1.6 Chemical equilibria, Kc
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When is a reaction at equilibrium?
• When the reactants are proportional to the products. • When the reaction is a reversible reaction. • Equilibrium can only be reached in a closed system. • Forwards reaction and backwards reaction happen at the same rate. • At equilibrium, the concentration of reactants and products stay constant.
What is La Chatelier’s principle?
If a system of equilibrium is disturbed, it shifts on the direction that reduces the disturbance.
What happens if you increase the temperature of a homogenous reaction?
The equilibrium will shift in favour of the endothermic reaction to oppose the change.
What happens if you decrease the temperature of a homogenous reaction?
The equilibrium will shift in favour of the exothermic reaction to oppose the change.
What happens if you decrease the pressure of a homogenous reaction?
The equilibrium will shift in favour of the reaction that makes more moles.
What happens if you increase the pressure of a homogenous reaction?
The equilibrium will shift in favour of the reaction that makes less moles.
What happens if you change the pressure of a homogeneous system, where equal amounts of moles are made and used up?
The equilibrium doesn’t shift at all.
What happens if you increase the concentration of reactants in a homogenous reaction?
Equilibrium will shift in favour of the forwards reaction, making more products.
What happens if you increase the concentration of products in a homogenous reaction?
Equilibrium will shift in favour of the backwards reaction, making more reactants.
What does the equilibrium constant (Kc) do?
Gives a value for the position of equilibrium, and how it changes with concentration.
What is the quantitive representation of Kc?
The bigger the Kc value, the closer it is to the products (favours the forwards reaction).
How do you formulate a Kc expression? eg equ: aA + bB ⇌ cC
[A]ª x [B]ᵇ
What are the units of concentration used in Kc?
moldm¯³
How do you find the Kc of a reaction? (step by step)
• Find initial moles • Find moles at equilibrium. • Find concentration at equilibrium. • Put into the Kc equation and find the correct units.
How does temperature affect Kc?
• Kc is bigger when reaction shifts right. - So depending on wether the forward reaction is exo/endo will show the effect of temperature.
How do catalysts or concentration affect Kc?
No effect on the Kc value, but concentration changes will shift the equilibrium.