酸的强度

酸是可以在反应中提供质子或接受电子对的分子或其他物质。当酸与 H ₂ O 反应时，它们会产生氢离子；酸的强度取决于溶液中氢离子的浓度。氢离子数越多表明酸越强，而氢离子数越少表明酸越弱。它们分为强酸和弱酸。

在化学中，强酸和弱酸是需要理解的重要概念。在水中，强酸完全分解成它们的离子，而弱酸仅部分解离。只有少数强酸，但有很多弱酸。

酸溶解产生离子的难易程度决定了它们的强弱。酸溶于水产生氢离子，而碱溶于水形成氢氧根离子。强酸和强碱离子迅速解离，完全溶解在水中，产生带正电荷的 H 氢离子或带负电荷的 OH ^-氢氧根离子。弱酸和碱仅部分解离，在溶液中留下更少的离子。酸中的氢离子和碱中的氢氧根离子决定了酸和碱的性质和强度。

什么是强酸？

Strong acids totally breakdown into their ions in water, producing one or more protons or hydrogen cations per molecule. Inorganic or mineral acids are often strong acids.

当暴露在水中时，强酸会完全分解成它们的离子。换句话说，酸完全电离并将H ⁺离子释放到溶液中。酸解离常数值定义了酸的强度 (K _a )。强酸通常具有非常高的 K _a值。

酸越强，它就越容易失去质子。由于 H 原子与分子其余部分之间的连接具有强极性，这些质子很容易被释放。参与这种连接的两个原子的电负性决定了它的极性。与质子连接的阴离子的极性和大小决定了强酸的去质子化（质子的去除）。

For example, if the acid H-A is considered, the dissociation of HA acid may be written as,

HA (aq) + H₂O (l)  ⇢  A (aq) + H₃O⁺ (aq)

However, if the acid molecule has more than one proton that may be released, it can be represented as illustrated below. The dissociation of a diprotic acid is demonstrated in the example below. This indicates it has the ability to emit two protons.

H₂B (aq) + H₂O (l)  ⇢  B₂ (aq) + H₃O⁺ (aq)

Strong acids, on the other hand, have a large impact on the pH of the solution because they release H⁺ ions into it. The pH is determined by the concentration of H⁺. The connection between H⁺ concentration and pH is shown below.

pH = -log [H⁺ (aq)]

If the acid is really powerful, the pH value is quite low. For example, if a strong monoprotic acid is dissolved in water at a concentration of 0.1 molL-1, the pH of the solution would be,

pH = -log [H⁺ (aq)]

      = -log [0.1 mol/L]

      = 1

强酸的例子：

• 氯酸 (HClO ₃ )
• 盐酸 (HCl)
• 硫酸（H ₂ SO ₄ ）
• 氢碘酸 (HI)
• 氢溴酸 (HBr)
• 高氯酸 (HClO ₄ )
• 硝酸 (HNO ₃ )

什么是弱酸？

有许多弱酸，但只有少数强酸。

Weak acids partially dissociate in water, resulting in an equilibrium state containing the weak acid and its ions. Hydrofluoric acid (HF), for example, is classified as a weak acid because some HF, in addition to H⁺ and F^– ions, persists in an aqueous solution.

在水溶液中，弱酸是部分溶解成离子的化合物。弱酸不会将所有 H+ 离子完全释放到溶液中。酸解离常数 (Ka) 低于强酸。溶液的pH值约为3-5。这是因为弱酸不会像强酸那样提高溶液中的 H ⁺含量。有H ⁺离子，分子的阴离子，弱酸分子存在于水系统中的弱酸中。

在描述弱酸的解离时，应使用双箭头而不是单箭头。这样做是为了证明反应处于平衡状态。

例如，乙酸的解离形成水合氢阳离子和乙酸根阴离子：

CH ₃ COOH + H ₂ O ⇆ H ₃ O ⁺ + CH ₃ COO ^–

弱酸的例子：

• 亚硝酸 (HNO ₂ )
• 亚硫酸（H ₂ SO ₃ ）
• 氢氟酸 (HF)
• 甲酸 (HCOOH)
• 磷酸（H ₃ PO ₄ ）
• 苯甲酸 (C ₆ H ₅ COOH)
• 乙酸 (CH ₃ COOH)

示例问题

问题 1：说出唯一一种弱酸的无机酸。

回答：

Carbonic Acid (H₂CO₃) is the only mineral acid that is a weak acid.

问题 2：定义电离度。

The degree of ionization is the ratio or percentage that gives tells the number of neutral molecules of a compound breaks into ions when dissolved into water.

问题3：解释硫酸如何形成H ₃ O ⁺ 。

回答：

When sulfuric acid is dissolved into water. It gets ionized into H+ and SO_4^2- ions as shown below:

H₂SO₄ ⇄ H⁺ + SO_4^2-  

Now the H+ ions cannot exist independently and then combine with water molecules forming H₃O⁺ ions.  

 H⁺  + H₂O → H₃O⁺

This is how sulfuric acid forms H3O+ ions when dissolved into water.

问题 4：如何使用 pH 标度区分酸性化合物与碱性和中性化合物？

回答：

We can distinguish between the acidic, basic and neutral compounds using the value of the pH scale as:

• If the value of pH is below 7, then the compound is acidic.
• If the value of pH is equal to 7, then the compound is neutral.
• If the value of pH is greater than 7, then the compound is basic.

问题 5：什么是 pH 值？

回答：

The pH scale is the scale used to measure the acidity or basicity of a solution. It has a value from the range of 0 to 14. If the value of the pH scale is 7, then the solution is neutral.