Equilibrium | Chemistry for Grade 11 (IGCSE) PDF Download

The Concept of Equilibrium

• We have already learned that a reversible reaction can occur in both directions.
• When the rates of the forward and reverse reactions become equal during a reaction, the entire reaction reaches a state of equilibrium.
• Equilibrium is a dynamic process where molecules on the left and right sides of the equation continually transform into each other through chemical reactions at an equal pace.
• The concentrations of reactants and products remain stable as long as there are no changes in factors like temperature and pressure.
• Equilibrium is only achievable in a closed system, preventing any of the chemical components from exiting the reaction vessel.

• An example of dynamic equilibrium is the reaction between hydrogen (H₂) and nitrogen (N₂) in the Haber process.
• Initially, with only nitrogen and hydrogen present, the forward reaction rate is highest due to their concentrations being at peak levels.
• As the reaction progresses, the concentrations of hydrogen and nitrogen decrease, leading to a decline in the forward reaction rate.
• Simultaneously, the concentration of ammonia increases, causing the backward reaction rate to rise (ammonia decomposes to reform hydrogen and nitrogen).
• Due to the interconnected nature of these reactions and the enclosed system, the forward and backward reaction rates eventually equalize, reaching equilibrium:
  3H₂ (g) + N₂ (g) ↔ 2NH₃ (g)
  
• The equilibrium position indicates the balance between reactant and product concentrations in the equilibrium state.
• Shifting the equilibrium to the left signifies an increase in reactant concentration.
• Conversely, a shift to the right indicates an increase in product concentration.

Le Chatelier's Principle

• The distribution of reactants and products at equilibrium varies based on reaction conditions, a concept elucidated by Le Chatelier's Principle, named after 19th-century French military engineer Henri Le Chatelier.
• This principle posits that a system at equilibrium adjusts in response to any changes in its conditions, opposing the change.
• Le Chatelier's Principle is applied to predict shifts in equilibrium position due to alterations in temperature, pressure, or concentration.
• Utilizing knowledge of energy changes, states, and concentrations, the principle enables manipulation of outcomes in reversible reactions.
• For instance, an increase in pressure prompts equilibrium to shift toward the side with fewer gaseous molecules.
• Equilibrium is deemed to shift rightward when the forward reaction is favored, resulting in increased product formation.
• Conversely, equilibrium shifts leftward when the reverse reaction is favored, leading to increased reactant formation.

Effects of Temperature on Equilibrium

Table showing the Effects of Temperature on Equilibrium

• Iodine monochloride reacts reversibly with chlorine to form iodine trichloride
  
• When the equilibrium mixture is heated, it becomes dark brown, indicating a shift towards endothermic reaction.
• The dark brown color signifies more ICl3 is produced, suggesting an endothermic backward reaction.
• Increasing temperature favors the endothermic direction of the equilibrium.

Effects of Pressure on Equilibrium

Table showing the Effects of Pressure on Equilibrium

• With an increase in pressure, the equilibrium position of the reaction between nitrogen dioxide and dinitrogen tetroxide shifts.
  
• Since there are two gas molecules on the left side and only one on the right, the shift occurs toward the direction with fewer gas molecules.
• Consequently, the equilibrium moves to the right, favoring the production of dinitrogen tetroxide.
• As a result, the reaction mixture becomes lighter in color as more colorless N₂O₄ is generated.

Effects of Concentration on Equilibrium

 Table showing the Effects of Concentration on Equilibrium

• Elevating the concentration of either ICl or Cl₂ leads to a shift in equilibrium toward the right.
• This shift favors the formation of the yellow product, ICl₃.
• Conversely, a decrease in the concentration of ICl or Cl₂ results in equilibrium shifting to the left.
• This favors the production of the dark brown reactant, ICl.

Effect of Catalyst on Equilibrium

• The presence of a catalyst doesn't change the equilibrium position but speeds up the rate of reaching equilibrium.
• This acceleration occurs because the catalyst lowers the activation energy for both forward and reverse reactions.
• Consequently, the concentrations of reactants and products remain the same at equilibrium as they would be without the catalyst.

Diagram showing the effect of a catalyst on the time taken for equilibrium to be established