Section 1 of 5Features of Dynamic Equilibrium
A dynamic equilibrium forms only in a reversible reaction where neither reactants nor products can escape.
Reversible reaction
- Reversible reaction A reaction that can go in both directions, shown by the symbol ⇌ in the equation.
Closed system
- Closed system Equilibrium is reached only when reactants and products cannot escape, such as in a sealed container.
Equal rates
- Equal rates The forward and reverse reactions proceed at the same rate, so the concentrations stop changing.
Dynamic
- Dynamic Both reactions are still happening, so the equilibrium is dynamic rather than static.
Section 2 of 5Reaching Equilibrium in the Haber Process
The Haber process makes ammonia from nitrogen and hydrogen, and it is a good example of a reversible reaction.
1
Nitrogen and hydrogen react to form ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g).
2
At the start the forward reaction is fast because the reactant concentrations are high.
3
As ammonia builds up, the reverse reaction speeds up until both rates become equal.
4
At equilibrium all three gases are present at fixed concentrations, and the reaction is still taking place.
Section 3 of 5Effect of Changes (HT only)
Le Chatelier's principle predicts how a changed condition moves the position of equilibrium.
Effect of Changes (HT only)
Le Chatelier's principle
- Le Chatelier's principle If a system at equilibrium is disturbed, the position of equilibrium shifts to oppose the change.
Pressure
- Pressure Higher pressure favours the side with fewer gas molecules, which is the ammonia side in the Haber process.
Temperature
- Temperature The forward Haber reaction is exothermic, so a higher temperature shifts the equilibrium towards the reactants.
| Change | Shift direction in the Haber process | Effect on ammonia yield |
|---|---|---|
| Increase pressure | Towards fewer gas molecules (right) | Increases |
| Increase temperature | Towards the endothermic reaction (left) | Decreases |
| Add a catalyst | No shift | No change to yield, only faster approach to equilibrium |
Section 4 of 5Worked Example
Question Predict the effect of increasing the pressure on the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g).
- Left side1 mol N₂ plus 3 mol H₂ gives 4 mol of gas molecules.
- Right side2 mol NH₃ gives 2 mol of gas molecules.
- Higher pressure favours the side with fewer gas molecules.
- The equilibrium shifts to the right, so more ammonia is produced.
Answer The equilibrium shifts to the right, and the yield of ammonia increases.
Section 5 of 5Exam Checklist & Tips
10.4.2 – Key Points for the Exam
- Reversible reaction A reaction in which products can react to re-form the reactants.
- Dynamic equilibrium A state in which forward and reverse reactions occur at equal rates in a closed system.
- Closed system A system in which no reactants or products can enter or leave.
- Equilibrium position The relative amounts of reactants and products present at equilibrium.
- Le Chatelier's principle A rule stating that equilibrium moves to oppose any change in conditions.
Common Mistakes
- Saying the reactions have stopped at equilibrium, when both forward and reverse reactions are still happening.
- Assuming the concentrations are equal at equilibrium, when only the rates of the two reactions are equal.
- Stating that a catalyst changes the position of equilibrium, when it only changes how quickly equilibrium is reached.
- Counting solids and liquids when comparing numbers of gas molecules for a pressure change.
كيمياء 3 ثانوي & IGCSE — الاتزان الكيميائي
قاعدة لوشاتيليه وموديل الإجابة
العوامل المؤثرة على موضع الاتزان وقيمة ثابت الاتزان Kc
- درجة الحرارة: العامل الوحيد الذي يغير قيمة Kc! في التفاعل الطارد (ΔH < 0) رفع الحرارة يزيح التفاعل عكسياً فتقل Kc. وفي الماص (ΔH > 0) رفع الحرارة يزيحه طردياً فتزيد Kc.
- الضغط: يؤثر فقط على التفاعلات الغازية غير المتساوية في عدد المولات؛ زيادة الضغط توجه التفاعل نحو عدد المولات الأقل (الحجم الأقل).
- العامل الحفاز: لا يغير موضع الاتزان ولا قيمة Kc؛ وظيفته تقليل طاقة التنشيط وزيادة سرعة التفاعلين الطردي والعكسي بنفس المقدار.