What are Acids &Bases?

Transcription

1 Chemistry 481(01) Spring 2013 Instructor: Dr. Upali Siriwardane Office: CTH 311 Phone Office Hours: 8:00-9:00 a.m. & 11:00-12:00 a.m. M, W; 8:00-10:00 a.m. Tu, Th, F. April 2, 2013: Test 1 (Chapters 1, 2, 3,) April 25, 2013: Test 2 (Chapters 5, 6 & 7) May 14, 2013: Test 3 (Chapters. 19 & 20) May 15, Make Up: Comprehensive covering all Chapters What are Acids &Bases? Definition? a) Arrhenius b) Bronsted-Lowry c) Lewis 1 Chapter 4-2 Arrhenius definitions Acid Anything that produces hydrogen ions in a water solution. HCl (aq) H Cl - Base Anything that produces hydroxide ions in a water solution. NaOH (aq) Na OH - Arrhenius definitions are limited to aqueous solutions. Acid base reactions: HCl(aq) NaOH(aq) NaCl(aq) H 2 O(l) Chapter 4-3 Expands the Arrhenius definitions Acid Brønsted-Lowry definitions Proton donor Base Proton acceptor This definition explains how substances like ammonia can act as bases. NH 3 (g) H 2 O(l) NH 4 OH - Eg. HCl(g) NH 3 (g) > NH 4 Cl(s) HCl (acid), NH 3 (base). Chapter 4-4 1

2 Proton in water Dissociation Equilibrium HCl(aq) H 2 O(l) H 2 SO 4 (aq) H 2 O(l) H 3 O(aq) Cl - (aq) H 3 O(aq) HSO 4- (aq) H 2 O(l) H 2 O(l) H 3 O(aq) OH - (aq) This dissociation is called autoionization of water. HC 2 H 3 O 2 (aq) H 2 O(l) H 3 O(aq) C 2 H 3 O 2- (aq) NH 3 (aq) H 2 O(l) NH 4 OH - (aq) Chapter 4-5 Chapter 4-6 Bronsted conjugate acid/base pairs in equilibria HCl(aq) H 2 O(l) H 3 O(aq) Cl - (aq) HCl(aq): acid H 2 O(l): base H 3 O(aq): conjugate acid Cl - (aq): conjugate base H 2 O/ H 3 O: base/conjugate acid pair HCl/Cl - : acid/conjugate base pair Chapter 4-7 Brønsted-Lowry definitions Conjugate acid-base pairs. Acids and bases that are related by loss or gain of H as H 3 O and H 2 O. Examples. Acid Base H 3 O H 2 O HC 2 H 3 O 2 C 2 H 3 O 2 - NH 4 NH 3 H 2 SO 4 HSO 4 - HSO 4- SO 4 2- Chapter 4-8 2

3 Select acid, base, acid/conjugate base pair, base/conjugate acid pair H 2 SO 4 (aq) H 2 O(l) acid base conjugate acid conjugate base base/conjugate acid pair acid/conjugate base pair H 3 O(aq) HSO 4- (aq) Types of Acids and Bases Binary acids Oxyacid Organic acids Acidic oxides Basic oxides Amine Polyprotic acids Chapter 4-9 Chapter 4-10 Binary Acids Compounds containing acidic protons bonded to a more electronegative atom. e.g. HF, HCl, HBr, HI, H 2 S The acidity of the haloacid (HX; X = Cl, Br, I, F) Series increase in the following order: HF < HCl < HBr < HI Oxyacids Compounds containing acidic - OH groups in the molecule. Acidity of H 2 SO 4 is greater than H 2 SO 3 because of the extra O (oxygens) The order of acidity of oxyacids from the a halogen (Cl, Br, or I) shows a similar trend. HClO 4 > HClO 3 > HClO 2 >HClO perchloric chloric chlorus hyphochlorus Chapter 4-11 Chapter

4 Aqua Acids Acidic proton is on a water molecule coordinated to a central metal ion [Fe(OH 2 ) 6 ] 3,Al(OH 2 ) 6 3, Si(OH) 4 Acidity increase with charge Acidity increase as metal become smaller Anhydrous oxides The Lux/Flood Definition Covers things which would become acids or bases if dissolved in water. Acidic Oxides These are usually oxides of non-metallic elements such as P, S and N. E.g. NO 2, SO 2, SO 3, CO 2 They produce oxyacids when dissolved in water Chapter 4-13 Chapter 4-14 Basic Oxides Oxides oxides of metallic elements such as Na, K, Ca. They produce hydroxyl bases when dissolved in water. e.g. CaO H 2 O --> Ca(OH) 2 Chapter 4-15 Protic Acids Monoprotic Acids: The form protic refers to acidity or protons. Monoprotic acids have only one acidic proton. e.g. HCl. Polyprotic Acids: They have more than one acidic proton. e.g. H 2 SO 4 - diprotic acid H 3 PO 4 - triprotic acid. Chapter

5 Amines Class of organic bases derived from ammonia NH 3 by replacing hydrogen by organic groups. They are defined as bases similar to NH 3 by Bronsted or Lewis acid/base definitions. What acid base concepts (Arrhenius/Bronsted/Lewis) would best describe the following reactions: a) HCl(aq) NaOH(aq) ---> NaCl(aq) H 2 O(l) b)hcl(g) NH 3 (g) ---> NH 4 Cl(s) c)bf 3 (g) NH 3 (g) ---> F 3 B:NH 3 (s) d)zn(oh) 2 (s) 2OH - (aq) ---> [Zn(OH) 4 ] 2- (aq) Chapter 4-17 Chapter 4-18 Common acids and bases Acids Formula Molarity* nitric HNO 3 16 hydrochloric HCl 12 sulfuric H 2 SO 4 18 acetic HC 2 H 3 O 2 18 Bases ammonia NH 3 (aq) 15 sodium hydroxide NaOH solid *undiluted. Acidic Acids and bases Basic Citrus fruits Baking soda Aspirin Detergents Coca Cola Ammonia cleaners Vinegar Tums and Rolaids Vitamin C Soap Chapter 4-19 Chapter

6 Equilibrium, Constant, K a & K b K a : Acid dissociation constant for a equilibrium reaction. K b : Base dissociation constant for a equilibrium reaction. Acid: HA H 2 O H 3 O A - Base: BOH H 2 O B OH - [H 3 O][ A - ] [B ][OH - ] K a = ; K b = [HA] [BOH] What is K a HCl(aq) H 2 O(l) <===> H 3 O(aq) Cl - (aq) Chapter 4-21 Chapter 4-22 What is K a1 and K a2? E.g. K a HCl(aq) H 2 O(l) [H 3 O][Cl-] K a = [HCl] [H ][Cl-] K a = [HCl] H 3 O(aq) Cl - (aq) H 2 SO 4 (aq) H 2 O(l) HSO 4- (aq) H 2 O(l) H 3 O(aq) HSO 4- (aq) H 3 O(aq) SO 4 2- (aq) Chapter 4-23 Chapter

7 What is K b H 2 SO 4 Dissociation NH 3 (aq) H 2 O(l) NH 4 OH - (aq) E.g. H 2 SO 4 (aq) H 2 O(l) HSO 4- (aq) H 2 O(l) H 3 O(aq) HSO 4- (aq) H 3 O(aq) SO 2-4 (aq) [H 3 O][HSO 4- ] H 2 SO 4 ; K a1 = [H 2 SO 4 ] [H 3 O][SO 4 2- ] H 2 SO 4 ; K a2 = [HSO 4- ] Chapter 4-25 Chapter 4-26 K a and K b Acidity/Basicity of HA and F - E.g. HC 2 H 3 O 2 (aq) H 2 O(l) H 3 O(aq) C 2 H 3 O 2- (aq) [H ][C 2 H 3 O 2- ] H C 2 H 3 O 2 ; K a = [H C 2 H 3 O 2 ] NH 3 (aq) H 2 O(l) NH 4 OH - (aq) [NH 4 ][OH - ] NH 3 ; K b = [ NH 3 ] Chapter 4-27 Chapter

8 Which is weaker? a. HNO 2 ; K a = 4.0 x b. HOCl 2 ; K a = 1.2 x c. HOCl ; K a = 3.5 x d. HCN ; K a = 4.9 x WEAKER/STRONGER Acids and Bases & K a and K b values A larger value of K a or K b indicates an equilibrium favoring product side. Acidity and basicity increase with increasing K a or K b. pk a = - log K a and pk b = - log K b Acidity and basicity decrease with increasing pk a or pk b. Chapter 4-29 Chapter 4-30 Autoionization of water Autoionization When water molecules react with one another to form ions. H 2 O (l) H 2 O (l) K w = [ H 3 O ] [ OH - ] H 3 O (aq) OH - (aq) (10-7 M) (10-7 M) = 1.0 x at 25 o C ion product of water Note: [H 2 O] is constant and is included in K w. H 2 O(l) H 2 O(l) What is K w? H 3 O(aq) OH - (aq) This dissociation is called autoionization of water. Autoionization of water: K w = [H 3 O][OH - ] K w is called ionic product of water K w = 1 x Chapter 4-31 Chapter

9 Why is water important for acid/base equilibria? Water is the medium/solvent for acids and bases. Acids and bases alter the dissociation equilibrium of water based on Le Chaterlier s principle H 2 O(l) H 2 O(l) H 3 O(aq) OH - (aq) Comparing K w and K a & K b Any compound with a K a value greater than K w of water will be a an acid in water. Any compound with a K b value greater than K w of water will be a base in water. Chapter 4-33 Chapter 4-34 ph and other p scales We need to measure and use acids and bases over a very large concentration range. ph and poh are systems to keep track of these very large ranges. ph = -log[h 3 O ] poh = -log[oh - ] ph poh = 14 ph scale A logarithmic scale used to keep track of the large changes in [H ] M 10-7 M 1 M Very Neutral Very Basic Acidic When you add an acid, the ph gets smaller. When you add a base, the ph gets larger. Chapter 4-35 Chapter

10 ph of some common materials Substance ph 1 M HCl 0.0 Gastric juices Lemon juice Classic Coke 2.5 Coffee 5.0 Pure Water 7.0 Blood Milk of Magnesia 10.5 Household ammonia M NaOH 14.0 What is ph? K w = [H 3 O][OH - ] = 1 x [H 3 O][OH - ] = 10-7 x 10-7 Extreme cases: Basic medium [H 3 O][OH - ] = x 10 0 Acidic medium [H 3 O][OH - ] = 10 0 x ph value is -log[h ] spans only 0-14 in water. Chapter 4-37 Chapter 4-38 ph, pk w and poh The relation of ph, K w and poh K w = [H ][OH - ] log K w = log [H ] log [OH - ] -log K w = -log [H ] -log [OH - ] ; previous equation multiplied by -1 pk w = ph poh; pk w = 14 since K w =1 x = ph poh ph = 14 - poh poh = 14 - ph Strong acids Weak acids Acid and Base Strength Ionize completely in water. HCl, HBr, HI, HClO 3, HNO 3, HClO 4, H 2 SO 4. Partially ionize in water. Most acids are weak. Strong bases Ionize completely in water. Strong bases are metal hydroxides - NaOH, KOH Weak bases Partially ionize in water. Chapter 4-39 Chapter

11 ph 3/28/2013 ph and poh calculations of acid and base solutions a) Strong acids/bases dissociation is complete for strong acid such as HNO 3 or base NaOH [H ] is calculated from molarity (M) of the solution b) weak acids/bases needs K a, K b or percent(%)dissociation Titration curves Indicator Transition Buffer region Overtitration Equivalence Point Chapter 4-41 % titration or ml titrant Chapter 4-42 Indicators Acid-base indicators are highly colored weak acids or bases. HIn In - H color 1 color 2 They may have more than one color transition. Example. Thymol blue Red - Yellow - Blue One of the forms may be colorless - phenolphthalein (colorless to pink) Chapter 4-43 Selection of an indicator for a titration a) strong acid/strong base b) weak acid/strong base c) strong acid/weak base d) weak acid/weak base Calculate the ph of the solution at he equivalence point or end point Chapter

12 Common Ion Effect Weak acid and salt solutions E.g. HC 2 H 3 O 2 and NaC 2 H 3 O 2 Weak base and salt solutions E.g. NH 3 and NH 4 Cl. H 2 O C 2 H 3 O 2 - <==> OH - HC 2 H 3 O 2 (common ion) H 2 O NH 4 <==> H 3 O NH 3 (common ion) Chapter 4-45 Buffers Solutions that resist ph change when small amounts of acid or base are added. Two types weak acid and its salt weak base and its salt HA(aq) H 2 O(l) H 3 O (aq) A - (aq) Add OH - Add H 3 O shift to right shift to left Based on the common ion effect. Chapter 4-46 Buffers The ph of a buffer does not depend on the absolute amount of the conjugate acid-base pair. It is based on the ratio of the two. Henderson-Hasselbalch equation. Easily derived from the K a or K b expression. Starting with an acid [A ph = pk a log - ] [HA] Starting with a base [HA] ph = 14 - ( pk b log [A - ] ) Chapter 4-47 Control of blood ph Buffers and blood Oxygen is transported primarily by hemoglobin in the red blood cells. CO 2 is transported both in plasma and the red blood cells. CO 2 (aq) H 2 O H 2 CO 3 (aq) The bicarbonate buffer is essential for controlling blood ph H (aq) HCO 3 - (aq) Chapter

13 Main Group Acid/Bases Amphoteric Oxides Chapter 4-49 Chapter 4-50 Strength of oxo-acids by Paulings Rules pk a Values of Oxy Acids For O p E(OH) q, pk a ~ 8-5p The successive pk a values of polyprotic acids (i.e. q >1) increase by 5 units for each successive proton transfer. Chapter 4-51 Chapter

14 Lewis Definition Lewis was successful in including acid and bases without proton or hydroxyl ions. Lewis Acids/Bases Lewis Acid: A substance that accepts an electron pair. Lewis base: A substance that donates an electron pair. E.g. BF 3 (g) :NH 3 (g) F 3 B:NH 3 (s) Chapter 4-53 Chapter 4-54 Hard and soft acids and bases Solvent leveling If the solvent contains ionizable protons it is said to be protonic, and if it is protonic, it will engage in acid-base reactions. All acids/bases which are stronger than the H 3 O (aq) or OH - (aq) ion will react to produce hydronium/hydroxide ion, and so their strength will be leveled to that of the H 3 O (aq) or OH - (aq) ion. In aqueous solution, the strongest acid/base which can exist is the H 3 O (aq) or OH - (aq) Chapter 4-55 Chapter

15 Acid-Base Discrimination Windows Levelling effect in other protic liquid all acids are levelled to the strength of the ammonium ion, NH 4, and all bases are levelled to the strength of the amide ion, NH NH 3 NH 4 NH 2-. 2HNO 3 H 2 NO 3 NO 2-3HF H 2 F HF 2-2H 2 SO 4 H 3 SO 4 HSO 4 - Chapter 4-57 Chapter 4-58 Polycation Formation Polyanion Formation:Zeolites The general method of zeolite production involves dissolving an aluminium source (metal or oxide) into an aqueous solution of sodium or potassium hydroxide. Once this solution has cooled the silica source (and organic template, if required) is added in The form of an aqueous slurry and the resulting gel stirred until homogenous. Na 12 [(AlO 2 )12(SiO 2 ) 12.27H 2 O NaAl 13 O 4 (OH) 24 (H 2 O) 12 (SO 4 ) 4.x H 2 O Chapter 4-59 Chapter