Effect of Concentration, Pressure and Temperature — Revision Notes

⚡ 30-Second Revision

Concentration — Add reactant/remove product $\implies$ shift right. Remove reactant/add product $\implies$ shift left.\n- Pressure (gases only): Increase P $\implies$ shift to fewer moles of gas. Decrease P $\implies$ shift to more moles of gas. (No effect if $\Delta n_g = 0$).\n- Temperature: Exothermic ($\Delta H < 0$): Increase T $\implies$ shift left (K decreases). Decrease T $\implies$ shift right (K increases). Endothermic ($\Delta H > 0$): Increase T $\implies$ shift right (K increases). Decrease T $\implies$ shift left (K decreases).\n- Catalyst: No effect on equilibrium position or K; only speeds up attainment of equilibrium.\n- Inert Gas (constant V): No effect on equilibrium position.

2-Minute Revision

Le Chatelier's Principle helps predict how a system at equilibrium responds to stress. \n\n1. Concentration: Increasing the concentration of a reactant or decreasing a product shifts the equilibrium towards the product side.

Conversely, decreasing a reactant or increasing a product shifts it towards the reactant side. The system always tries to consume what's added or replenish what's removed. \n\n2. Pressure: This primarily affects gaseous reactions where the number of moles of gas changes.

Increasing pressure (by decreasing volume) shifts the equilibrium to the side with fewer moles of gas to reduce the pressure. Decreasing pressure shifts it to the side with more moles of gas. Adding an inert gas at constant volume has no effect, as partial pressures of reacting gases remain unchanged.

\n\n3. Temperature: Temperature is unique as it changes the equilibrium constant (K). For exothermic reactions (heat released, $\Delta H < 0$), increasing temperature shifts the equilibrium to the left (endothermic direction), decreasing K.

Decreasing temperature shifts it to the right, increasing K. For endothermic reactions (heat absorbed, $\Delta H > 0$), increasing temperature shifts the equilibrium to the right (endothermic direction), increasing K.

Decreasing temperature shifts it to the left, decreasing K. \n\nRemember, a catalyst only speeds up the reaction to reach equilibrium but does not alter the equilibrium position or K.

5-Minute Revision

Le Chatelier's Principle is a cornerstone for understanding chemical equilibrium, stating that a system at equilibrium will adjust itself to counteract any applied stress. This principle is crucial for predicting the outcome of changes in concentration, pressure, and temperature.

\n\nConcentration Effects: If you increase the concentration of a reactant, the system will shift to the right (forward reaction) to consume the excess reactant and produce more products. If you decrease a product's concentration (e.

g., by removing it), the system will also shift right to replenish it. The opposite holds true: decreasing a reactant or increasing a product will shift the equilibrium to the left (reverse reaction).

This is quantitatively explained by the reaction quotient (Q) trying to return to the equilibrium constant (K).\n\nPressure Effects (for Gaseous Reactions): Pressure changes are significant only when there's a difference in the total number of moles of gaseous reactants and products ($\\Delta n_g \neq 0$).

If the pressure is increased (e.g., by reducing volume), the system shifts to the side with fewer moles of gas to reduce the overall pressure. If the pressure is decreased, it shifts to the side with more moles of gas.

A common trap is the addition of an inert gas: if added at constant volume, it has no effect on equilibrium because the partial pressures of the reacting gases remain unchanged. If added at constant pressure (implying volume increase), it effectively decreases the partial pressures of reacting gases, causing a shift to the side with more moles of gas.

\n\nTemperature Effects: Temperature is the only factor that alters the numerical value of the equilibrium constant (K). For exothermic reactions ($\Delta H < 0$, heat is a product), increasing temperature shifts the equilibrium to the left (endothermic direction) to absorb the added heat, thereby decreasing K.

Decreasing temperature shifts it right, increasing K. For endothermic reactions ($\Delta H > 0$, heat is a reactant), increasing temperature shifts the equilibrium to the right (endothermic direction) to absorb heat, increasing K.

Decreasing temperature shifts it left, decreasing K.\n\nCatalyst's Role: A catalyst accelerates both the forward and reverse reactions equally by lowering the activation energy. It helps the system reach equilibrium faster but does not change the equilibrium position or the value of K.

It simply reduces the time required to achieve equilibrium.

Prelims Revision Notes

1
Le Chatelier's Principle — System counteracts stress to re-establish equilibrium.\n2. Concentration Changes:\n * Increase reactant: Shift right (products increase).\n * Decrease reactant: Shift left (reactants increase).\n * Increase product: Shift left (reactants increase).\n * Decrease product: Shift right (products increase).\n * *K remains unchanged by concentration changes.*\n3. Pressure Changes (Gaseous Reactions Only):\n * Count moles of gaseous reactants ($\Sigma n_R$) and products ($\Sigma n_P$).\n * Increase Pressure (decrease V): Shift to side with fewer moles of gas (if $\Sigma n_P < \Sigma n_R$, shift right; if $\Sigma n_P > \Sigma n_R$, shift left).\n * Decrease Pressure (increase V): Shift to side with more moles of gas.\n * If $\Sigma n_P = \Sigma n_R$ (i.e., $\Delta n_g = 0$): No effect of pressure change.\n * Inert Gas Addition:\n * At constant volume: No effect on equilibrium (partial pressures of reacting gases unchanged).\n * At constant total pressure: Shifts to side with more moles of gas (due to decrease in partial pressures of reacting gases).\n * *K remains unchanged by pressure changes.*\n4. Temperature Changes:\n * Exothermic Reaction ($\Delta H < 0$, Heat is a product):\n * Increase T: Shift left (K decreases).\n * Decrease T: Shift right (K increases).\n * Endothermic Reaction ($\Delta H > 0$, Heat is a reactant):\n * Increase T: Shift right (K increases).\n * Decrease T: Shift left (K decreases).\n * *Temperature is the only factor that changes the value of K.*\n5. Catalyst: No effect on equilibrium position or K. Only increases the rate of both forward and reverse reactions, thus speeding up the attainment of equilibrium.

Vyyuha Quick Recall

To remember Le Chatelier's Principle effects: 'CPT'

Concentration: Consume what's Crowded, Create what's Clear.

Pressure: Push to People (moles) Poorer (fewer).

Temperature: Take Thermal (heat) side for Temp increase; Take Thermal side for Temp decrease (opposite of what's added/removed).