Understanding Points
Reactivity 2.3.1—A state of dynamic equilibrium is reached in a closed system when the rates of
forward and backward reactions are equal.
Reactivity 2.3.2—The equilibrium law describes how the equilibrium constant, K, can be
determined from the stoichiometry of a reaction.
Reactivity 2.3.3—The magnitude of the equilibrium constant indicates the extent of a reaction at
equilibrium and is temperature dependent.
Reactivity 2.3.4—Le Châtelier’s principle enables the prediction of the qualitative effects of changes in concentration, temperature and pressure to a system at equilibrium.
Dynamic Equilibrium
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A state of equilibrium is reached in a closed system when the rates of the forward and reverse reactions are equal
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State of dynamic equilibrium will only be established in a closed system
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Concentrations of reactants and products remain constant
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Equilibrium constant Kc
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Consider the following reversible reaction: mA + nB ⇌ pC + qD
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Once it reaches equilibrium, the concentration of the product and reactants remain constant
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If Kc increases when temperature increases, the forward reaction is endothermic
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If Kc decreases when temperature increases, the forward reaction is exothermic
Position of Equilibrium
Le Chȃtelier’s Principle
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In a dynamic equilibrium, changes in conditions are counteracted by changes in positions of equilibrium
Condition | Effects | ||
Concentration | ConcentrationSystem will want to…Direction of equilibrium Increase in reactants Decrease [reactants]Right Decrease in reactants Increase [reactants] Left Increase in productsDecrease [products]LeftDecrease productsIncrease [products]Right | ||
Decrease in pressure, the system will try and increase the pressure by increasing the number of moles of gas present, hence, equilibrium shifts to the side with more moles of gas | |||
Temperature | Increase in temperature, the system will try to decrease the temperature by absorbing heat, hence, equilibrium will shift in the endothermic direction | ||
Decrease in temperature, the system will try to increase the temperature by producing heat, hence, equilibrium will shift in the exothermic direction | |||
Catalyst | Adding a catalyst lowers the activation energy of both the forward and reverse reaction by the same magnitude, hence, catalysts do not affect Kc nor the position of equilibrium |
Summary
Condition | Forward vs Backward | Position of Equilibrium | Kc |
“System counteracts ↑ in…” | “by favouring…” | “As a result, P.O.E. shift to” | “with …. in Kc value” |
Concentration [reactant] | Forward rxn to ↓ [reactant] | Product side | No change |
Pressure | direction to produce less no. of moles(g) to ↓ pressure | Side with less no. of moles | No change |
Temperature | Endothermic rxn to ↓ temp. | Reactant side (-ve ∆H) / Product side (+ve ∆H) | ↓ (-ve ∆H) / ↑ (+ve ∆H) |
Catalyst | Both forward and backward | No change | No change |
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Temperature is the only condition that can change the value of Kc and the position of equilibrium permanently
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N.B.// pressure and [conc.] changes P.O.E. which, in turn, changes the Kc value but only temporarily before reverting back to what it was before because the system successfully restores the original balance (in contrast to temp. ↑↓ establishing a new balance)