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Chemistry 12 Equilibrium Cheat Sheet

BC Chemistry 12: Dynamic Equilibrium – The Ultimate Cheat Sheet

1. The Core Concept: Dynamic Equilibrium

Equilibrium is a dynamic state where forward and reverse reactions occur at equal rates.

  • Requirements for Equilibrium:

    • Closed System: No matter enters or leaves.

    • Constant Temperature: Since Keq is temperature-dependent.

    • Reversibility: The reaction must proceed in both directions.

  • The Balance:

    • Macroscopic: Properties like color, pressure, and concentration are constant.

    • Microscopic: Forward Rate = Reverse Rate (Rate_f = Rate_r).

2. Le Chatelier’s Principle (The "Reflex" Rule)

If a system at equilibrium is stressed, it shifts to partially counteract that stress.

Stress AppliedSystem ResponseShift DirectionEffect on Keq
Increase [Concentration]Consume added speciesAway from addition

None

Decrease [Concentration]Replace removed speciesToward removal

None

Increase TemperatureAbsorb heatToward Endothermic side

Changes!

Decrease TemperatureRelease heatToward Exothermic side

Changes!

Increase Pressure (Gas)Reduce pressureSide with fewer gas moles

None

Decrease Pressure (Gas)Increase pressureSide with more gas moles

None

Add CatalystIncrease both ratesNo Shift

None

Add Inert GasNo partial pressure changeNo ShiftNone

  • Temperature Rule: If a shift to the Right is caused by temperature, Keq increases. If a shift to the Left is caused by temperature, Keq decreases.

3. The Equilibrium Constant (Keq) & Trial K (Q)

Law of Mass Action: For aA + bB ⇌ cC + dD, the constant is: Keq = ([C]^c ×^d) / ([A]^a ×^b)

  • Exclusion Rule: Never include solids (s) or pure liquids (l) in the expression. Their concentrations are constant.

  • Trial K (Q): Compare current concentrations (Q) to the constant (Keq) to predict shift:

    • Q < Keq: Too many reactants; shifts Right.

    • Q > Keq: Too many products; shifts Left.

    • Q = Keq: System is at Equilibrium.

4. Solving Problems: The ICE Table

Use this when you have initial amounts and need equilibrium values.

  1. I (Initial): Starting concentrations (mol/L).

  2. C (Change): Use mole ratios (coefficients). Reactants are –x, products are +x.

  3. E (Equilibrium): Sum of I and C. Plug these into the Keq expression to solve for x.

5. Common "Wall" Pitfalls

  • The Catalyst Trap: Catalysts lower activation energy for both directions equally. They help reach equilibrium faster but do not change the yield or the Keq value .

  • The Solid/Liquid Distraction: Adding more of a solid does not shift equilibrium because its density (concentration) does not change.

  • Rate vs. Yield: Higher temperature increases Rate for all reactions (more collisions), but it can decrease Yield for exothermic reactions (shifts left).

6. Real World Applications

  • Haber Process: N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g) + 92 kJ

    • Maximize Yield: Use high pressure and constant removal of NH₃.

  • Hemoglobin (Hb): Hb + 4O₂ ⇌ Hb(O₂)₄

    • Lungs: High [O₂] shifts right (Hb picks up oxygen).

    • Tissues: Low [O₂] shifts left (Hb releases oxygen).

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