What is Qc?

What is Qc?

• A. The rate constant at equilibrium
• B. The value obtained by inserting concentrations into the Kc expression
• C. The maximum possible concentration
• D. The sum of product and reactant concentrations

Answer: (B)

Qc is the reaction quotient, the value you get when you plug the current, instantaneous concentrations into the same expression used for the equilibrium constant Kc. For a generic reaction aA + bB ⇌ cC + dD, Qc = [C]^c [D]^d / [A]^a [B]^b. This tells you where the system sits relative to equilibrium and helps predict which direction the reaction will shift to reach equilibrium by comparing Qc to Kc. It’s not the rate constant at equilibrium (that’s Kc), nor a maximum possible concentration, nor a simple sum of concentrations.

Qc is the reaction quotient, the value you get when you plug the current, instantaneous concentrations into the same expression used for the equilibrium constant Kc. For a generic reaction aA + bB ⇌ cC + dD, Qc = [C]^c [D]^d / [A]^a [B]^b. This tells you where the system sits relative to equilibrium and helps predict which direction the reaction will shift to reach equilibrium by comparing Qc to Kc. It’s not the rate constant at equilibrium (that’s Kc), nor a maximum possible concentration, nor a simple sum of concentrations.