Hess’s Law | 赫斯定律

📚 Hess’s Law | 赫斯定律

Hess’s law is one of the most useful tools in A Level chemical energetics. It allows chemists to calculate enthalpy changes that cannot be measured directly by constructing alternative routes between reactants and products.

赫斯定律是 A Level 化学能量学中最有用的工具之一。它允许化学家通过构建反应物与生成物之间的替代路径，来计算无法直接测量的焓变。

1. What Is Hess’s Law? | 什么是赫斯定律

Hess’s law states that the total enthalpy change for a chemical reaction is independent of the route taken, provided the initial and final conditions are the same.

赫斯定律指出，只要反应的始态和终态相同，化学反应的总焓变与所采取的路径无关。

Enthalpy H is a state function. This means ΔH depends only on the initial and final states, not on the intermediate steps.

焓 H 是一个状态函数。这意味着 ΔH 只取决于始态和终态，而不取决于中间步骤。

For a direct route A → B and an indirect route A → C → B, Hess’s law gives:

对于直接路径 A → B 和间接路径 A → C → B，赫斯定律给出：

ΔH₁ = ΔH₂ + ΔH₃

2. Enthalpy Change and State Symbols | 焓变与状态符号

Enthalpy change, ΔH, is the heat transferred at constant pressure. Its unit is kJ mol⁻¹, where the “per mole” refers to the molar quantities in the balanced equation.

焓变 ΔH 是恒压条件下传递的热量。其单位为 kJ mol⁻¹，其中“每摩尔”指的是配平方程中的摩尔量。

State symbols are essential because changing state involves an enthalpy change. For example, vaporising liquid water requires +44 kJ mol⁻¹, so H₂O(l) and H₂O(g) cannot be used interchangeably.

状态符号至关重要，因为状态变化涉及焓变。例如，液态水汽化需要 +44 kJ mol⁻¹，因此 H₂O(l)

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