Chapter 14

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Chapter 14 Acids and Bases: 

Chapter 14 Acids and Bases Arrhenius Concept: Acids produce H+ in solution, bases produce OH ion. Brønsted-Lowry: Acids are H+ donors, bases are proton acceptors. HCl + H2O  Cl + H3O+ acid base conjugate conjugate base acid Proton is transferred from the HCl molecule to the water molecule.

Slide2: 

Figure 14.1 The Reaction of HCI and H2O Figure 14.2 The Reaction of an Acid with Water Figure 14.3 The Reaction of NH3 with HCI to Form NH4+ and CI-

Conjugate Acid/Base Pairs: 

Conjugate Acid/Base Pairs HA(aq) + H2O(l)  H3O+(aq) + A(aq) conj conj conj conj acid 1 base 2 acid 2 base 1 Conjugate base: everything that remains of the acid molecule after a proton is lost. Conjugate acid: formed when the proton is transferred to the base. A conjugate acid-base pair consists of two substances related to each other by the donating and accepting of a single proton.

Acid Dissociation Constant (Ka): 

Acid Dissociation Constant (Ka) HA(aq) + H2O(l)  H3O+(aq) + A(aq) Where, Ka is the acid dissociation constant. In dilute solution we can assume that the concentration of liquid water remains essentially constant when an acid is dissolved.

Acid Strength: 

Acid Strength The strength of an acid is defined by the equilibrium position of its dissociation (ionization) reaction: HA(aq) + H2O(l) H3O+(aq) + A-(aq) Strong Acid: Its equilibrium position lies far to the right. (HNO3) Yields a weak conjugate base. (NO3) Common strong acids are H2SO4, HCl, HNO3, HClO4

Slide6: 

Figure 14.4 Graphic Representation of the Behavior of Acids of Different Strengths in Aqueous Solution

Slide7: 

Figure 14.5 Acid Strength Versus Conjugate Base Strength

Acid Strength (continued): 

Acid Strength (continued) Weak Acid: Its equilibrium lies far to the left. (CH3COOH) Yields a much stronger (it is relatively strong) conjugate base than water. (CH3COO) Common weak acids are H3PO4, HNO2, HOCl, organic acids (-COOH).

Slide9: 

Figure 14.6 A Strong Acid (a) and a Weak Acid (b) in Water

Slide10: 

Monoprotic acid: One acidic proton (HCl) HCl(aq) H+(aq) + Cl-(aq) Diprotic acid: Two acidic protons (H2SO4) H2SO4(aq) H+(aq) + HSO4-(aq) HSO4- (aq) H+(aq) + SO4 2-(aq) Oxyacids: Acidic proton is attached to an oxygen atom (H2SO4) Organic acids: Those with a carbon atom backbone, contain the carboxyl group (-COOH). CH3-COOH, C6H5-COOH

Water as an Acid and a Base: 

Water as an Acid and a Base A substance is said to be amphoteric if it can behave either as an acid or as a base. Water is amphoteric (it can behave either as an acid or a base). H2O + H2O  H3O+ + OH conj conj acid 1 base 2 acid 2 base 1 Kw = [H3O+][OH-] = [H+][OH-] = 1  1014 at 25°C Where, Kw is the ion-product constant or dissociation constant for water. [H+] = [OH-] = 1.0 x 10-7 M at 25oC in pure water.

Slide12: 

Figure 14.7 Two Water Molecules React to Form H3O+ and OH-

The pH Scale: 

The pH Scale The pH scale provides a convenient way to represent solution acidity. The pH is a log scale based on 10. pH  log[H+] pH in water ranges from 0 to 14. The pH decreases as [H+] increases. Kw = 1.00  1014 = [H+] [OH] pKw = -log Kw = 14.00 = pH + pOH As pH rises, pOH falls (sum = 14.00). pOH = -log [OH-]

Slide14: 

Figure 14.8 The pH Scale and pH Values of Some Common Substances

Calculating the pH of Strong Acid Solutions: 

Calculating the pH of Strong Acid Solutions Calculate the pH of 1.0 M HCl. Since HCl is a strong acid, the major species in solution are H+, Cl- and H2O To calculate the pH we will focus on major species that can furnish H+. The acid is completely dissociates in water producing H+ and water also furnishes H+ by autoionization by the equilibrium H2O(l) H+(aq) + OH-(aq) In pure water at 25oC, [H+] is 10-7M and in acidic solution even less than that. So the amount of H+ contributed by water is negligible compared with the 1.0M H+ from the dissociation of HCl. pH = -log [H+] = -log (1.0) = 0

Solving Weak Acid Equilibrium Problems: 

Solving Weak Acid Equilibrium Problems List major species in solution. Choose species that can produce H+ and write reactions. Based on K values, decide on dominant equilibrium. Write equilibrium expression for dominant equilibrium. List initial concentrations in dominant equilibrium.

Solving Weak Acid Equilibrium Problems (continued): 

Solving Weak Acid Equilibrium Problems (continued) Define change at equilibrium (as “x”). Write equilibrium concentrations in terms of x. Substitute equilibrium concentrations into equilibrium expression. Solve for x the “easy way.” Verify assumptions using 5% rule. Calculate [H+] and pH.

Percent Dissociation (Ionization): 

Percent Dissociation (Ionization) It is useful to specify the amount of weak acid that has dissociated in achieving equilibrium in an aqueous solution. The percent dissociation is defined as follows: For a given weak acid, the percent dissociation increases as the acid becomes more dilute.

Slide19: 

Figure 14.10 The Effect of Dilution on the Percent Dissociation and (H+) of a Weak Acid Solution

Bases: 

Bases Arrhenius concept: A base is a substance that produces OH- ions in aqueous solution. Bronsted-Lowry concept: A base is a proton acceptor. “Strong” and “weak” are used in the same sense for bases as for acids. strong = complete dissociation (hydroxide ion supplied to solution) NaOH(s)  Na+(aq) + OH(aq)

Bases (continued): 

Bases (continued) weak = very little dissociation (or reaction with water) H3CNH2(aq) + H2O(l)  H3CNH3+(aq) + OH(aq) H3CNH2 molecule accepts a proton and thus functions as a base. Water is the acid in this reaction. Methyl amine contains no hydroxide ion, it still increases the concentration of hydroxide ion to yield a basic solution.

Polyprotic Acids: 

Polyprotic Acids . . . can furnish more than one proton (H+) to the solution. A polyprotic acid always dissociates in a stepwise manner, one proton at a time. For a typical weak polyprotic acid, Ka1 > Ka2 > Ka3

Acid-Base Properties of Salts: 

Acid-Base Properties of Salts

Structure and Acid-Base Properties: 

Structure and Acid-Base Properties When a substance is dissolved in water, it produces an acidic solution if it can donate protons and produces a basic solution if it can accept protons. Two factors for acidity in binary compounds: Bond Polarity (high is good) Bond Strength (low is good)

Slide25: 

Figure 14.11 The Effect of the Number of Attached Oxygens on the O-H Bond in a Series of of Chlorine Oxyacids

Oxides: 

Oxides Acidic Oxides (Acid Anhydrides): When a covalent oxide dissolves in water an acidic solution forms. OX bond is strong and covalent. SO2, NO2, CO2, CrO3 Basic Oxides (Basic Anhydrides): When an ionic oxide dissolves in water a basic solution results. OX bond is ionic. K2O, CaO

Lewis Acids and Bases: 

Lewis Acids and Bases Lewis Acid: electron pair acceptor Lewis Base: electron pair donor Lewis acid has an empty atomic orbital that it can use to accept an electron pair from a molecule that has a lone pair of electrons. Lewis Lewis acid base

Slide28: 

Figure 14.13 The AI(H2O)63+ Ion

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