3.1.6 Chemical equilibria, Le Chatelier’s principle and K_c

Define a Reversible Reaction

A reaction where the products can react together to reform the reactants.

Define Dynamic Equilibrium

A state in a closed system where the rates of the forward and backward reactions are equal, and the concentrations of reactants and products remain constant.

What are the two necessary conditions for a system to reach equilibrium

The reaction must be reversible and it must occur in a closed system (where no reactants or products can escape).

State Le Chatelier’s Principle

If a system at equilibrium is disturbed by a change in conditions, the position of equilibrium shifts to counteract or oppose the change.

How does increasing the concentration of a reactant affect equilibrium

The equilibrium shifts to the right (towards the products) to remove the added reactant.

How does increasing the pressure affect a gaseous equilibrium

The equilibrium shifts to the side with the fewest moles of gas to reduce the pressure.

How does decreasing the pressure affect a gaseous equilibrium

The equilibrium shifts to the side with the most moles of gas to increase the pressure.

How does increasing the temperature affect equilibrium

The equilibrium shifts in the endothermic direction (\Delta H is positive) to absorb the added heat.

How does decreasing the temperature affect equilibrium

The equilibrium shifts in the exothermic direction (\Delta H is negative) to release heat and oppose the change.

What effect does a catalyst have on the position of equilibrium

It has no effect on the position of equilibrium or the yield of products.

Why is a catalyst used in industrial equilibrium reactions

It increases the rate at which equilibrium is reached by speeding up both the forward and backward reactions equally.

Define the Equilibrium Constant (K_c)

A value that expresses the relationship between the concentrations of products and reactants at equilibrium for a specific temperature.

Write the general expression for K_c for the reaction aA + bB \rightleftharpoons cC + dD

K_c = \frac{[C]^c [D]^d}{[A]^a [B]^b}.

What is the only factor that changes the value of Kc (and Kp)

Temperature.

How do you determine the units for K_c

By substituting the units of concentration (mol dm⁻³) into the K_c expression and cancelling them out.

If a reaction is exothermic in the forward direction, how does increasing temperature affect K_c

K_c will decrease because the equilibrium shifts to the left, decreasing the concentration of products.

If a reaction is endothermic in the forward direction, how does increasing temperature affect K_c

K_c will increase because the equilibrium shifts to the right, increasing the concentration of products.

What does a large value of K_c (>1) indicate about the equilibrium position

The equilibrium lies to the right, meaning there are more products than reactants at equilibrium.

What does a small value of K_c (<1) indicate about the equilibrium position

The equilibrium lies to the left, meaning there are more reactants than products at equilibrium.

Why does changing concentration or pressure not affect the value of K_c

Because the system adjusts the concentrations of all species to maintain the same ratio defined by K_c at that specific temperature.