Le Chatelier's Principle — Revision Notes

⚡ 30-Second Revision

Principle: — System shifts to relieve stress.\n- Concentration: Add reactant $\implies$ shift right; Remove product $\implies$ shift right.\n- Pressure (gases only): Increase P $\implies$ shift to fewer moles gas; Decrease P $\implies$ shift to more moles gas. (Ignore solids/liquids).\n- Temperature:\n * Exothermic ($\Delta H < 0$): Increase T $\implies$ shift left, $K_{eq}$ decreases.\n * Endothermic ($\Delta H > 0$): Increase T $\implies$ shift right, $K_{eq}$ increases.\n- Catalyst: No effect on equilibrium position or $K_{eq}$; only speeds up attainment.\n- Inert Gas:\n * Constant Volume: No effect.\n * Constant Pressure: Shifts to more moles gas (due to dilution).

2-Minute Revision

Le Chatelier's Principle is your guide to predicting how a chemical system at equilibrium reacts to disturbances. Think of it as the system's way of fighting back against change. If you add more of a reactant, the system will consume it by making more products, shifting right.

If you remove a product, it will make more to replenish it, also shifting right. For reactions involving gases, pressure is key. Increase pressure (by decreasing volume), and the system will try to reduce it by favoring the side with fewer gas molecules.

Decrease pressure, and it favors the side with more gas molecules. Remember to only count gaseous moles! Temperature is special: it's the only factor that changes the equilibrium constant ($K_{eq}$). For exothermic reactions (which release heat), increasing temperature shifts the equilibrium left and decreases $K_{eq}$.

For endothermic reactions (which absorb heat), increasing temperature shifts it right and increases $K_{eq}$. Finally, catalysts are like express lanes; they get you to equilibrium faster but don't change where you end up.

Adding an inert gas at constant volume does nothing, but at constant pressure, it dilutes the reacting gases, causing a shift towards the side with more moles of gas.

5-Minute Revision

Le Chatelier's Principle is a powerful tool for understanding and predicting the behavior of reversible reactions at equilibrium. The core idea is that a system at equilibrium, when subjected to a 'stress' (a change in conditions), will adjust itself to counteract that stress and establish a new equilibrium.

\n\n1. Concentration Changes:\n * Adding a reactant: The equilibrium shifts to the right (towards products) to consume the added reactant. Example: $N_2 + 3H_2 \rightleftharpoons 2NH_3$. Adding $N_2$ shifts right.

\n * Removing a product: The equilibrium shifts to the right to replenish the removed product. Example: Removing $NH_3$ shifts right.\n * Conversely, adding a product or removing a reactant shifts the equilibrium to the left.

\n\n2. Pressure Changes (for gaseous reactions only):\n * Only reactions with a change in the number of moles of gas ($\Delta n_g \neq 0$) are affected. \n * Increasing pressure (by decreasing volume): The system shifts to the side with *fewer* moles of gas to reduce the pressure.

Example: $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ (4 moles $\rightarrow$ 2 moles). Increasing pressure shifts right.\n * Decreasing pressure (by increasing volume): The system shifts to the side with *more* moles of gas.

Example: Decreasing pressure shifts the Haber process left.\n * Adding an inert gas: \n * At constant volume: No effect, as partial pressures of reacting gases remain unchanged.\n * At constant pressure: The volume increases, decreasing partial pressures of reacting gases.

The equilibrium shifts to the side with *more* moles of gas to counteract this dilution effect.\n\n3. Temperature Changes:\n * Temperature is the *only* factor that changes the value of the equilibrium constant ($K_{eq}$).

\n * **Exothermic reactions ($\Delta H < 0$, heat is a product):** \n * Increasing temperature: Shifts left (to consume heat), $K_{eq}$ decreases.\n * Decreasing temperature: Shifts right (to produce heat), $K_{eq}$ increases.

\n * **Endothermic reactions ($\Delta H > 0$, heat is a reactant):** \n * Increasing temperature: Shifts right (to consume heat), $K_{eq}$ increases.\n * Decreasing temperature: Shifts left (to produce heat), $K_{eq}$ decreases.

\n\n4. Effect of a Catalyst:\n * A catalyst speeds up both forward and reverse reactions equally. \n * It helps the system reach equilibrium *faster* but does not change the equilibrium position or the value of $K_{eq}$.

It has no impact on the final yield of products at equilibrium. This is a crucial point for NEET.

Prelims Revision Notes

Le Chatelier's Principle is a qualitative rule predicting the direction of equilibrium shift. \n\nI. Effect of Concentration:\n* Increase Reactant: Equilibrium shifts right (towards products) to consume excess reactant.

\n* Decrease Reactant: Equilibrium shifts left (towards reactants) to replenish reactant.\n* Increase Product: Equilibrium shifts left (towards reactants) to consume excess product.\n* Decrease Product: Equilibrium shifts right (towards products) to replenish product.

\n* Pure Solids/Liquids: Changes in their amount do NOT affect equilibrium position as their concentrations are constant.\n\nII. Effect of Pressure (for gaseous reactions only):\n* Increase Pressure (decrease volume): Equilibrium shifts to the side with FEWER moles of gas.

\n* Decrease Pressure (increase volume): Equilibrium shifts to the side with MORE moles of gas.\n* **$\Delta n_g = 0$ (moles of gaseous reactants = moles of gaseous products):** No effect of pressure change.

\n* Adding Inert Gas:\n * At Constant Volume: No effect on partial pressures of reacting gases, hence no shift.\n * At Constant Pressure: Volume increases, partial pressures of reacting gases decrease.

Equilibrium shifts to the side with MORE moles of gas (to counteract dilution).\n\nIII. Effect of Temperature:\n* Temperature is the ONLY factor that changes the value of the Equilibrium Constant ($K_{eq}$).

\n* **Exothermic Reaction ($\Delta H < 0$, Heat is a product):**\n * Increase T: Shifts LEFT (reactants), $K_{eq}$ DECREASES.\n * Decrease T: Shifts RIGHT (products), $K_{eq}$ INCREASES.\n* **Endothermic Reaction ($\Delta H > 0$, Heat is a reactant):**\n * Increase T: Shifts RIGHT (products), $K_{eq}$ INCREASES.

\n * Decrease T: Shifts LEFT (reactants), $K_{eq}$ DECREASES.\n\nIV. Effect of Catalyst:\n* NO EFFECT on equilibrium position.\n* NO EFFECT on $K_{eq}$.\n* Only increases the rate of both forward and reverse reactions equally, thus speeding up the attainment of equilibrium.

Vyyuha Quick Recall

LCP: Can People Think Clearly? \n\n* Concentration: Add $\rightarrow$ shift away; Remove $\rightarrow$ shift towards.\n* Pressure: Increase P $\rightarrow$ fewer moles gas; Decrease P $\rightarrow$ more moles gas.\n* Temperature: Exothermic (heat out) $\rightarrow$ cool for products; Endothermic (heat in) $\rightarrow$ heat for products.\n* Catalyst: No shift, just faster.