可逆反应与化学平衡
化学· 3.17–3.22（S3 子主题 3(c)）· 15 分钟阅读
1. 可逆反应（核心内容，所有级别均要求）★★☆☆☆⏱ 5 min
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可逆反应
可同时向正反应（反应物 → 生成物）与逆反应（生成物 → 反应物）方向进行的化学反应，在配平的方程式中使用 ⇌ 符号标识。
你必须牢记两个考试要求的核心可逆反应：五水合硫酸铜的脱水反应，以及氯化铵受热发生的反应。
写出五水合硫酸铜可逆脱水反应的配平方程式，需标注状态符号。
- 1
- 确定反应物：五水合硫酸铜为蓝色晶体，化学式为
- 2
- 确定加热后的生成物：白色无水硫酸铜 与水蒸气
- 3
- 添加可逆符号（该方程式已配平）：
- 4
描述在密封试管中加热氯化铵时观察到的可逆变化现象。
- 1
- 加热白色固态氯化铵，会使其发生热分解，生成无色的氨气与氯化氢气体
- 2
- 当气体在密封试管顶部冷却时，会重新反应生成白色固态氯化铵，附着在玻璃管壁上
- 3
Exam tip:
考试可能要求你将硫酸铜脱水反应与水的检验关联：向白色无水硫酸铜中加水，固体变为蓝色，即可证明水的存在。
2. 动态平衡（仅高阶/化学方向要求）★★★☆☆HL 专属⏱ 4 min
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动态平衡
在封闭（密封）体系中达到的状态，此时正反应速率与逆反应速率相等，因此反应物与生成物的浓度不随时间发生变化。
注意“动态”指两个反应仍在持续进行，并非反应已经停止。反应速率相等意味着平衡状态下观测不到任何宏观变化。
一名学生在敞口烧杯中加热五水合硫酸铜，声称体系达到了平衡。解释该学生的说法为何错误。
- 1
- 动态平衡要求体系为封闭体系，所有物质均无法进入或离开体系。
- 2
- 在敞口烧杯中，脱水反应生成的水蒸气会逸散到空气中，因此逆反应（重新生成五水合硫酸铜）的速率无法与正反应速率相等。
- 3
- 因此该反应在敞口容器中永远无法达到平衡状态。
3. 影响平衡位置的因素（仅高阶/化学方向要求）★★★★☆HL 专属⏱ 6 min
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平衡位置描述了平衡状态下混合物中反应物占比更高还是生成物占比更高。温度或压强的变化可以使平衡位置发生移动，而催化剂不会对其产生影响。
催化剂对平衡的影响
催化剂会同等程度地加快正反应和逆反应的速率，因此它不会改变平衡位置，只会缩短反应达到平衡所需的时间。
Temperature change: An increase in temperature shifts equilibrium in the direction of the endothermic reaction (absorbs heat). A decrease in temperature shifts equilibrium in the direction of the exothermic reaction (releases heat).
Pressure change (only applies to gas-phase reactions): An increase in pressure shifts equilibrium towards the side with fewer moles of gas. A decrease in pressure shifts equilibrium towards the side with more moles of gas.
针对下述可逆反应，预测(a)升高温度、(b)增大压强带来的影响： ΔH = -92 kJ/mol（正反应为放热反应）
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(a) 升高温度：平衡向吸热反应方向移动。由于正反应放热，逆反应吸热，因此平衡向左移动，生成更多的和。
- 2
(b) 计算两侧的气体物质的量：左侧 = 1 + 3 = 4 mol 气体，右侧 = 2 mol 气体。增大压强会使平衡向气体分子数更少的方向移动，因此平衡向右移动，生成更多的。
Exam tip:
预测平衡移动时务必写明推理过程：例如要写“平衡向左移动，因为逆反应为吸热反应”，而不要仅写“向左移动”，才能拿到全部分数。
4. 核心回顾与概念检测★★★☆☆⏱ 2 min
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Which symbol indicates a reversible reaction?
→
↔
⇌
=
显示答案
2 —Correct. The ⇌ symbol is used exclusively for reversible reactions in Edexcel IGCSE Chemistry.
(Higher only) Which statement about dynamic equilibrium is true?
Reactions stop when equilibrium is reached
Forward reaction rate is higher than reverse rate
Concentrations of reactants and products are equal
Equilibrium only occurs in closed systems
显示答案
3 —Correct. A closed sealed system is required so no reactants or products can escape, allowing rates to equalise.
5. 常见陷阱
错误做法:
Using → instead of ⇌ for reversible reactions
原因:
The single arrow indicates a reaction that goes to completion, not reversible, so you lose the mark for correct symbol use.
正确做法:
Always use ⇌ for all reversible reaction equations, including the two named core examples.
错误做法:
Stating that reactions stop at dynamic equilibrium
原因:
Equilibrium is dynamic, meaning both forward and reverse reactions are still occurring at equal rates, with no net change.
正确做法:
Explicitly state that both reactions continue at the same rate at equilibrium.
错误做法:
Claiming catalysts shift equilibrium position
原因:
Catalysts speed up forward and reverse rates equally, so there is no change to equilibrium position or product yield.
正确做法:
State that catalysts only reduce the time taken to reach equilibrium, with no effect on the equilibrium position.
错误做法:
Forgetting to state a closed system is required for equilibrium
原因:
Exam answers require explicit mention of a closed/sealed system to award full marks for equilibrium conditions.
正确做法:
Always include reference to a closed system when explaining how equilibrium is achieved.
错误做法:
Applying pressure shift rules to reactions with no gaseous reactants/products
原因:
Pressure only affects equilibrium of gas-phase reactions, not reactions with only solid or liquid reactants/products.
正确做法:
First check for gas state symbols before applying pressure shift rules, and count only moles of gas on each side of the equation.
6. 速查表
Content Area | Core Tier Requirement | Higher Tier Requirement |
|---|---|---|
Reversible reaction symbol | Recall ⇌ means reversible | Same as core |
Named reactions | Describe copper sulfate dehydration and ammonium chloride heating reversibility | Same as core |
Dynamic equilibrium | Not assessed | Closed system, equal forward/reverse rates, constant reactant/product concentrations |
Catalyst effect | Not assessed | No change to equilibrium position, only speeds up reaching equilibrium |
Temperature shift | Not assessed | Increase temp → shift to endothermic direction; decrease temp → shift to exothermic direction |
Pressure shift | Not assessed | Increase pressure → shift to side with fewer gas moles; decrease pressure → shift to side with more gas moles |
7. 常见问题
我需要为该考试学习勒夏特列原理吗？
不需要，爱德思 4CH1 考纲明确说明不要求掌握勒夏特列原理。你仅需应用给定的温度、压强平衡移动规则作答即可。
在敞口容器中能否达到平衡状态？
不能，动态平衡要求体系为密封（封闭）状态，确保反应物与生成物均无法逸出，使正逆反应速率能够最终相等。
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下一步
Now that you have mastered reversible reactions and equilibria, you can move on to applying these rules to context-based exam questions, and link this content to other physical chemistry topics such as reaction rates and energetics. This knowledge will also form a strong foundation if you progress to A-Level Chemistry, where you will learn about quantitative equilibrium calculations. Be sure to practice past paper questions specifically for this topic to test your ability to apply shift rules correctly under timed exam conditions.