and poh Find the ph of a STRONG acid or Base a) Calculate the [H 3 O + ], [OH - ], ph, and poh of a 3.87x10-4 M HNO 3 solution. **Remember, Strong Acids or Bases dissociate completely HNO 3 H + + NO g of NaOH is added to 7.50 L of water. Calculate the [H 3 O + ], [OH - ], ph, and poh 10

11 Pg. 509 #1,2, 5 c f, 6, 7,8 Pg. 527 #1,2 K a for weak acids Weak acids do not completely dissociate into hydronium ions in water, and this affects how acidic the solution is. HX weak acid, conjugate base is X HX + H 2 O > H 3 O + + X (l) The amount of H 3O + ions depends on the extent of dissociation, and therefore Ka 11

12 Measuring strength of acid: K a K a acid equilibrium constant Larger the K a, the greater the acid will dissociate into hydronium ions (increasing acidity) K a for strong acids is so large that the reaction favours products so much that it is considered an irreversible reaction HX + H 2 O > H 3 O + + X (l) K a = [H 3 O + ][X ] [HX] (at equilibrium) Once you know [H 3 O + ], you can calculate ph and you can calculate the : % dissociation= [H 3 O + ] HX X

13 Calculate the ph and the % dissociation of a M acetic acid solution. Calculate the ph and % dissociation of a M NH 3 solution. 13

14 The ph of a M solution of HNO 2 is 2.1. Calculate the value for the acid ionization constant for HNO 2. H/W pg. 516 #1,2 pg. 520 #1,2 pg. 525 #3,5,6, 14