Genchem 15.5

Chemical Equilibrium

• System at Equilibrium

  • A chemical reaction is at equilibrium when the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products.

  • The system will maintain this state indefinitely unless disturbed.

• Disturbance of Equilibrium

  • When external factors are applied to a system at equilibrium, the system responds by shifting the position to counteract the disturbance. This phenomenon is governed by Le Chatelier's Principle.

Le Chatelier's Principle

• Definition: When a system at equilibrium experiences a disturbance, it will shift in a direction that minimizes the disturbance and establishes a new equilibrium.

• Factors that Disturb Equilibrium:

  1. Concentration

  2. Pressure

  3. Volume

  4. Temperature

Concentration Changes

• Modifying the concentration of reactants or products results in either a shift to the right (favoring products) or to the left (favoring reactants).

• Adding Reactants:

  • Adding more of a reactant shifts the equilibrium to the right.

  • Example: Adding N₂ or H₂ leads to the formation of more products.

• Removing Reactants:

  • Removing a reactant results in the equilibrium shifting to the left to produce more reactant.

• Adding Products:

  • More product increases concentration and shifts equilibrium to the left, favoring reactants.

• Removing Products:

  • Removing a product shifts the equilibrium to the right, producing more products.

Pressure Changes

• Applicable only to gas reactions.

• Increasing Pressure:

  • Shifts the equilibrium towards the side with fewer moles of gas to decrease pressure.

• Decreasing Pressure:

  • Shifts the equilibrium towards the side with more moles of gas to increase pressure.

• Same Number of Moles:

  • If both sides have the same number of gas moles, changing pressure alters the pressure but does not shift the equilibrium.

Volume Changes

• Volume changes also apply only to gases.

• Inversely related to pressure (Boyle's Law):

  • $P<em>1 V</em>1 = P<em>2 V</em>2$

• Increasing Volume:

  • Decreases pressure, shifting equilibrium towards the side with more moles of gas.

• Decreasing Volume:

  • Increases pressure, shifting equilibrium towards the side with fewer moles of gas.

Temperature Changes

• Determines the shift based on whether the reaction is endothermic or exothermic.

• Endothermic Reactions:

  • Heat behaves as a reactant.

  • Increasing temperature (adding heat) shifts equilibrium to the right.

  • Decreasing temperature shifts equilibrium to the left.

• Exothermic Reactions:

  • Heat behaves as a product.

  • Increasing temperature shifts equilibrium to the left.

  • Decreasing temperature shifts equilibrium to the right.

Catalysts and Inert Gases

• Catalysts:

  • Speed up both the forward and reverse reactions without affecting the position of the equilibrium.

  • Do not cause any shift in equilibrium once it has been established.

• Inert Gases:

  • Adding inert gases increases pressure but does not affect the equilibrium shift as they do not participate in the reaction.

Practical Applications

• Understanding these concepts is vital in industry, especially in chemical manufacturing.

• Adjusting conditions (temperature, pressure, concentration) allows controlled production of desired products.

Example Reactions

• Reaction Setup:

  • For example, consider $PCl<em>3 + Cl</em>2 \rightleftharpoons PCl_5$

• Adding PCl₃: shifts to the right.

• Removing Cl₂: shifts to the left.

• Removing PCl₅: shifts to the right.

• Decrease in Volume: shifts to the right (less moles of gas).

• Adding Inert Gas: No shift, but pressure increases.

Summary of Shifts Based on Disturbances

Conclusion

• The equilibrium concept and Le Chatelier's Principle provide essential insights into chemical reactions and their behavior under various conditions.

• Understanding and applying these principles enable predictions regarding the shifts in equilibrium and facilitate effective chemical production strategies.