第七章 酸與鹼 Chapter 7 Acids & Bases Jwu-Ting Chen 陳 竹 亭 第七章 酸與鹼 Jwu-Ting Chen 陳 竹 亭 Department of Chemistry, National Taiwan University Taipei, Taiwan 106, jtchen@ntu.edu.tw

What is acid? What is base? 何謂酸？何謂鹼？ Are acids and bases two classes of substances that have opposite properties, and their character could be canceled off by meeting each other? 酸與鹼是兩種性質相反的物質，相遇 時彼此的性質就相互抵銷嗎？

Background 酸鹼的歷史 By tastes 感官的認識 Acids are a class of substances that taste sour 酸嚐起來是「酸」的 Bases (alkalis) are a class of substances that taste bitter 鹼嚐起來是「苦」的

By Alchemist Experience 煉金術士對酸與鹼的認識 HCl was called salt spirit HCl稱作鹽精 Many metal containing minerals gave alkalis in water 許多含金屬的礦物在水中呈現鹼性 Minerals without metal often give acidic solutions 許多不含金屬的礦物在水中呈現酸性 能舉例嗎？

Arrhenius concept – The First Modern Theory 阿瑞尼士理論 1884 阿瑞尼士理論 1884 Acids produce hydrogen ion in aqueous solutions 酸在水溶液中產生氫離子(H+) Bases produce hydroxide ion in aqueous solutions 鹼在水溶液中產生氫氧根離子(OH－)

The Content of Arrhenius Concept 阿瑞尼士理論的精義 Theory of Ionization 游離說 HCl(aq) → H+(aq) + Cl－(aq) NaOH(aq) → Na+(aq) + OH－(aq) NaCl(aq) → Na+(aq) + Cl－(aq) Electrolytes & Non-electrolytes 電解質與非電解質

Brønsted-Lowry concept 布忍司特―羅瑞理論 Acid is proton donor in aqueous solution 酸在水溶液中是質子供給者 Base is proton acceptor in aqueous solution 鹼在水溶液中是質子接受者 Conjugate acid-base pair 共軛酸鹼對酸是相互競爭 Chemical equilibrium for acid-base reactions 酸鹼反應是共軛酸鹼對的化學平衡，反應傾向 從強共軛酸鹼往弱共軛酸鹼的方向進行

Arrhenius vs Brønsted-Lowry concepts ionization 游離 Brønsted-Lowry proton transfer 質子傳遞

Self- Ionization of Water 水的自身游離 The chemistry of aqueous solution is unique 水溶液有獨特的化學行為 Rapid proton exchange via H-bonding 水分子間藉由氫鍵進行快速的質子交換 H3O＋ is hydrated proton, and called hydronium ion 質子在水溶液中呈水合的狀態稱為鋞離子(H3O＋) Actually, either H+ or OH－is hyrdated 事實上，大部分溶在水中的物質都是呈水合的狀態

Self ionization of water is in chemical equilibrium 水的自身游離常呈化學平衡狀態 Kw = [H3O＋][OH－], and is called ion product constant Kw = [H3O＋][OH－,稱為離子積常數 At 25℃, [H3O＋] = [OH－] = 1.0×10-7 M; Kw = 1.0×10-14

Acid-Base in Other Solutions 其他溶液中的酸鹼平衡 in ethanol 乙醇中 Ethoxide is a stronger base than water, and protonated ethanol is a stronger acid than water. 乙烷氧離子在水中是強鹼，質子化的乙醇則是強酸 Basicity: EtO－ > OH－ > H2O > EtOH > H3O+ > EtOH2+

in ammonia 氨水中 Bacicity: NH2- > OH - > NH3 > H2O > NH4+ > H3O+

sulfate (tetrahedral)硫酸根(正四面體) Structure of Oxyacids 常見含氧酸的分子結構 sulfuric acid 硫酸 hydrogen sulfate 硫酸氫根 sulfate (tetrahedral)硫酸根(正四面體)

The higher oxidation state of the central atom, the stronger acid it will be.中央原子的氧化態越高的酸酸性愈強 nitrous acid 亞硝酸 nitrate (trigonal planar) 硝酸根(平面三角形) nitric acid 硝酸 nitrite (angular) 亞硝酸根(角形)

hypochlorite (linear) hypochlorous acid 次氯酸 perchloric acid 過氯酸 perchlorate (tetrahedral) 過氯酸根(正四面體) hypochlorite (linear) 次氯酸根

Organic Acids 有機酸 acetic acid 醋酸 acetate 醋酸根 benzoic acid 苯酸 benzoate 苯酸根

Acid Strength 酸的強度 Ka － acid dissociation constant, indicating the extent of dissociation of acids Ka是酸的解離常數 the equilibrium of strong acid goes toward its complete dissociation 強酸完全解離 the weak acid reaches its equilibrium with partial dissociation 弱酸部分解離 pH = －log [H+] the pH scale of aqueous solutions is <14 水溶液的pH值範圍<14 pH meter－Saturated Calomel Electrode (SCE) Hg + Cl- = ½ Hg2Cl2 + e-

Acid Strength pH of strong acid, [H＋] = [HA]0 Leveling Effect 平準效應 All strong acids or bases show the same strength in the same solvent. In aqueous solution, the strongest acid allowed is H+, the strongest base allowed is OH-. 水中的最強酸是H+，最強鹼是OH–

Acid Strength pH of weak acid, before after

Percent Dissociation 解離率

Percent Dissociation 計算解離率 before after ,

The pH of mixture of weak acids depends on the relative acidity Example: Calculate the pH of a solution that contains 1.00 M HCN (6.2×10-10) and 5.00 M HNO2 (4.0×10-4) and calculate [NO2-] and [CN-]. Nitrous acid is stronger than cyanic acid to the 6th order of magnitude, it will dominate [H+]. , [H+] = [NO2-] = 4.5x10-2 M pH = 1.35 The dissociation of cyanic acid is suppressed by nitrous acid.

The pH of very dilute acid ([H+] < 1.0x10-6 M) ……(eq 1) ……(eq 2) Kw = [H+][OH-] = 1.0x10-14 , mass balance: [HA]0 = [HA] + [A-] ……(eq 3) charge balance: [H+] = [OH -] + [A-] ……(eq 4) Unknowns: [H+], [OH-], [HA], [A-] Knowns: [HA]0, Ka, Kw

learn how to set up equations 會設定方程式 learn mass and charge balance 會使用質量及電荷平衡的概念 learn how to solve the unknowns 找出未知量的參數 learn how to do approximation 使用概算法簡化計算 learn the meaning of approximation 檢驗概算的假設 if [HA]0 >> ([H+]2 – Kw)/[H+] [H+] = (Ka[HA]0 + Kw)1/2

Polyprotic (Polybasic) Acids 多質子酸 phosphoric acid (tribasic) 磷酸(三質子酸) phosphorous acid (dibasic) 亞磷酸(二質子酸) hypophosphorous acid (monobasic) 次磷酸(單質子酸)

The pH of a diprotic acid – H2CO3 …(eq 1) …(eq 2) mass balance: CH2A = [H2A] + [HA-] + [A2-] ……(eq 3) , charge balance: [H3O+] = [OH -] + [HA-] + 2[A2-] ……(eq 4) Unknowns: [H+], [OH-], [H2A], [HA-], [A2-] Knowns: CH2A, K1, K2, Kw

if K1>>K2 (more than 1000 fold larger) [HA-]>>[A2-] [H3O+] ≈ [HA-] [A2-] ≈ K2 (double checked later) [H2A] = [H2A] + [HA-] = [H2A] + [H3O+]

Calculate pH of 5.00 M H3PO4 and the concentrations of anions. ,

[H3O+] ≈ [HSO4-] = 0.0100 M approximation Calculate [H3O+] for 0.0100 M H2SO4 . [H3O+] ≈ [HSO4-] = 0.0100 M approximation , [SO42-] <<[HSO4-] or [SO42-] <<[H3O+] thus is not a proper assumption.

…Mass balance …charge balance ,

Inorganic Bases 無機鹼 ammonia 氨 metal hydroxides 金屬氫氧化物: NaOH, KOH, Ca(OH)2 metal oxides 金屬氧化物: CaO, BaO metal hydrides 金屬氫化物: NaH, KH metal amides 金屬氨化物: NaNH2, LiNMe2, NaOMe weak acid salts 金屬弱酸鹽: NaOAc, NaCN, Na2CO3

Organic amines 有機鹼 primary (1º) amine 一級胺 methyl amine 甲胺 aniline 苯胺

Organic amines secondary (2º) amine 二級胺 dimethyl amine 二乙胺 piperidine 環戊胺

Organic amines tertiary (3º) amine 三級胺 trimethyl amine 三甲胺 Pyridine 吡啶

Biofunctional amines 生物胺 epinephrine norepinephrine dopamine 多巴胺 amphetamine 安非他命

Salts 鹽類 Neutral Salts: salts formed from strong acid & strong base are always of neutral salts, pH = 7. 強酸與強鹼形成的鹽為中性鹽 Example:

Acidic Salts: salts formed from strong acid & weak base 強酸與弱鹼形成酸性鹽 Ka = 5.6x10-10 For 1M NH4Cl at 25 ℃  pH=4.6 The solution is slightly acidic.

Solution of acidic salts of ampholyte 兩性鹽 Mass balance: CNaHA = [H2A] + [HA-] + [A2-] Charge balance: [Na+] + [H3O+] = [OH-] + [HA-] + 2[A2-] [H2A] = [A2-] + [OH-] - [H3O+]

if both Kb”, K2 are small [HA-] ≈ CNaHA and K1 << [HA-]= CNaHA Kw<< CNaHA

Salts formed from weak acid & weak base (NH4OAc) 弱酸與弱鹼形成的鹽 Mass balance: CNH4OAc = [NH4+] + [NH3] = [HOAc] + [OAc-] Charge balance: [NH4+] + [H3O+] = [OH-] + [OAc-] [NH3] - [H3O+] = [HOAc] - [OH-]

弱酸及其鹽類的同離子效應 Solution of weak acid & Its Salt HA/NaA H2A/ NaHA or H2A/Na2A NaHA/Na2A

0.0100 M phthalic acid/0.200 M KHP [H2P] = 0.100 - [H3O+] ≈ 0.100 M [HP-] = 0.200 - [H3O+] ≈ 0.200

Method 1. Method 2.

總結 酸、鹼、鹽都是在水中會游離的物質，水會發生自身解離，所以酸與鹼都會和水分子競爭質子，鹽類則發生水合的行為 。 不同的物質解離程度和競爭質子的能力均不同，利用測量酸、鹼、鹽在水中的平衡的情形，可以計算各種化學成分在水溶液中的含量。 有機、無機與生化物質都可能在水中有酸鹼鹽的性質。物質的結構與組成都影響其酸或鹼的性質。