Chapter 10 - Acids & Bases

Transcription

1 Chapter 10 - Acids & Bases 10.1-Acids & Bases: Definitions Arrhenius Definitions Acids: substances that produce hydrogen ions when dissolved in H 2 O Common Strong Acids: Common Weak acids: Organic carboxylic acids Which subatomic particles are present in H +? (Why called protic?) Bases: substances that produce hydroxide ions when dissolved in H 2 O Common Strong Bases: Common Weak Bases Page1

2 Bronsted-Lowry Acids & Bases Acid: Proton donor Base: Proton acceptor Implications of the Bronsted-Lowry definition: 1) Free H + does not actually exist in H 2 O 2) Reactions between an acid and base involve proton transfer. i. NH 3 + HCl à ii. HO - + HCN à Conjugated acid/base pair: In an acid/base reaction an acid loses a H + to become its conjugated base and a base gains a H + to become its conjugated acid. The formula for a conjugate acid/base pair differ by Page2

3 Problem: Write the conjugate base for HClO 3. Problem: Write the conjugate acid for HS -. Problem: Identify the following conjugate pairs: a) The conjugated base of hydrogen sulfate ion is. b) The conjugated acid of a cyanide ion is. c) The conjugated base of a bicarbonate ion is. d) The conjugated acid of a bicarbonate ion is. A substance that has a conjugate acid is a: A substance that has a conjugate base is an: A substance that can act as both an acid and a base is: Other amphoteric substances: Identify the conjugate acid-base pairs in the following reactions. HC 2 H 3 O 2(aq) + H 2 O (l) D H 3 O + (aq) + C 2 H 3 O 2 - (aq) NH 3(aq) + H 2 O (l) D NH 4 + (aq) + OH - (aq) Page3

4 What do you notice that is different about the previous reactions? Problem:: Write a balanced acid/base reaction between hydrochloric acid and the phosphate ion. Identify the conjugate acid base pairs Acids and Base Strength When describing the strength of acids and bases, we are referring to the degree of dissociation of the H + or OH - ions. Strong acids and bases completely dissociate. Weak acids and bases only partially dissociate. Strong Acids & Bases. Strong Acids Strong Bases HCl LiOH HBr NaOH HI KOH H 2 SO 4 * Ca(OH) 2 HNO 3 Ba(OH) 2 HClO 4 * Sr(OH) 2 ** HClO 3 OH - are insoluble except 1A ions, Ba 2+. NH4OH is a weak base; *Ca(OH)2 and Sr(OH)2 are slightly soluble in water (depends on temp). **HClO3 is borderline; only about 99% dissociated. Page4

5 - - - Problem: Use the table above to identify the stronger acid in each pair. + a) H 2 O or NH 4 b) HCN or H 2 CO 3 Problem: Use the above table to identify the weaker base in each pair. a) OH - or NH 3 b) CN - - or HCO 3 Page5

6 Equilibrium of Acid-Base Reactions Proton transfer ALWAYS favors reaction of the stronger acid and base to FORM the weaker acid and base. (lies to the side of the weaker ones) In General: Problem: Write a balanced equation for the Rxn between HPO 4 2- and OH -. Identify each acid-base conjugate pair, and determine in which direction the equilibrium is favored Acid Dissociation Constants Do ALL weak acid dissociate the same amount? (Or is there only 1 level of weak?) How do we compare weak acid strengths? Since it is based on the extent of the equilibrium, we need to compare their equilibrium constants! Acid dissociation constant, Ka Page6

7 Problem: Write the Ka expression for phosphoric acid. What is the relationship between the value of Ka and the degree of dissociation? Comparing Weak Acids Because there are different strengths of weak acids, we can compare their Ka values. However, Ka values are awkward (they span large orders of magnitude), so we look at the power of the Ka or by taking log of the Ka to simplify the numerical expression. Compound Name Ka pka H 3 O + Hydronium ion H 3 PO 4 Phosphoric acid 7.5 x HF Hydrofluoric acid 3.5x CH 3 COOH Acetic acid 1.8 x H 2 CO 3 Carbonic acid 4.3 x H 2 PO 4 Dihydrogen phosphate ion 6.2 x NH 4 Ammonium ion 5.6 x HCN Hydrocyanic acid 4.9 x HCO 3 Bicarbonate ion 5.6 x HPO 4 Hydrogen phosphate ion 2.2 x H 2 O Water 1.0 x Page7

8 What is the relationship between acid strength and pka values? Problem: Which acid is stronger? aspirin, pka = 3.49 or vitamin C, pka = 4.1 Problem: The Ka for benzoic acid is 6.5 x What is the pka? Problem: The pka of lactic acid is 3.9. What is the Ka? Problem: Use pka values to predict the direction of the equilibrium in the following reactions. Strategy: 1) Identify the conjugate acid-base pairs. 2) Compare pka values to determine the strongest acid. HC 2 H 3 O 2(aq) + H 2 O (l) D H 3 O + (aq) + C 2 H 3 O 2 - (aq) HC 2 H 3 O 2(aq) + NH 3(aq) D NH 4 + (aq) + C 2 H 3 O 2 - (aq) Page8

9 10.4 Water as Both an Acid and a Base Identify the conjugate acid/base pairs in the following reactions. HF (aq) + H 2 O (l) H 3 O + (aq) + F - (aq) NH 3(aq) + H 2 O (l) D OH - (aq) + NH 4 + (aq) Water is amphoteric, it can be both as an acid or a base. Self-ionization of water: H 2 O (l) + H 2 O (l) D OH - (aq) + H 3 O + (aq) Ion-product constant for water: 2 H 2 O (l) «H 3 O + (aq) + OH - (aq) Neutral: [H 3 O + ] = 10-7 M [OH - ] = 10-7 M Acidic: [H 3 O + ] > 10-7 M [OH - ] < 10-7 M Basic: [H 3 O + ] < 10-7 M [OH - ] > 10-7 M Page9

10 Problem: Is water an acid or a base in the following reaction? H 2 CO 3(aq) + H 2 O (l) «HCO 3 - (aq) + H 3 O + (aq) Problem: Are the following solutions acidic, basic or neutral? Beer; [H 3 O + ] = 3.2 x 10-5 M Milk of Magnesia; [H 3 O + ] = 1.0 x 10-9 M Problem: What is the [OH - ] in a solution of 0.10 M HBr? 10.5 Measuring Acidity with ph Scale Once again, we have encountered awkward large and small numbers. We will use logarithms to simplify the numerical values. ph = power of the hydrogen or hydronium ion concentration. ph= [H 3 O + ] = [H 3 O + ] > 1.0 x 10-7 [H 3 O + ] = 1.0 x 10-7 [H 3 O + ] < 1.0 x 10-7 Page10

11 In H 2 O: ph ranges from Remember: Logarithms are powers of ph unit change Þ 3 ph unit change Þ As ph goes, Acidity goes: We can also work with the [OH - ] on a log scale. poh= [OH - ] = As [H 3 O + ] goes, the [OH - ] goes ph + poh = As [H 3 O + ] goes, the acidity goes At the ph goes, the acidity goes As [OH - ] goes, the alkalinity goes and the acidity goes. As the poh goes up, the alkalinity goes and the acidity goes. Page11

12 Problem: Which solution has a higher H 3 O + concentration, one with a ph of 5 or one with a ph of 9? Which one has the higher OH - concentration? Measuring ph Early measurements were made with indicators, which we still use today. Indicators Indicators are substances (dyes) that change color at various ph values. They are usually weak acids. HA + H 2 O D H 3 O + + A - Early scientists discovered the fiber litmus. The most common acid/base indicator: phenolphthalein. Page12

13 Picture above shows universal indicator: ph Meter A more accurate way to measure ph is with an electronic ph meter Working with ph Page13

14 Problem: What is the ph of a solution with a [H 3 O + ] = 1 x 10-5 M? Problem: What is the ph of a solution with a [H 3 O + ] = 4.3 x 10-3 M? Problem: What is the ph of a solution with a [OH - ] = 2.4 x 10-8 M? Problem: a)what is the H 3 O + concentration of saliva, with a ph of 6.70? b)is it acidic or basic? Problem: What is the ph of a M solution of HCl? Problem: What is the H 3 O + concentration of saliva, with a poh of 3.42? Is it acidic or basic? Problem: What is the poh of a 0.024M solution of Ba(OH) 2? Page14

15 10.7 presented later 10.8 Common Acid-Base Reactions 1) Acid Base Neutralization reactions: Acid + Base salt + water 2) Acids with active Metals (single replacement reaction) Mg (s) + 2 HCl (aq) MgCl 2(aq) + H 2(g) 3) Acids with Carbonates and Bicarbonates Similar result for bicarbonate: Can also be written with hydronium Page15

16 4) Acids with Amines + HCl (aq) + RNH 2(aq) RNH 3 (aq) + Cl - (aq) Amines react with acids in the same way that ammonia would: 10.9 Acidity and Basicity of Salt solution: ions of strong acids and strong bases do not react with water (don t undergo hydrolysis) and therefore have no effect on the ph of a solution. ions of weak acids react with water to form OH - ions, forming basic soln. e.g. F- from the weak acid HF ions of weak bases react with water to form H 3 O + ions, forming acidic soln. e.g. NH 4 + from the weak base NH 4 OH Examples in salts. Salt of strong acid and weak base : Salt of strong base and weak acid : Salt of strong acid and strong base : Salt of weak acid and weak base : Page16