1. 1 Chapters 18 Acids and Bases 18.1. Properties and names of acids and bases Arrhenius, Brønsted-Lowry, and Lewis models 18.2. Strength of acids and bases Acid and base ionization constants K a, K b 18.3 Hydrogen ion concentration and pH 18.4 Neutralization and Titration

2. 2 Acids and Bases are considered opposites They both… are electrolytes in aqueous solution (conduct electricity), and cause indicators to change color (Litmus in acid = red, in base = blue) Neutralize each other, Acid + base  salt + water (salt ~ ionic compound) pH meter

3. 3 Acids have several distinctive properties and they are in many common items. They give foods a tart or sour taste. (vinegar contains acetic acid, citrus fruits citric acid, tea tannic acid, vitamin C is ascorbic acid etc.) Properties of Acids Nitric acid, HNO 3, oxidizes copper metal to produce copper nitrate, the green solution, and nitrogen dioxide, an orange-red gas.

4. 4 Acids react with active* metals, like zinc (Zn) and magnesium (Mg), to produce hydrogen gas (H 2 ). Generic equation Generic equation M + x HX  MX x + n H 2 Example Example Mg + 2 HCl  MgCl 2 + H 2 Zn + 2 HCl  ZnCl 2 + H 2 active* = higher on the activity series of metals than hydrogen

5. 5 CaCO 3 + 2 HC 2 H 3 O 2  CO 2 + Ca(C 2 H 3 O 2 ) 2 + H 2 O Eggshell in Vinegar Acids and carbonates: Acids react with… carbonate ions (CO 3 2- ), to form carbon dioxide (CO 2 ), water and a salt CaCO 3 + 2 HCl  CO 2 + CaCl 2 + H 2 O (problems of buildings being damaged by acid rain). Geologists identify limestone because it produces bubbles of carbon dioxide when exposed to hydrochloric acid.

6. 6 Two types of antacids: a) carbonates. CaCO 3 + 2 HCl  CO 2 + CaCl 2 + H 2 O b) poorly soluble hydroxides (Mg, Al) Mg(OH) 2 + 2 HCl  MgCl 2 + H 2 O important that hydroxides are not soluble in water. Why? Insoluble = slow release in stomach as it reacts with HCl Soluble base = reacts with saliva in mouth and throat, damaging soft tissue.

7. 7 Naming acids

8. 8 Memorize these acids HCl hydrochloric acid HF hydrofluoric acid HNO 3 nitric acid H 2 CO 3 carbonic acid HC 2 H 3 O 2 acetic acid (or CH 3 COOH) H 2 SO 4 sulfuric acid H 3 PO 4 phosphoric acid

9. 9Bases Aqueous base solutions taste bitter and feel slippery. Everyday bases are NaOH (lye): Drain cleaner Milk of magnesia Mg(OH) 2 : heartburn Ammonia (NH 3 ): Windex Lime (CaO): Neutralize acidic soil Simple bases are metal hydroxides such as NaOH.

10. 10 All water solutions contain a small amount of hydrogen ions (H + ), or rather hydronium ions (H 3 O + ), and hydroxide ions (OH – ). An acidic solution contains more hydrogen ions than hydroxide ions. A basic solution contains more hydroxide ions than hydrogen ions. H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH – (aq)

11. 11 Know these ions: hydrogen ion also called proton H + hydronium ion H 3 O + hydroxide ion OH –

12. Arrhenius model An Acid contains acidic hydrogen and ionizes to produce hydrogen ions in aqueous solution, A Base contains a hydroxide group and dissociates to produce a hydroxide ion in solution. Arrhenius acids and bases –HCl ionizes to produce H + ions. –HCl(g) → H + (aq) + Cl – (aq) –NaOH dissociates to produce OH – ions. –NaOH(s) → Na + (aq) + OH – (aq) –Some solutions produce hydroxide ions even though they do not contain a hydroxide group.

13. 13 Brønsted-Lowry Model a more inclusive model of acids and bases An acid is a hydrogen ion donor, and a base is a hydrogen ion acceptor. Acid Base

14. A conjugate acid is produced when a base accepts a hydrogen ion. A conjugate base is produced when an acid donates a hydrogen ion. A conjugate acid-base pair consists of two substances related to each other by donating and accepting a single hydrogen ion. acid base conjugate acid conjugate base A weak acid has a strong conjugate base etc. etc.

15. 15 Ammonia— Brønsted base NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH – (aq) Water and other substances that can act as acids or bases are called amphoteric. HBr(aq) + H 2 O(l)  H 3 O + (aq) + Br – (aq) Base acid conjugate acid conjugate base H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH – (aq)

16. Only ionizable hydrogen atoms can be donated, which depends on the bond polarity (  EN) What is an acidic hydrogen?

17. 17 Monoprotic acids contain only one acidic hydrogen, HCl  H + + Cl - Polyprotic Acids: Diprotic acids contain two acidic hydrogens, H 2 SO 4  2H + + SO 4 2- Triprotic acids contain three acidic hydrogens, H 3 PO 4  3H + + PO 4 3- Monoprotic and Polyprotic Acids

18. Lewis acid Lewis base

19. 19 A strong acid (or base) is completely (or almost completely) ionized in water A weak acid or base is only slightly ionized in water HCl is a strong acid Section 18.2 Strength of acids and bases HCl(aq) + H 2 O(l)  H 3 O + (aq) + Cl – (aq) HF(aq) + H 2 O(l) H 3 O + (aq) + F – (aq) HF is a weak acid A weak acid has a strong conjugate base A strong acid has a weak conjugate base etc.

20. 20 Acid Ionization constant K a HCN(aq) + H 2 O(l) H 3 O + (aq) + CN - (aq) Keq = [H 3 O + ] [CN - ] [HCN] [H 2 O] Since H 2 O concentration in dilute solutions is constant, it is incorporated into Keq as Ka Ka = [H 3 O + ] [CN - ] [HCN] Weak: equilibrium (note arrow)… calculate! Ka = [H 3 O + ] [X - ] generic expression [HX]

21. 21 Base Ionization constant K b Group 1 + 2 metal hydroxides are strong bases, all others consider to be weak CH 3 NH 2 (aq) + H 2 O(l) CH 3 NH 3 + (aq) + OH - (aq) H 2 O concentration in dilute solutions again part of Kb K b = [CH 3 NH 3 + ] [OH - ] [CH 3 NH 2 ] K b = [BH + ] [OH - ] generic expression [B]

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24. 24 Section 18.3 Self-ionization of water and pH Concept H 2 O(l) + H 2 O(l) H 3 O + (aq) + OH – (aq) Or short: H 2 O(l) H + (aq) + OH – (aq)

25. 25 occurs only to a very small extent; in pure water at 25 °C, [H + ] = [OH - ] = 1.0 x 10 -7 M [H + ] = [OH - ]neutral [H + ] > [OH - ]acidic (more H+) [H + ] < [OH - ]basic(more OH-) Keq = [H + ] x [OH - ][H 2 O] part of Keq => Kw [H 2 O] The self-ionization of water H 2 O(l) H + (aq) + OH – (aq) Ion-product constant for water Kw: Kw = [H + ] x [OH - ] = 1.0 x 10 -14

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27. 27 What is pH????? pH = -log [H + ] pOH = -log [OH - ] pH + pOH = 14 pH = 14 - pOHpOH = 14 - pH What is the hydrogen concentration [H + ] at pH = 4.00 at pH = 11.00 the hydroxide concentration [OH - ] at pH = 4.00 at pH = 11.00 …?

28. 28 The pH Scale For strong dilute acids [H + ] = normality, but For weak acids use ionization constant K a Ka = [H + ] [X - ] [HX]

29. 29 Acids (HX) react with bases in aqueous solution to form water and a salt. (double replacement reaction) Generic equation Generic equation MOH + HX  MX + H 2 O Example Example NaOH + HCl  NaCl + H 2 O (Na + OH - H + Cl - ) Section 18.4 “Neutralization” A salt is an ionic compound made up of a cation from a base and an anion from an acid

30. 30 Titration is a method for determining the concentration of a solution by reacting a known volume of that solution with a solution of known concentration. A measured volume of an acid or base of unknown concentration is placed in a beaker with an indicator. A buret is filled with the titrating solution of known concentration, called a titrant.

31. 31 Titrant is added slowly and mixed into the solution in the beaker until the reaction reaches the equivalence point, at which moles of H + ion from the acid equals moles of OH – ion from the base. An abrupt change in pH occurs at the equivalence point.

32. 32 End point is the point at which an indicator used in a titration changes color.

33. 33 Acid-base indicators are chemical dyes whose color are affected by acidic and basic solutions.

34. 34 Normality Instead of molarity an older concentration unit often used in acid base titrations is normality (N), which is the number of equivalents per liter of solution. The equation is always: N A V A = N B V B If there is only one ionizable hydrogen or hydroxide ion in a substance, M = N. Otherwise, N = (M)(#of ionizable H + or OH - ions). Example for HCl 1 M = 1 NNaOH 1 M = 1 N but for H 2 SO 4 1M = 2 N or Ca(OH) 2 1M = 2 N

35. 35 Salt hydrolysis Anions of a dissociated salt accept hydrogen ions from water or the cations of the dissociated salt donate hydrogen ions to water (reverse of neutralization). Salts that produce basic solutions KF is the salt of a strong base (KOH) and a weak acid (HF). KF(s) → K + (aq) + F – (aq) K + (aq) + OH - (aq) + H + (aq) F – (aq)  K + (aq) + OH - (aq) + HF(aq) Excess of hydroxide ions in solution = basic

36. 36 Salts that produce acidic solutions NH 4 Cl is the salt of a weak base (NH 3 ) and strong acid (HCl). When dissolved in water, the salt dissociates into ammonium ions and chloride ions. NH 4 Cl(s) → NH 4 + (aq) + Cl – (aq) Salts that produce neutral solutions NaNO 3 is the salt of a strong acid (HNO 3 ) and a strong base (NaOH). Little or no salt hydrolysis occurs because neither Na + nor NO 3 – react with water.

37. 37 Buffers are solutions that resist changes in pH when limited amounts of acid or base are added. A buffer is a solution made up of a weak species and its conjugate. ( Ex: HF/F- from NaF) Ions and molecules in a buffer solution resist changes in pH by reacting with any hydrogen ions or hydroxide ions added to the buffered solution. HF(aq) H + (aq) + F – (aq) When acid is added, the equilibrium shifts to the left. Additional H + ions react with F – ions to form undis- sociated HF molecules but the pH changes little.

38. 38 The amount of acid or base that a buffer solution can absorb without a significant change in pH is called the buffer capacity. A buffer is most effective when the concentrations of the conjugate acid-base pair are equal or nearly equal. Buffers stabilize the pH of a solution

39. 39 Acidic anhydrides Nonmetal oxides are “Acidic anhydrides”, be- cause they form acids when reacting with waterExamples CO 2 (g)+ H 2 O (l)  H 2 CO 3 (aq) carbonic acid SO 3 (g)+ H 2 O (l)  H 2 SO 4 (aq) (sulfuric acid) Problem: acid rain Anhydrides Anhydrides are oxides, which contain besides oxygen only one other element.

40. 40 Basic anhydrides Metal oxides are “Basic anhydrides” because they form bases when they react with water Examples CaO (s)+ H 2 O (l)  Ca(OH) 2 (aq) K 2 O(s)+ H 2 O (l)  2 KOH (aq) Wood ash was used to make soapWood ash was used to make soap CaO (Lime) slowly reacting to Ca(OH) 2 can regenerate acidic soilCaO (Lime) slowly reacting to Ca(OH) 2 can regenerate acidic soil MgO to neutralize SO3 in smokestacksMgO to neutralize SO3 in smokestacks