3.6
一元酸/碱滴定过程中的溶液组成
用强碱来滴定弱酸会形成水和酸的共轭碱。例如，用氢氧化钠溶液来滴定乙酸会形成水和乙酸钠。乙酸和乙酸钠溶液会构成缓冲液。在滴定不同阶段，其相对浓度由α来进行表示，α能够代表弱酸及其共轭碱的百分比。
α0 和α1 分别代表了乙酸和乙酸根离子的相对平衡浓度。在添加碱基之前，α0 和 α1 的值分别为 0.9…
请记住，用氢氧化钠滴定乙酸会生成乙酸钠和水，从而形成缓冲溶液。
滴定过程中缓冲溶液中不同组分的相对浓度由 alpha 值表示。
虽然曲线表示乙酸与氢氧化钠的滴定过程，但标为 alpha-knot 和 alpha-one 的直线分别表示乙酸和乙酸根离子的相对平衡浓度。
最初，α-结值为0.987，表明98.7%的乙酸未解离，其余1.3%为乙酸根离子。
在交叉点或半当量点时，pH 等于 pKa。因此，溶液中醋酸与醋酸根离子的浓度相等。
随着滴定的进行，乙酸的浓度（α₀）在等当点处降低至接近零。相反，乙酸根离子的浓度（α₁）则增加并趋近于1。
Q1: What happens to acetic acid and acetate ions during titration with sodium hydroxide?
During titration of acetic acid with sodium hydroxide, the weak acid is gradually converted to its conjugate base, sodium acetate, forming a buffer solution. The alpha values track this composition change: initially, alpha-0 (acetic acid) is 0.987 and alpha-1 (acetate ions) is 0.013. As titration proceeds, alpha-0 decreases while alpha-1 increases, reaching equal concentrations at the half-equivalence point where pH equals pKa.
Q2: What do alpha values represent in an acid-base titration?
Alpha values represent the relative equilibrium concentration or fractional composition of species in a buffer solution during titration. Alpha-0 (α0) indicates the fraction of undissociated weak acid, while alpha-1 (α1) indicates the fraction of conjugate base. These values are expressed as decimals or percentages, allowing visualization of how solution composition changes throughout the titration process.
Q3: Why is the half-equivalence point significant in weak acid titrations?
At the half-equivalence point, the pH of the solution equals the pKa of the weak acid. At this critical point, the concentrations of acetic acid and acetate ions are equal, with alpha-0 and alpha-1 both equal to 0.5. This relationship between pH and pKa makes the half-equivalence point essential for understanding buffer behavior and predicting solution pH during titration.
Q4: What is the composition of the solution at the equivalence point?
At the equivalence point, the titration is complete and nearly all acetic acid has been converted to acetate ions. The alpha-0 value drops to nearly zero, while alpha-1 approaches unity (1.0), indicating the solution is predominantly sodium acetate. This conjugate base solution exhibits basic properties due to hydrolysis of acetate ions.
Q5: How does the buffer solution form during the titration of acetic acid?
As sodium hydroxide is added to acetic acid, a buffer forms because both the weak acid (acetic acid) and its conjugate base (acetate ions) are present in solution. The buffer composition changes continuously: initially dominated by acetic acid with trace acetate, then becoming increasingly acetate-rich. This coexistence of weak acid and conjugate base maintains relatively stable pH across much of the titration curve.
Q6: What percentage of acetic acid remains undissociated before titration begins?
Before any sodium hydroxide is added, the alpha-0 value is 0.987, meaning 98.7% of the acetic acid remains undissociated. Only 1.3% of the acetic acid has dissociated into acetate ions and hydrogen ions. This initial composition reflects the weak nature of acetic acid and its limited tendency to ionize in aqueous solution.
Q7: How do alpha values change as the titration progresses toward equivalence?
As titration proceeds, alpha-0 (acetic acid fraction) continuously decreases from 0.987 toward zero, while alpha-1 (acetate fraction) increases from 0.013 toward unity. The rate of change accelerates near the equivalence point. This inverse relationship between the two alpha values reflects the conversion of weak acid to conjugate base and is fundamental to understanding buffer capacity and solution composition changes.