IB Chemistry: Hess’s Law Principles & Calculation Skills | IB化学：盖斯定律原理与计算技巧

📚 IB Chemistry: Hess’s Law Principles & Calculation Skills | IB化学：盖斯定律原理与计算技巧

Hess’s Law is one of the core ideas in IB Chemistry energetics. It states that the enthalpy change of a reaction depends only on the initial and final states, not on the route taken. This article explains why this rule works and shows you step-by-step methods to solve Hess’s Law problems.

盖斯定律是IB化学能量学中的核心概念之一。它指出：一个反应的焓变只取决于反应的始态和终态，而与反应途径无关。本文将从原理出发，手把手演示如何用盖斯定律进行准确计算。

1. What Is Hess’s Law? | 什么是盖斯定律？

Hess’s Law says: the enthalpy change of a reaction is the same whether the reaction takes place in one step or in several steps, provided the initial and final conditions are identical.

盖斯定律指出：在初始和最终条件相同的情况下，无论反应一步完成还是分几步完成，反应的总焓变都相同。

This law is also called the law of constant heat summation. It was proposed by the Russian chemist Germain Hess in 1840. For example, the enthalpy change of converting graphite to diamond can be calculated indirectly using combustion data, because direct conversion is difficult to perform in a laboratory.

这一定律又称“热量总和守恒定律”，由俄国化学家盖斯于1840年提出。例如，石墨转化为金刚石的焓变难以直接在实验室中测定，但可以通过燃烧焓数据间接计算。

2. Why Does the Route Not Matter? | 为什么反应途径不影响焓变？

Enthalpy H is a state function. A state function depends only on the current state of the system, not on how that state was reached. Therefore, the change ΔH = H(products) − H(reactants) only uses the start and end states.

焓H是一个状态函数。状态函数只由系统当前的状态决定，而不管系统是如何到达该状态的。因此，ΔH = H(生成物) − H(反应物) 只取决于始态和终态。

A useful analogy is hiking: the altitude difference between two peaks depends only on their heights, not on the trail you choose. Similarly, the enthalpy difference depends only on the initial and final chemical states.

一个有用的类比是登山：两座山峰之间的高度差只取决于它们的高度，而与所走的路线无关。同样，焓差也只取决于起始和最终的化学状态。

If two different routes gave different enthalpy changes, we could go one way and return the other to create or destroy energy. This would violate the

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