4.6.2.4,4.6.2.5,4.6.2.6,4.6.2.7 - Changing equilibrium conditions
The composition of a reversible mixture at equilibrium depends on its conditions. Learn to use Le Chatelier's principle to predict the effects of concentration, temperature and gas pressure, then separate equilibrium yield from the speed of reaching it.
Le Chatelier's principle and equilibrium position (Higher)
A change to an equilibrium system makes it respond in a direction that counteracts the change.
At equilibrium, the forward and reverse rates are equal and composition is constant. Changing a condition can make the rates unequal temporarily. One direction then occurs faster overall until a new equilibrium is established, with equal rates again but potentially different relative amounts of reactants and products.
Le Chatelier's principle
If a system is at equilibrium and a change is made to its conditions, the system responds to counteract the change.
The equilibrium position describes the relative amounts of reactants and products at equilibrium. A shift to the right means more products relative to reactants. A shift to the left means more reactants relative to products. Write the reaction in a stated direction before using these descriptions, because reversing the written equation reverses what “right” means.
For , a change favouring the forward reaction initially produces C and D faster than they are converted back. As composition changes, the rates become equal again. The new equilibrium remains dynamic; the system does not permanently abandon the reverse reaction.
Counteracting does not mean completely undoing the imposed change. After adding a reactant, the final concentration of that reactant may still be higher than before addition even though some of it is consumed. Similarly, the system does not necessarily return to the original temperature or pressure. The principle predicts the direction of adjustment, not the exact amount of a new substance without further data.
Keep the questions “Which equilibrium composition is favoured?” and “How quickly does the reaction happen?” separate. A condition can increase rate while decreasing the equilibrium proportion of a desired product. The remaining parts teach which property of the reaction is needed for each prediction.
Changing reactant or product concentration (Higher)
Changing one concentration disturbs equilibrium, so the mixture adjusts its concentrations until equilibrium is reached again.
For , adding A increases its concentration. The system responds by consuming some of the added A, so the forward reaction is favoured and the equilibrium position shifts right. More C and D form while A and B are consumed during adjustment.
Removing product C decreases its concentration. The system responds by making more C, again favouring the forward direction. More reactants react until equilibrium is re-established. Product removal can therefore help a process form further desired product, although product being removed is not simply accumulating in the reacting vessel.
The opposite changes also follow the principle. Removing A favours the reverse reaction that replaces some A. Adding C favours the reverse reaction that consumes some C. Read the identity of the changed substance carefully: a concentration increase does not always mean a shift right.
| Change in | Direction favoured | Adjustment that counteracts it |
|---|---|---|
| Add A or B | Forward/right | Consumes some added reactant |
| Remove C or D | Forward/right | Makes more of the removed product |
| Remove A or B | Reverse/left | Makes more of the removed reactant |
| Add C or D | Reverse/left | Consumes some added product |
If a concentration–time graph is supplied, distinguish the immediate imposed change from the subsequent reaction adjustment. Removing C produces an immediate drop in C. Afterwards, C rises as it is remade, while reactant concentrations fall. The final curves become horizontal at the new equilibrium; they need not return to their old heights.
For a real equation such as , adding nitrogen at fixed other relevant conditions favours ammonia formation, while adding ammonia favours its breakdown. The coefficients do not reverse the concentration rule; they describe the amounts participating in the same directional change.
Temperature and endothermic or exothermic directions (Higher)
Increasing temperature favours the endothermic direction, while decreasing temperature favours the exothermic direction.
The endothermic direction takes in energy, so favouring it counteracts an imposed temperature increase. The exothermic direction transfers energy to the surroundings, so favouring it counteracts cooling. Identify the energy transfer for the forward direction before predicting product yield.
For an exothermic forward reaction, such as
increasing temperature favours the endothermic reverse reaction. The equilibrium position shifts left and the relative amount of ammonia decreases. Decreasing temperature favours the forward exothermic reaction, shifts the position right and increases the equilibrium proportion of ammonia.
For an endothermic forward reaction, the conclusions reverse. Increasing temperature shifts the position right and increases the relative amount of products. Decreasing temperature shifts it left and decreases the relative amount of products. “Higher temperature makes more product” is therefore incomplete until the energy direction is known.
| Forward reaction | Temperature increased | Temperature decreased |
|---|---|---|
| Exothermic | Fewer products at equilibrium | More products at equilibrium |
| Endothermic | More products at equilibrium | Fewer products at equilibrium |
Given appropriate data, you can infer an energy direction. Suppose an illustrative gas reaction, at the same pressure and starting composition, has an equilibrium product percentage of 70% at , 45% at , and 25% at . Heating reduces product proportion, so it favours the reverse direction. The reverse is endothermic and the forward is exothermic. This inference concerns an equilibrium dataset; an amount measured after one fixed short reaction time could instead reflect differing rates.
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