Chemical Equilibria Notes

Overview of Reversible Reactions

• Chemical reactions can be reversible, meaning:
  • Reactants create products, which can revert to reactants.
  • There are both forward and reverse reactions.
• Dynamic equilibrium occurs when:
  • Rates of forward and reverse reactions are equal.
  • Chemistry is ongoing, but no net change is observed due to balance.

Stoichiometry in Reactions

• Traditional stoichiometry focuses on unidirectional reactions:
  • Assumes all reactants convert to products before ending.
• Equilibrium calculations are more complex:
  • Concentrations of each substance at equilibrium need to be calculated.

ICE Box Method for Equilibrium Calculations

• ICE Box: Stands for Initial, Change, and Equilibrium.
  • Initial: Record initial amounts of reactants and products.
    • Example: Start with 1 mole of PCl5 (others at 0).
  • Change: Represent unknown changes as variables (e.g., X).
    • Reactants: -X (depleted), Products: +X (formed).
  • Equilibrium: Sum of initial and change.
    • Reactants: 1 - X; Products: X.
• Example Calculation:
  • Given: 0.135 moles of PCl3 at equilibrium, thus X = 0.135.
  • Calculate other substances using X.

Equilibrium Constant (Kc)

• Each equilibrium has a constant Kc defined as:
  • [ K_c = \frac{[Products]}{[Reactants]} ]
  • Each concentration raised to the power of its stoichiometric coefficient.
• Interpretation of Kc:
  • If Kc > 1: Products are favored (more products).
  • If Kc < 1: Reactants are favored (more reactants).
• Exclude solids and pure liquids in Kc expression.
  • Example: Carbon (solid) not included in expression.

Reaction Quotient (Q)

• To predict direction of reaction:
  • Calculate Q using non-equilibrium concentrations.
• Compare Q with Kc:
  • If Kc > Q: Reaction shifts to products.
  • If Kc < Q: Reaction shifts to reactants.
  • If Kc = Q: System is at equilibrium.

Advanced ICE Box Example

• For given information, calculate equilibrium concentrations using molarity.
  • Set up ICE box considering stoichiometry:
    • Reactant: Initial value; Products: 0.
  • Change values based on stoichiometry:
    • For every 2 moles of reactant, 1 mole of each product is formed.
  • Calculate equilibrium concentrations and plug into Kc expression.
  • Solve for X:
    • If quadratic, might need quadratic equation.

Conclusion

• Understanding chemical equilibria is key to predicting reaction behavior.
• Practice calculations using the ICE box and Kc expressions.

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