// CHEMISTRY L5 — EQUILIBRIUM CONSTANTS
Put a number on the balance point.
Le Chatelier tells you which way equilibrium shifts; Kc tells you exactly how far. Set the equilibrium concentrations for the hydrogen–iodine reaction and compute its constant.
// FIG. 01 — THE Kc CALCULATOR · H₂(g) + I₂(g) ⇌ 2HI(g)
Kc = [HI]² ÷ ( [H₂] × [I₂] )
Kc = {{ hi }}² ÷ ({{ h2 }} × {{ i2 }}) = {{ numer }} ÷ {{ denom }}
Kc =
{{ kc }}
{{ interpretation }}
Here HI has no units in Kc because the concentration powers cancel (2 on top, 1+1 on the bottom). For this reaction Kc is dimensionless; for others it carries units you work out from the expression.
// WRITING Kc
For a general equilibrium aA + bB ⇌ cC + dD, the equilibrium constant is Kc = [C]ᶜ[D]ᵈ ÷ [A]ᵃ[B]ᵇ — products on top, reactants on the bottom, each raised to the power of its balancing number. You only ever put in the concentrations at equilibrium. Pure solids and liquids are left out entirely (their concentration is effectively constant).
// WHAT THE VALUE TELLS YOU
// Kp — FOR GASES, AND WHAT CHANGES Kc
For gas reactions we often use Kp, built the same way but from partial pressures instead of concentrations. The big idea to hold onto: for a given reaction, K only changes with temperature. Changing concentration or pressure, or adding a catalyst, shifts the position of equilibrium but the system always re-adjusts back to the same K. Only heating or cooling gives a genuinely different value — which is Le Chatelier and Kc telling the same story in two languages.
// REFERENCES & FURTHER READING
Cross-check against your own exam board's specification (AQA, Edexcel, OCR, SQA).