3.3.2Physical Chemistry
Dynamic Equilibrium — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 8 of 9 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Reversible Reactions
Some reactions do not go to completion because products can react to re-form the reactants.
Meaning
- Meaning A reversible reaction can go in both directions, shown by the symbol ⇌ in equations.
Forward and reverse
- Forward and reverse Reactants form products in the forward reaction, and products change back to reactants in the reverse reaction.
Example
- Example In the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), nitrogen and hydrogen make ammonia and ammonia also decomposes.
Heating hydrated copper(II) sulfate
- Heating hydrated copper(II) sulfate Blue CuSO₄·5H₂O loses water to form white CuSO₄, and adding water reverses the change.
Section 2 of 5Reaching Equilibrium
The graph of rates shows the two lines converging.
1
At the start only reactants are present, so the forward reaction has its highest rate and the reverse rate is zero.
2
As reactants are used up, their concentrations fall and the forward reaction slows.
3
As products build up, the reverse reaction speeds up.
4
Eventually the forward and reverse rates become equal, and this is dynamic equilibrium.
5
From then on the concentrations of reactants and products stay constant.
Section 3 of 5Features of Dynamic Equilibrium
Features of Dynamic Equilibrium
Equal rates
- Equal rates The rate of the forward reaction equals the rate of the reverse reaction.
Constant concentrations
- Constant concentrations The concentrations of reactants and products stay constant, though not necessarily equal.
Dynamic
- Dynamic Both reactions are still happening all the time, so molecules keep changing in both directions.
Closed system
- Closed system Equilibrium is reached only in a closed system where nothing can escape or enter.
| System | Equilibrium possible? | Reason |
|---|---|---|
| Sealed container | Yes | No substances escape |
| Open beaker | No | Gaseous product escapes |
Section 4 of 5Common Misunderstandings
Common Misunderstandings
Reaction stopped
- Reaction stopped At equilibrium the reactions have not stopped;
- they continue at the same rate in both directions.
Equal amounts
- Equal amounts There is no requirement for equal concentrations of reactants and products.
Time to reach
- Time to reach Reaching equilibrium takes time, and a catalyst makes it quicker without changing the final balance.
Section 5 of 5Exam Checklist & Tips
3.3.2 – Key Points for the Exam
- Reversible reaction A reaction that can proceed in both the forward and reverse directions.
- Dynamic equilibrium A state in which forward and reverse reactions occur at equal rates.
- Closed system A system in which no substances enter or leave.
- Equilibrium sign The ⇌ symbol that shows a reaction is reversible.
Common Mistakes
- Saying that nothing happens at equilibrium; the reactions continue in both directions.
- Claiming the concentrations of reactants and products must be equal.
- Forgetting to mention that the system must be closed.
- Writing a single arrow for a reversible reaction.
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3 & IGCSE —
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- : ( ).
- : Kc .