Acids and Bases. Dr. Diala Abu-Hassan, DDS, PhD Lecture 2 Nursing First Semester 014. Dr. Diala Abu-Hassan 1

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1 science.lotsoflessons.com Acids and Bases, DDS, PhD Lecture 2 Nursing First Semester 014 1

2 Outline Definitions of acids and bases Acid and base strength The dissociation constant Conjugate pair strength Water dissociation ph Monoprotic and polyprotic acids 2

3 Different Definitions of Acids and Bases - Arrhenius - Bronsted-Lowry - Lewis 3

4 Arrhenius Definition of Acids and Bases and Their Reactions Arrhenius Acids and Bases Acids are Acids in H 2 O are H + donors Bases in H 2 O are OH - donors Neutralization of acids and bases produces salt and water. NaOH H 2 O Na + + OH - HCL H 2 O Cl - + H + H + + H Ö: H 3 O + H Drawbacks: 1. Reactions has to happen in aqueous solutions 2. H 3 O + is released but not H + Arrhenius 1903 Nobel Prize 4

5 Bronsted-Lowry Definition of Acids and Bases and Their Reactions Bronsted-Lowry Acids and Bases (1923) Acids donate H + Bases accept H + (non-bonding pairs) Conjugate acid-base pair B: + H A BH + + A - Base Acid Acid Base Conjugate acid-base pair 5

6 Lewis Definition of Acids and Bases and Their Reactions Acids accept electrons Bases donate electrons (non-bonding pairs) BF 3 + : NH 3 F 3 B : NH 3 Acid base 6

7 Common Acids HCl- hydrochloric- stomach acid H 2 SO 4 - sulfuric acid - car batteries HNO 3 nitric acid - explosives HC 2 H 3 O 2 - acetic acid - vinegar H 2 CO 3 -carbonic acid sodas H 3 PO 4 - phosphoric acid -flavorings 7

8 Common Bases NaOH- sodium hydroxide (LYE) soaps, drain cleaner Mg (OH) 2 - magnesium hydroxide-antacids Al(OH) 3 -aluminum hydroxide-antacids, deodorants NH 4 OH-ammonium hydroxide- ammonia 8

9 Water as both an acid and a base Amphoterism - an ion or molecule can act as an acid or base depending upon the reaction conditions Water in NH 3 serves as an acid H 2 O + NH 3 NH OH - acid base acid base Water in acetic acid serves as a base H 2 O + CH 3 CO 2 H H 3 O + + CH 3 CO 2 - base acid acid base Conjugate acid of H 2 O Conjugate base of acetic acid 9

10 Measuring concentrations Molarity: the number of moles in a liter of solution Unit: Mole/Liter = M mm = 10-3 M, um = 10-6 M Symbol: [X] Concentration = Amount of solute Amount of solvent Dissolve 2 moles of glucose in 5 liters of H 2 0. what is the concentration? [Glucose] = 2/5 = 0.4 M 10

11 Acid and Base Strength Some acids can cause burns if come in contact with skin, other acids are safe. why? How easy can the acid produce proton Strong acid: gives up H + easily (100% dissociated in water) HCl H + + Cl - Weak acid: gives up H + with difficulty ( less than 100% dissociated) CH 3 COOH H + + CH 3 COO -

12 K a Acid dissociation constant, K a The general ionization of an acid is as follows: HA + H 2 O H 3 O + + A - So the acid dissociation constant is as follows: [H 3O ][A ] [HA][H O] 2 [H 2 O] = 55.5 M and is constant in all equations [H O ][A K 3 a [HA] ] 12

13 Acid dissociation constant K a = [H + ][A - ] [HA] Larger K a means: More dissociation Smaller pk a Stronger acid For simplicity: pk a = log 1/K a = - log 10 K a The larger the value of the pk a, the smaller the extent of dissociation. 13

14 Decrease acid strength Increase acid strength 14

15 Acid dissociation constant 15

16 Base dissociation constant B + H 2 O BH + + OH - K b [BH ][OH [B] ] Reverse the reaction: BH + B + H + K a [B][H [BH ] ] 16

17 Weak Bases 17

18 Strong Acids - Dissociate readily - Ka is very large - Examples: Hydrochloric, Nitric; Sulfuric Ex. HCl H + + Cl - Ka = [H + ] [Cl - ] [HCl] [H + ] = [acid] Ex.1 M solution of HCl has a [H + ] of 1 mm HCl solution has a [H + ] of 0.1 M H 2 SO 4 solution has a [H + ] of 1 M 1 mm 0.2 M 18

19 Weak Acids - Dissociate slightly - Ka is smaller than strong acids - Examples: Acetic, Boric, Nitrous, Phosphoric, Sulfurous Ex. CH 3 COOH + H 2 O CH 3 COO - + H 3 O + Ka = [H+] [CH 3 COO - ] [CH 3 COOH] What is the H+ of a 0.1 M solution of acetic acid? Ka= 1.74 X X 10-5 = x 2 /[0.1 - x] x 2 = 1.74 X 10-6, or x = 1.32 X 10-3 M 19

20 Conjugate pair strength HA + H 2 O H 3 O + + A - HA + H 2 O H 3 O + + A - CH 3 COOH + H 2 O CH 3 COO - + H 3 O + Weak acid weak base strong base strong acid CH 3 COOH + OH - CH 3 COO - + H 2 O Stronger acid strong base weak base weak acid Strong acid + strong base Weak base + Weak acid 20

21 Equilibrium constant and the ph of water H 2 O dissociates to a slight extent to form hydrogen (H + ) and hydroxyl (OH - ) ions. H 2 O H + + OH - [H+] of pure water is only M 21

22 K w = [H + ] [OH - ] = Dissociation of water Example: A solution has an [OH - ] = 10-9 M [H 3 O + ] = 10-5 M

23 Example: Problem solving What is the [H + ] of a 0.01 M NaOH solution? Kw = [H + ] x [OH - ] = [H + ] x 10-2 = [H + ] = M Example: What is the [OH - ] of a 0.01 M HCl solution? Kw = [H + ] x [OH - ] = 10-2 x [OH - ] = [OH - ] = M

24 Example : The [H + ] of a 0.03 M weak base solution is 1 x M. Calculate pkb? B + H 2 O BH + + OH - [OH - ] = 10-4 K b = (10-4 x 10-4 ) / 0.03 = 3.33 x 10-7 M pk b = -log K b =

25 Measuring the acidity of solutions, ph ph = -log 10 [H + ] [H 3 O + ] = 10 -ph Example 1: [H 3 O + ] in household bleach is M ph = - log [10-12 ] = 12 Example 2: Orange juice has a ph of 4 [H 3 O + ] = 10-4 M 25 Garrett et al

26 Acidic ph in = solutions more H+ Basic = more OH - Acids Neutral Bases 26

27 The ph scale is a logarithmic scale. ph One ph unit difference implies a 10-fold difference in [H+]. Example: lemon juice at ph 2.0 contains more than 100 times as much H+ as orange juice at ph 4.0 Garrett 27 et al

28 ph problem solving Example 1: What is the ph of a solution whose hydrogen ion concentration is 3.2 x 10 4 mol/l? ph= -log [H + ] = -log (3.2 x 10-4 ) = -log (3.2) log (10-4 ) = =

29 Enzyme activity and ph Campbell 6 th edition 29

30 Example: Find the K a of a 0.04 M weak acid HA whose ph is 4.0? HA H + + A - K a = [A - ] [H + ] / [HA] = [H + ] 2 / [HA] = 10-4 x10-4 / 0.04 = 2.5 x 10-7 Example 2: What is the ph of a 0.05 M Ba(OH) 2? Ba(OH) 2 Ba + 2OH - [OH - ] = 2x 0.05 = 0.10 M = 1 x 10-1 poh = - log 1 x 10-1 = 1 Remember poh = -log [OH - ] ph= 14-1 = 13 [H + ] = 1x

31 Example: The K b for ammonia is 1.8 x 10-5 M. What is the ph of 1 x 10-2 M of ammonia? NH 3 + H 2 O NH OH - K b = [NH 4+ ] [OH - ] / [NH 3 ] 1.8 x 10-5 = [OH - ] 2 / 0.01 [OH - ] = 4.24 x 10-4 M poh = -log 4.24 x 10-4 = 3.37 ph = =

32 Monoprotic- and PolyproticAcids 1. Monoprotic acids have only one ionizable proton. 2. Polyprotic acids have more than one ionizable proton. 3. The protons are removed in steps, not all at once. CH 3 COOH + NaOH CH 3 COO - Na + + H 2 O pk a1 = 2.14 pk a2 = 7.20 pk a3 = 12.4 H 3 PO 4 H 2 PO - 4 HPO -2 4 PO H + H + H + 32

33 Polyprotic Acids - It is always easier to remove the first proton in a polyprotic acid than the second. - K a1 > K a2 > K a3 33