A-Level Chemistry: The Ionic Product of Water, Kw and Its Significance | A-Level化学：水的离子积 Kw 及其意义

📚 A-Level Chemistry: The Ionic Product of Water, Kw and Its Significance | A-Level化学：水的离子积 Kw 及其意义

Water is a very weak electrolyte that undergoes self-ionisation to a tiny extent. The equilibrium constant for this process is called the ionic product of water, Kw. Understanding Kw is essential for pH calculations, especially for strong bases and buffer solutions.

水是一种极弱的电解质，会发生极小程度的自电离。该过程的平衡常数称为水的离子积 Kw。理解 Kw 对 pH 计算至关重要，尤其是在强碱和缓冲溶液的计算中。

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1. The Self-Ionisation of Water | 水的自电离

Water molecules can transfer a proton from one molecule to another, forming hydronium ions and hydroxide ions:

水分子之间可以转移质子，形成水合氢离子和氢氧根离子：

H₂O(l) + H₂O(l) ⇌ H₃O⁺(aq) + OH⁻(aq)

This equilibrium lies very far to the left, so pure water contains only a tiny concentration of ions.

该平衡非常偏向左方，因此纯水中仅含有极微量的离子。

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2. The Ionic Product Expression | 离子积表达式

For the self-ionisation equilibrium, the equilibrium constant Kc can be written as:

对于自电离平衡，平衡常数 Kc 可写为：

Kc = [H₃O⁺][OH⁻] / [H₂O]²

Because the concentration of water is effectively constant in dilute aqueous solutions, it is incorporated into the equilibrium constant. This gives the ionic product of water, Kw:

因为在稀水溶液中，水的浓度实际上保持不变，故将其并入平衡常数中，得到水的离子积 Kw：

Kw = [H₃O⁺][OH⁻]

Often H₃O⁺ is written simply as H⁺, so Kw = [H⁺][OH⁻].

通常 H₃O⁺ 简写为 H⁺，因此 Kw = [H⁺][OH⁻]。

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3. The Value of Kw at 25 °C | 25°C 时的 Kw 值

At 25 °C, the ionic product of water is:

在 25 °C 时，水的离子积为：

Kw = 1.00 × 10⁻¹⁴ mol² dm⁻⁶

In pure water at this temperature, [H⁺] = [OH⁻] = 1.00 × 10⁻⁷ mol dm⁻³, which gives a pH of 7.

在此温度下，纯水中 [H⁺] = [OH⁻] = 1.00 × 10⁻⁷ mol dm⁻³，因此 pH 为 7。

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4. Temperature Dependence of Kw | Kw 的温度依赖性

The self-ionisation of water is an endothermic process. According to Le Chatelier’s principle, increasing the temperature shifts the equilibrium to the right, increasing the value of Kw.

水的自电离是一个吸热过程。根据勒夏特列原理，升高温度会使平衡向右移动，从而增大 Kw 的值。

At 50 °C, Kw ≈ 5.48 × 10⁻¹⁴ mol² dm⁻⁶, and the pH of neutral water is about 6.63, not 7. This does not mean the water is acidic; it means neutral pH changes with temperature.

在 50 °C 时，Kw ≈ 5.48 × 10⁻¹⁴ mol² dm⁻⁶，中性水的 pH 约为 6.63，而不是 7。这并不意味着水呈酸性，而是说明中性 pH 随温度变化而改变。

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5. Relationship Between [H⁺] and [OH⁻] in Aqueous Solutions | 水溶液中 [H⁺] 与 [OH⁻] 的关系

In any aqueous solution at a given temperature, the product of the hydrogen ion concentration and the hydroxide ion concentration is constant and equal to Kw.

在任意温度下，任何水溶液中氢离子浓度与氢氧根离子浓度的乘积都是常数，等于 Kw。

If [H⁺] increases, [OH⁻] must decrease to keep Kw constant. This allows us to calculate one from the other.

如果 [H⁺] 增大，为了保持 Kw 不变，[OH⁻] 必须减小。这让我们可以由一种离子浓度计算另一种。

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6. Calculating [H⁺] and [OH⁻] in Pure Water | 纯水中 [H⁺] 和 [OH⁻] 的计算

In pure water, the only source of ions is the self-ionisation of water, so [H⁺] = [OH⁻].

在纯水中，离子的唯一来源是水的自电离，所以 [H⁺] = [OH⁻]。

Using Kw = [H⁺][OH⁻] and the equality above:

利用 Kw = [H⁺][OH⁻] 以及上述相等关系：

[H⁺]² = Kw → [H⁺] = √Kw

At 25 °C, [H⁺] = √(1.00 × 10⁻¹⁴) = 1.00 × 10⁻⁷ mol dm⁻³.

在 25 °C 时，[H⁺] = √(1.00 × 10⁻

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