Equilibrium
Section: 6. Chemical Reactions | Syllabus: Cambridge AS Level Physics 9702
What is Dynamic Equilibrium? Dynamic equilibrium occurs in a reversible reaction when the rate of the forward reaction equals the rate of the backward reaction. The concentrations of reactants and products remain constant, but both reactions continue to occur.
At equilibrium: • Rate of forward reaction = Rate of backward reaction • Concentrations of reactants and products stay constant • Both reactions are still happening (it's DYNAMIC, not static) Why is it Called "Dynamic"?
The word "dynamic" means that the reactions are still occurring - they haven't stopped. The forward and backward reactions continue at the same rate, giving the appearance of no change. Think of it like this: Imagine two escalators between two floors: one going up, one going down.
If the same number of people go up as come down every minute, the number of people on each floor stays constant - but people are still moving! This is dynamic equilibrium. Aspect Static (Not Moving) Dynamic (Moving) Reactions happening?
NO - everything stopped YES - both reactions continuing Concentrations Constant because nothing happens Constant because rates are equal Example Completed reaction (irreversible) Equilibrium in reversible reaction Conditions for Equilibrium For equilibrium to be established, certain conditions must be met: Reversible reaction: Must be able to go both ways (⇌) Closed system: No substances can enter or leave Constant conditions: Temperature and pressure must remain constant Sufficient time: System must be left long enough to reach equilibrium Important: If the system is open (e.g., gases can escape), equilibrium CANNOT be reached!
Characteristics of Equilibrium Property At Equilibrium Rate of forward reaction EQUALS rate of backward reaction Concentrations Remain CONSTANT (not necessarily equal!) Macroscopic properties No observable change (color, pressure, etc.
stay constant) Microscopic level Both reactions continue (molecules still reacting) Position of equilibrium Can favor reactants, products, or be in middle Position of Equilibrium The position of equilibrium describes the relative amounts of reactants and products at equilibrium.
Position What It Means Amounts Equilibrium lies to the RIGHT Favors products More products than reactants at equilibrium Equilibrium lies in the MIDDLE Balanced Similar amounts of reactants and products Equilibrium lies to the LEFT Favors reactants More reactants than products at equilibrium Important: At equilibrium, concentrations are CONSTANT but NOT necessarily EQUAL.
You can have much more product than reactant (or vice versa) and still be at equilibrium! Le Chatelier's Principle Le Chatelier's Principle states: If a system at equilibrium is subjected to a change in conditions (temperature, pressure, or concentration), the equilibrium position will shift to counteract that change.
Think of it as the system trying to "undo" the change you made! Effect of CONCENTRATION on Equilibrium Le Chatelier's Principle for concentration: If you INCREASE the concentration of a substance, equilibrium shifts AWAY from it.
If you DECREASE the concentration of a substance, equilibrium shifts TOWARDS it. Examples: Reaction: A + B ⇌ C + D • Add more A → equilibrium shifts RIGHT (makes more C and D) • Remove C → equilibrium shifts RIGHT (makes more C and D to replace what was removed) • Add more D → equilibrium shifts LEFT (converts D back to A and B) Change Made System Response Direction of Shift Increase reactant concentration Use up excess reactant Shifts RIGHT (→ products) Decrease reactant concentration Make more reactant Shifts LEFT (← reactants) Increase product concentration Use up excess product Shifts LEFT (← reactants) Decrease product concentration Make more product Shifts RIGHT (→ products) Effect of PRESSURE on Equilibrium (Gases Only) Le Chatelier's Principle for pressure: If you INCREASE pressure, equilibrium shifts to the side with FEWER gas molecules.
If you DECREASE pressure, equilibrium shifts to the side with MORE gas molecules. How to count molecules: Look at the balanced equation Count the coefficient (big number in front) of each gas Add up left side, add up right side Compare totals Example: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) Left side: 1 + 3 = 4 molecules of gas Right side: 2 molecules of gas Increase pressure → shifts RIGHT (toward fewer molecules) Decrease pressure → shifts LEFT (toward more molecules) Special case: If both sides have the SAME number of gas molecules, pressure has NO EFFECT on equilibrium position.
Example: H₂(g) + I₂(g) ⇌ 2HI(g) (2 molecules each side → no shift) Effect of TEMPERATURE on Equilibrium Le Chatelier's Principle for temperature: If you INCREASE temperature, equilibrium shifts in the ENDOTHERMIC direction.
If you DECREASE temperature, equilibrium shifts in the EXOTHERMIC direction. Step-by-step approach: Identify which direction is exothermic (releases heat) and which is endothermic (absorbs heat) Remember: If forward is exothermic, backward is endothermic (and v…
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