Equilibria-physical chemistry Flashcards
What is a reversible reaction?
A reversible reaction is a chemical reaction that can proceed in both the forward and reverse directions under the same conditions.
What happens at dynamic equilibrium?
The forward and reverse reactions occur at the same rate.
The concentrations of reactants and products remain constant (but are not necessarily equal).
It only occurs in a closed system (no substances enter or leave).
What is Le Chatelier’s Principle?
Le Chatelier’s Principle states that if a system at equilibrium is disturbed (by a change in temperature, pressure, or concentration), the position of equilibrium shifts to counteract the change.
How does changing concentration affect equilibrium?
Increasing reactant concentration → Equilibrium shifts right (more products formed).
Increasing product concentration → Equilibrium shifts left (more reactants formed).
Decreasing a reactant or product → Equilibrium shifts to replace what was removed.
How does changing pressure affect equilibrium?
Increasing pressure → Equilibrium shifts to the side with fewer gas molecules.
Decreasing pressure → Equilibrium shifts to the side with more gas molecules.
If moles of gas are equal on both sides, pressure has no effect.
How does changing temperature affect equilibrium?
Increasing temperature shifts equilibrium in the endothermic direction (absorbs heat).
Decreasing temperature shifts equilibrium in the exothermic direction (releases heat).
How does a catalyst affect equilibrium?
A catalyst speeds up both the forward and reverse reactions equally.
It does not shift equilibrium or change concentrations.
It helps the system reach equilibrium faster.
Why are compromise conditions used in industry?
High yield vs. fast rate vs. cost efficiency must be balanced.
Example: Haber Process (N₂ + 3H₂ ⇌ 2NH₃):
High pressure (200 atm) → Increases NH₃ yield (but expensive and dangerous).
Moderate temperature (~450°C) → Balances reaction rate and yield.
Iron catalyst → Speeds up reaction without affecting equilibrium.
How is
𝐾𝑐 expressed for a general reaction?
For a reaction:
𝑎𝐴+𝑏𝐵⇌𝑐C+𝑑𝐷
The expression for 𝐾𝑐 is:
𝐾𝑐 = [𝐶]𝑐[𝐷]𝑑/[𝐴]𝑎[𝐵]𝑏
where [X] represents the concentration of species X in mol dm−3
What factors affect the value of Kc?
✅ Temperature: Yes, changes in temperature can change Kc
❌ Concentration changes: No effect. The system shifts to restore equilibrium, but Kc stays the same.
❌ Adding a catalyst: No effect. A catalyst speeds up both forward and reverse reactions equally but does not alter equilibrium concentrations.
How does temperature affect Kc?
If the forward reaction is exothermic (
Δ𝐻<0
Increasing temperature shifts equilibrium left (more reactants), so Kc
decreases.
Decreasing temperature shifts equilibrium right (more products), so
increases.
If the forward reaction is endothermic (
Δ𝐻>0
Increasing temperature shifts equilibrium right, so Kc
increases.
Decreasing temperature shifts equilibrium left, so Kc
decreases.
How do you calculate Kc from equilibrium concentrations?
Write the balanced equation.
Set up the Kc expression.
Insert the equilibrium concentrations (in mol dm−3 ).
Calculate Kc using the equation.
