Chapter 7: Phenomena Phenomena: Scientists dissolved different substances in water and then measured the [H + ] and [OH - ] concentrations in each solution. What patterns do you notice about the substances? Substance Amount of substance dissolved in 1 L of water [H + ] [OH - ] HCl 1.0 mol 1.0 M 1.0 10-14 M KOH 1.0 mol 1.0 10-14 M 1.0 M HC 2 H 2 ClO 2 1.0 mol 0.031 M 3.2 10-13 M HBr 1.0 mol 1.0 M 1.0 10-14 M HOCl 1.0 mol 1.9 10-4 M 5.3 10-11 M CH 3 NH 2 1.0 mol 4.8 10-13 M 0.021 M NH 3 1.0 mol 2.4 10-12 M 0.0042 M H 3 1.0 mol 1.0 M 1.0 10-14 M NaOH 1.0 mol 1.0 10-14 M 1.0 M C 5 H 5 N 1.0 mol 2.4 10-10 M 4.1 10-5 M H 2 O N/A 1.0 10-7 1.0 10-7 Big Idea: A Bronsted-Lowry acid is a proton donor and a Bronsted-Lowry base is a proton acceptor. After an acid/base loses/gains its proton it becomes a conjugate base/acid. Acids and bases can either completely dissociate (strong) or incompletely dissociate (weak). An equilibrium problem must be set up to solve for the ph of a weak acid or base. Chapter 7 Acids and Bases Conjugate Acids/Bases ph/poh Scales ph/poh of Strong Acids/Bases ph/poh of Weak Acids/Bases Acid/Base Properties of Salts 2 Acid Sour taste (lemon citric acid) Dissolve many metals (Acid(aq) + metal(s) salt(aq) + H 2 (g)) Turn litmus paper red Base Bitter taste (unsweetened baker s chocolate) Slippery feel (cleaning products) Turn litmus paper blue Arrhenius (1884) Acid: A compound that forms hydrogen ions (H + ) in water. Examples: HCl(aq) acid CH 4 (aq) not an acid because it does not release (H + ) ions in solution Base: A compound that forms hydroxide ions (OH - ) in water. Examples: NaOH(aq) base NH 3 base because NH 3 (aq) + H 2 O(l) NH 4+ (aq) + OH - (aq) Bronsted-Lowry (1923) Acid: A proton donor. Base: A proton acceptor. 3 4 Deprotonation: The loss of a proton from a Bronsted-Lowry acid Note: First deprontination is the loss of the first H, the second deprotination is the loss of a second H, and the third deprotination is the loss of a third H. Conjugate Acid Base Pair: Two substances that are related to each other by the transfer of one proton Amphoteric: A substance that can act as an acid or base Example: H 2 O 5 6 1
Conjugate Acids/Bases Which of the following represent conjugate acid-base pairs? For those pairs that are not conjugates, write the correct conjugate acid or base for each species in the pair. a) H 2 SO 4 and SO 4 b) H 2 PO 4- and HPO 4 c) HClO 4 and Cl - d) NH 4+ and NH 2 - Strong Acid or Base: An acid/base that completely ionizes in solution. Weak Acid or Base: An acid/base that does not completely ionize in solution. Strong Acids HCl H 3 HBr HClO 4 HI HClO 3 HBrO 3 HBrO 4 H 2 SO 4 HIO 4 Strong Bases LiOH Sr(OH) 2 NaOH Ca(OH) 2 KOH Ba(OH) 2 RbOH Mg(OH) 2 CsOH 7 8 Name Formula K a Hydrogen Sulfate Ion HSO - 4 1.2 10-2 Chlorous Acid HClO 2 1.2 10-2 Monochloracetic Acid HC 2 H 2 ClO 2 1.35 10-3 Hydrofluoric Acid HF 7.2 10-4 Nitrous Acid H 2 4.0 10-4 Acetic Acid HC 2 H 3 O 2 1.8 10-5 Hydrated Aluminum(III) Ion [Al(H 2 O) 6 ] 3+ 1.4 10-5 Hypochlorous Acid HOCl 3.5 10-8 Hydrocyanic Acid HCN 6.2 10-10 Ammonium Ion NH + 4 5.6 10-10 Phenol HOC 6 H 5 1.6 10-10 Name Formula K b Ammonia NH 3 1.8 10-5 Methylamine CH 3 NH 2 4.38 10-4 Ethylamine C 2 H 5 NH 2 5.6 10-4 Aniline C 6 H 5 NH 2 3.8 10-10 Pyridine C 5 H 5 N 1.7 10-9 Note: A strong acid is defined as an acid that has a K a larger than 1. Not all strong acids have the same K a. For example the K a of H 3 O + is 55, whereas the K a of HCl is 1 10 6. Therefore, H 3 O + is one of the weakest strong acids. Note: The stronger the base, the larger the K b. 9 10 The K a values for HPO 4 and HSO 3- are 4.8 10-13 and 6.3 10-8 respectively. Therefore, it follows that HPO 4 is a acid than HSO 3- and PO 4 3- is a base than SO 3. a) weaker, weaker b) weaker, stronger c) stronger, weaker d) stronger, stronger Is there a relationship between K a and K b? General Weak Acid Equilibrium Equation HA(aq) H + (aq) + A - (aq) General Weak Base Equilibrium Equation B(aq) + H 2 O(l) BH + (aq) + OH - (aq) Water Equilibrium Equation H 2 O(l) H + (aq) + OH - (aq) 1.0 10 Note: K W is known as the ion product constant. 11 12 2
ph/poh Scale How do you tell if a solution acidic, basic, or neutral? [H + ] = [OH - ] neutral [H + ] > [OH - ] acidic [H + ] < [OH - ] basic Are the [H + ] and [OH - ] related? 1.0 10 For neutral solutions 1.0 10 [H + ] > 1.0 10-7 and [OH - ] < 1.0 10-7 acidic [H + ] < 1.0 10-7 and [OH - ] > 1.0 10-7 basic How do you calculate ph? ph=-log[h + ] ph = 7 neutral ph < 7 acidic ph > 7 basic ph/poh Scale Lye Milk of magnesia Ammonia Average seawater Alkaline soil Neutral (above 7) Baking soda Theoretical ph of rain Milk Human blood Acidic soil (5.5-6.5) Most acidic rainfall recorded in U.S. Vinegar Tomato juice Battery acid Apples Lemon juice 13 14 ph/poh of Strong Acids/Bases Calculate the ph of 0.25 M Ba(OH) 2. a) 0.60 b) 13.10 c) 13.40 d) 13.70 e) None of the above ph/poh of Weak Acids/Bases What is the ph of a 0.18 M monoprotic acid solution whose conjugate base has a K b = 2.8 x10-8? a) 3.22 b) 2.85 c) 3.90 d) 4.70 e) None of the above 15 16 Acid/Base Properties of Salts Is NH 4 C 2 H 3 O 2 : Helpful Information: K b NH 3 = 1.8 10-5 and K a HC 2 H 3 O 2 = 1.8 10-5 a) Acid b) Base c) Neutral d) More information needed ph water ~7, ph of unpolluted rain ~5.7 ph of rain in industrial areas has been recorded at ~2.4 17 18 3
What are the natural causes of acids in rain? Source CO 2 Causes Decomposition/Respiration/Fires Electrical Discharge Volcanic Gases CO 2 (produced from the combustion of C or C x H y ) Coal C(s) + O 2 (g) CO 2 (g) Gas CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) CO 2 (g) + H 2 O(l) H 2 CO 3 (aq) What are the man made causes of acids in rain? Source CO 2 Causes Fossil Fuel Combustion/Fires High Temperature Air Combustion Fossil Fuel Combustion (formed from N at high temperatures) N 2 (g) + O 2 (g) heat 2(g) 2(g) + O 2 (g) 2 2 (g) 3 2 (g) + H 2 O(l) 2H 3 (aq) + (g) Note: The majority of the emissions come from automobiles. 19 20 (formed from the combustion of S) S(s) + O 2 (g) (g) 2 (g) + O 2 (g) 2SO 3 (g) SO 3 (g) + H 2 O(l) H 2 SO 4 (aq) Three Way Catalytic converters (1981) 2(g) Pt or Rh N 2 (g) + O 2 (g) Note: The majority of all emissions come from the production of electricity. Note: Why are we more worried about controlling and emissions for acid rain? Scrubbers (in the 1990) (limestone slurries are put into the smoke stacks) CaCO 3 (s) + H 2 SO 4 (aq) CaSO 4 (aq) + H 2 O(l) +CO 2 (g) Note: Acid rain level have dropped 65% since 1976. 21 22 23 Big Idea: A Bronsted-Lowery acid is a proton donor and a Bronsted-Lowery base is a proton acceptor. After an acid/base loses/gains its proton it becomes a conjugate base/acid. Acids and bases can either completely dissociate (strong) or incompletely dissociate (weak). An equilibrium problem must be set up to solve for the ph of a weak acid or base. Know the different acid/base definitions. Arrhenius Acid: Forms H + in water Base: Forms OH - in water Bronsted-Lowery Acid: Proton donor Base: Proton acceptor 24 Conjugate Acids/Bases (20) Be able to recognize conjugate acid base pairs. Pairs that differ by 1 hydrogen atom (H 3 PO 4 and H 2 PO 4- ). Memorize the strong acids and bases. (24,33) Strong Acids: HCl, HBr, HI, H 3, HBrO 3, HBrO 4, HIO 3, HIO 4 HClO 4, HClO 3, and H 2 SO 4 (1 st deprotonation) Strong Bases: LiOH, NaOH, KOH, RbOH, CsOH, Sr(OH) 2, Ca(OH) 2, Ba(OH) 2, and Mg(OH) 2 Know the relationship between [H + ] and [OH - ] (40,41) [H + ][OH - ]=1.0 10-14 Be able to change between K a and K b values. Know that the stronger the acid/base the weaker the conjugate acid/base. (31) Be able to order acids/bases in increasing strength using K a and/or K b values (27,28,29,30,69,70) 4
ph/poh Scale (74,75) Be able to calculate the ph of a solution. Be able to calculate the poh of a solution. Be able to convert between ph and poh 14 ph/poh of Strong Acids/Bases Be able to calculate the ph and poh of strong acids and strong bases. ph/poh of Weak Acids/Bases (79) Be able to identify the major species in solution. (51) Be able to calculate the ph and poh of weak acids and bases (ICE table). (47,48,49,52,53,55,61,65) Be able to calculate the water solubility of a substance given the ph of a saturated solution. (56) Acid/Base Properties of Salts (101,103,105,109,131) Be able to determine if a salt is an acidic, basic, or neutral Be able to calculate the ph of a salt with only one acid anion or cation 25 26 5