Ionization of Water, Weak Acids, and Weak Bases: -Weak Acids and Bases Have Characteristic Dissociation Constants

Hydrochloric, sulfuric, and nitric acids, commonly called strong acids, are completely ionized in dilute aqueous solutions; the strong bases NaOH and KOH are also completely ionized. Of more interest to biochemists is the behavior of weak acids and bases—those not completely ionized when dissolved in water. These are common in biological systems and play important roles in metabolism and its regulation. The behavior of aqueous solutions of weak acids and bases is best understood if we first define some terms. Acids may be defined as proton donors and bases as proton acceptors. A proton donor and its correspon ding proton acceptor make up a conjugate acid-base pair (Fig. 2–16). Acetic acid (CH₃COOH), a proton donor, and the acetate anion (CH₃COO-), the corresponding proton acceptor, constitute a conjugate acid base pair, related by the reversible reaction

Each acid has a characteristic tendency to lose its proton in an aqueous solution. The stronger the acid, the greater its tendency to lose its proton. The tendency of any acid (HA) to lose a proton and form its conjugate base (A-) is defined by the equilibrium constant (Keq) for the reversible reaction

Equilibrium constants for ionization reactions are usu ally called ionization or dissociation constants, often designated Ka. The dissociation constants of some acids are given in Figure 2–16. Stronger acids, such as phosphoric and carbonic acids, have larger dissociation constants; weaker acids, such as monohydrogen phosphate (HPO₄^2-), have smaller dissociation constants.

FIGURE 2–16 Conjugate acid-base pairs consist of a proton donor and a proton acceptor. Some compounds, such as acetic acid and ammonium ion, are monoprotic; they can give up only one proton. Others are diprotic (H₂CO₃ (carbonic acid) and glycine) or triprotic (H₃PO₄ (phosphoric acid)). The dissociation reactions for each pair are shown where they occur along a pH gradient. The equilibrium or dis sociation constant (Ka) and its negative logarithm, the pKa, are shown for each reaction.

Also included in Figure 2–16 are values of pKa, which is analogous to pH and is defined by the equation

The stronger the tendency to dissociate a proton, the stronger is the acid and the lower its pKa. As we shall now see, the pKa of any weak acid can be determined quite easily.

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بعد مرور أربع سنوات على تأسيسه قسم شؤون المعارف يعلن منجزات مركز التراث الإسلامي في إيران

قسم الشؤون الفكرية يختتم مسابقة (معز المؤمنين الثقافية) ويكرِّم الفائزين في كربلاء وبابل

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الشاشات ليست مجرد ترفيه.. تأثيرات طويلة المدى على دماغ طفلك

الأسباب الرئيسية لنقص الطاقة في الجسم

الكاكاو وتأثيره على العين.. نتائج واعدة من تجارب مخبرية

طبيب يحذر: أطعمة شائعة الاستهلاك أخطر على صحتنا من التدخين

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الآخبار التكنلوجية

باحثون في "نيويورك أبوظبي" يطورون تقنيات لرصد وعلاج الأورام

جيتكس إفريقيا.. المغرب يجمع "رواد التكنولوجيا" من 103 دول

طاقم أرتميس 2 يحطم رقما قياسيا ويستعرض الجانب البعيد للقمر

"أرتيميس 2" تنطلق حول القمر وتحطم الرقم القياسي

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مواضيع ذات صلة

Amino Acids: - Amino Acids Share Common Structural Features

Buffering against pH Changes in Biological Systems: -A Simple Expression Relates pH, pKa, and Buffer Concentration

Buffering against pH Changes in Biological Systems:- Buffers Are Mixtures of Weak Acids and Their Conjugate Bases

Ionization of Water, Weak Acids, and Weak Bases: -Titration Curves Reveal the pKa of Weak Acids

Ionization of Water, Weak Acids, and Weak Bases: -The pH Scale Designates the H+ and OH- Concentrations

Ionization of Water, Weak Acids, and Weak Bases: -The Ionization of Water Is Expressed by an Equilibrium Constant

Ionization of Water, Weak Acids, and Weak Bases: - Pure Water Is Slightly Ionized

Weak Interactions in Aqueous Systems: -Solutes Affect the Colligative Properties of Aqueous Solutions

Weak Interactions in Aqueous Systems: -Weak Interactions Are Crucial to Macromolecular Structure and Function

Weak Interactions in Aqueous Systems: - van der Waals Interactions Are Weak Interatomic Attractions

Weak Interactions in Aqueous Systems: -Nonpolar Compounds Force Energetically Unfavorable Changes in the Structure of Water

Weak Interactions in Aqueous Systems: -Entropy Increases as Crystalline Substances Dissolve