Chance and Spontaneous Change | 概率与自发变化

📚 Chance and Spontaneous Change | 概率与自发变化

Some changes happen on their own without any continuous external help: a gas spreads through a room, a hot metal block cools to room temperature, and an ice cube melts in a warm drink. In A-Level Chemistry, such processes are called spontaneous changes. Their direction is determined not only by whether heat is released, but also by the probability of energy and particles becoming more spread out.

有些变化无需持续外部帮助就能自行发生：气体会在房间中扩散，热的金属块会冷却至室温，冰块会在温热的饮料中融化。在 A-Level 化学中，这类过程称为自发变化。其方向不仅取决于是否放热，还取决于能量和粒子变得更加分散的概率。

This article explains how chance, entropy, and Gibbs free energy combine to predict whether a reaction or physical change is spontaneous under specified conditions.

本文将解释概率、熵和吉布斯自由能如何共同用于预测一个反应或物理变化在指定条件下是否自发。

1. What Is a Spontaneous Change? | 什么是自发变化？

In thermodynamics, a spontaneous change is one that proceeds on its own once it has been given a start, without needing a continuous input of energy from the surroundings.

在热力学中，自发变化是指一旦开始就能自行进行、无需外界持续输入能量的变化。

Spontaneous does not mean rapid. The conversion of diamond to graphite is spontaneous at 298 K and 1 atm, but the process is extremely slow because of a very high activation energy barrier.

自发并不意味着快速。金刚石在 298 K 和 1 atm 下转变为石墨是自发的，但由于活化能垒极高，该过程极其缓慢。

2. Energy Release Alone Cannot Predict Direction | 仅凭能量释放无法预测方向

Many spontaneous reactions are exothermic, such as combustion or neutralisation. It is tempting to think that a reaction will go if it gives out heat, but that idea fails for several familiar processes.

许多自发反应是放热的，例如燃烧或中和反应。人们容易认为放热反应一定会进行，但这一想法对若干常见过程并不成立。

For example, ice melting above 0 °C is spontaneous even though it is endothermic. Ammonium nitrate dissolving in water is also spontaneous and absorbs heat, making the solution cold. These examples show that enthalpy change alone is not enough to predict spontaneity.

例如，冰在 0 °C 以上融化是自发的，但该过程吸热。硝酸铵溶于水也是自发的，并吸收热量使溶液变冷。这些例子说明仅靠焓变不足以预测自发性。

3. Probability and Particle Distribution | 概率与粒子分布

Imagine a gas confined to one bulb of a two-bulb flask. When the tap is opened, the gas spreads into the other bulb. It never spontaneously returns to only one bulb.

想象一个气体被限制在双球烧瓶的一个球中。当阀门打开时，气体会扩散到另一个球中。它绝不会自发地回到只占据一个球的状态。

This is a matter of chance. The number of ways of arranging gas particles evenly across both bulbs is vastly greater than the number of ways of keeping them all in one bulb. The most probable arrangement is the one in which the particles are spread out.

这是一个概率问题。将气体粒子均匀分布在两个球中的排列方式数远大于把它们全部留在一个球中的排列方式数。最有可能的排列方式是粒子分散开来。

For a large number of particles, the chance of finding all of them in one bulb is so small that it is effectively impossible. Spontaneous expansion to fill the available volume is therefore a probability-driven change.

对于大量粒子而言，发现所有粒子都处于一个球中的概率极小，实际上不可能发生。因此，气体自发膨胀充满可用体积是概率驱动的变化。

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