Le chatilier’s Principle , Chemical Equilibria And kc Flashcards
Fundamentals of Dynamic Equilibrium
Definition: A dynamic equilibrium is established in a closed system when the rate of the forward reaction equals the rate of the reverse reaction and the concentrations of reactants and products remain constant over time.
Key Points:
• Only occurs in closed systems.
• Dynamic = reactions continue at the molecular level.
• Macroscopically, system appears static.
Le Chatelier’s Principle (LCP)
Definition: When a system at equilibrium is subjected to a change in conditions, the position of equilibrium shifts to counteract the change.
a) Effect of Concentration
• Increase in concentration of a reactant → shifts to the right to use up excess.
• Decrease in concentration of a product → shifts right to form more product.
b) Effect of Pressure (only for gaseous reactions)
• Increase in pressure → shifts to side with fewer gas molecules.
• Decrease in pressure → shifts to side with more gas molecules.
• Note: Pressure has no effect if the number of gaseous moles is the same on both sides.
c) Effect of Temperature
• Increase in temperature favours the endothermic direction.
• Decrease in temperature favours the exothermic direction.
• Important: Changing temperature also changes Kc.
d) Effect of a Catalyst
• No effect on position of equilibrium.
• Increases rate of both forward and reverse reactions equally.
• Equilibrium is achieved faster.
Equilibrium Constant, Kc
a) Definition:
• Kc is the ratio of the concentrations
e) Calculations:
1. Use ICE table (Initial, Change, Equilibrium).
2. Insert equilibrium concentrations into the Kc expression.
3. Solve algebraically or rearrange to find unknowns.
Effect of Temperature on Kc
If temperature changes, Kc changes (unlike pressure/concentration).
• Endothermic reaction: ↑T → ↑Kc (more product formed).
• Exothermic reaction: ↑T → ↓Kc (less product formed).
Required Practical 7: Measuring Kc for an Equilibrium Reaction
Example: Ethanoic acid + ethanol ⇌ ethyl ethanoate + water
• Method Overview:
1. Mix known volumes/concentrations of acid and alcohol.
2. Leave in sealed container to reach equilibrium.
3. Titrate to find concentration of one component at equilibrium.
4. Use stoichiometry and Kc expression to calculate Kc.
• Assumptions:
• No side reactions.
• Accurate and consistent volume and temperature.
Common Exam Pitfalls
Not using square brackets in Kc expression (must show concentrations).
• Incorrect powers (must match stoichiometry).
• Forgetting to convert cm³ to dm³ when calculating molarities.
• Assuming catalysts change Kc (they don’t).
• Not stating “position of equilibrium shifts…” clearly in explanations.
Write the general expression for an equilibrium constant, Kc.
For the reaction: aA + bB ⇌ cC + dD
Kc = [conc] of reactant * n of reactant ( multiplied together )divided by [conc] of product * n of product
where square brackets indicate equilibrium concentrations in mol dm⁻³.
Units ?
Method: Insert units (mol dm⁻³) into the expression and simplify.
Magnitude of Kc
• Kc»_space; 1 → equilibrium lies to the right (products favoured).
• Kc «_space;1 → equilibrium lies to the left (reactants favoured).
• Kc ≈ 1 → significant amounts of both reactants and products present.
How do you calculate equilibrium concentrations from moles and volume?
[Concentration] = moles at equilibrium/ volume (dm³)
Then use these concentrations in the Kc expression.
