Volume and Pressure Changes

Introduction

Chemical Equilibrium: Achieved when the forward and reverse reactions occur at the same rate, maintaining constant concentrations of reactants and products.

• Closed System: Refers to a system where neither matter nor energy is exchanged with the surroundings, similar to a sealed container.
• Dynamic Equilibrium: Although no visible changes occur, molecular activities continue at the microscopic level.

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Le Chatelier's Principle

Le Chatelier's Principle: States that a system at equilibrium will adjust to counteract any changes imposed on it.

• Example - Pressure Change: An increase in pressure shifts the equilibrium towards fewer moles of gas.
• Example - Temperature Change: Reducing the temperature of an exothermic reaction will shift the equilibrium towards the production of more products.

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Factors Affecting Chemical Equilibrium

• Concentration, Temperature, Catalysts:
  • Decreasing volume induces an equilibrium shift towards fewer gas molecules.
  • Catalysts accelerate reaction time but do not alter the equilibrium position.

Application of Le Chatelier's Principle: Volume and Pressure Changes

• When Volume Decreases:

  • Pressure increases.
  • Equilibrium moves towards the side with fewer gas molecules.
  • Example 1: $A_2 (g) + B_2 (g) \rightleftharpoons 2AB (g)$
  • Example 2: $N_2 (g) + 3H_2 (g) \rightleftharpoons 2NH_3 (g)$

• When Volume Increases:

  • Pressure decreases.
  • Equilibrium shifts towards the side with a greater number of gas molecules.

Understanding Partial Pressures

• Partial Pressure: Plays a crucial role in equilibrium shifts when conditions such as volume and pressure vary.
  • Contributes to the total pressure of the system.

Reaction Quotient (Q)

Reaction Quotient (Q): Utilised to determine how a reaction will adjust under varying conditions.

$Q = \frac{[C]^c[D]^d}{[A]^a[B]^b}$

• Q > K: Indicates a shift towards reactants.
• Q < K: Indicates a shift towards products.
• Q = K: Denotes the system is at equilibrium.

Safety Protocols & Equipment

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Practical Implications

• Example: In ammonia synthesis, increased pressure favours the formation of fewer gas molecules.

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Misconceptions and Addressing Student Misunderstandings

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Worked Examples

Example 1: Predict equilibrium shifts when the volume is halved for $N_2 (g) + 3H_2 (g) \rightleftharpoons 2NH_3 (g)$.

Solution: When the volume is halved, pressure increases. Looking at the reaction:

• Reactant side: 4 moles of gas (1 mole $N_2$ + 3 moles $H_2$)
• Product side: 2 moles of gas (2 moles $NH_3$)

Since there are fewer moles of gas on the product side, the equilibrium will shift towards the formation of more $NH_3$ to reduce pressure and counteract the change.

Example 2: Examine $PCl_5 (g) \rightleftharpoons PCl_3 (g) + Cl_2 (g)$ under doubled pressure conditions.

Solution: When pressure is doubled, the system adjusts to reduce pressure:

• Reactant side: 1 mole of gas ($PCl_5$)
• Product side: 2 moles of gas (1 mole $PCl_3$ + 1 mole $Cl_2$)

The equilibrium will shift towards fewer gas molecules (reactant side), increasing the concentration of $PCl_5$ and decreasing $PCl_3$ and $Cl_2$.

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Chemical Equation and Colour Change Example

• Reaction:
  • Iron(III) ions interacting with thiocyanate ions can be represented as: $Fe^{3+} + SCN^- \rightleftharpoons [Fe(SCN)]^{2+}$
• Colour Change: A deep red colour indicates an increased concentration of $[Fe(SCN)]^{2+}$.

Conclusion

Understanding the effects of pressure and volume changes on chemical equilibrium provides insights into how systems respond to maintain balance according to Le Chatelier's Principle.