Applications of Equilibrium Constants
Using the equilibrium constant to predict the extent and direction of a reaction (reaction quotient, Q).
What is Applications of Equilibrium Constants?
A measure of the relative amounts of products and reactants present in a reaction at any given point in time, calculated using the same formula as the equilibrium constant but with non-equilibrium concentrations or partial pressures.
Key formula / rule: Reaction Quotient (Concentration)
Key points
- Define and calculate the reaction quotient (Q).
- Compare Q with K to predict the direction of a reaction.
- Interpret the magnitude of K to determine the extent of a reaction.
- Relate Kc and Kp for gaseous reactions.
Common exam trap
Confusing Kc and Kp, especially when gases are involved.
Definitions
- Term
Reaction Quotient (Q)
- Meaning
A measure of the relative amounts of products and reactants present in a reaction at any given point in time, calculated using the same formula as the equilibrium constant but with non-equilibrium concentrations or partial pressures.
- Term
Equilibrium Constant (K)
- Meaning
A ratio of the product concentrations (or partial pressures) to reactant concentrations (or partial pressures) at equilibrium, each raised to the power of their stoichiometric coefficient. It indicates the extent to which a reaction proceeds towards products at a given temperature.
Learning objectives
Define and calculate the reaction quotient (Q).
Compare Q with K to predict the direction of a reaction.
Interpret the magnitude of K to determine the extent of a reaction.
Relate Kc and Kp for gaseous reactions.
Formulae
- Name
Reaction Quotient (Concentration)
- Note
Where [X]t represents the concentration of species X at time t.
- Expression
Qc = ([C]t^c [D]t^d) / ([A]t^a [B]t^b)
- Name
Reaction Quotient (Pressure)
- Note
Where PX_t represents the partial pressure of species X at time t.
- Expression
Qp = (PC_tc PD_td) / (PA_ta PB_tb)
- Name
Equilibrium Constant (Concentration)
- Note
Where [X]eq represents the concentration of species X at equilibrium.
- Expression
Kc = ([C]eq^c [D]eq^d) / ([A]eq^a [B]eq^b)
- Name
Equilibrium Constant (Pressure)
- Note
Where PX_eq represents the partial pressure of species X at equilibrium.
- Expression
Kp = (PC_eqc PD_eqd) / (PA_eqa PB_eqb)
Prerequisites
Understanding of reversible reactions.
Concept of chemical equilibrium.
Calculation of equilibrium concentrations/partial pressures.
Stoichiometry.
Common mistakes
Confusing Kc and Kp, especially when gases are involved.
Incorrectly calculating Q using equilibrium concentrations instead of current concentrations.
Forgetting to raise concentrations/pressures to their stoichiometric coefficients when calculating K or Q.
Assuming K is independent of temperature.
Keywords
Equilibrium Constant
Reaction Quotient
Kc
Kp
Chemical Equilibrium
Reversible Reaction
Forward Reaction
Reverse Reaction
Extent of Reaction
Direction of Reaction
Practice preview
For the reaction: A + B ⇌ C + D. If at equilibrium, [A] = 0.5 M, [B] = 0.5 M, [C] = 0.5 M, and [D] = 0.5 M, what is the value of the equilibrium constant (Kc)?…
easy
Consider the reaction: N2(g) + 3H2(g) ⇌ 2NH3(g). If the reaction quotient (Qc) is greater than the equilibrium constant (Kc), which of the following is true?…
medium
For the reaction PCl5(g) ⇌ PCl3(g) + Cl2(g), Kc = 0.04 at a certain temperature. If initially, we have 2 moles of PCl5 in a 1-liter flask, what will be the direction of the reaction at the start?…
medium