Reaction Quotient Calculator
Calculate reaction quotient with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.
Reviewed by Manoj Kumar, Mathematics Educator
Formula
Q = [C]^c [D]^d / [A]^a [B]^b
The reaction quotient Q uses the same expression as Keq but with current (non-equilibrium) concentrations. If Q < Keq, the reaction shifts forward. If Q > Keq, it shifts in reverse. If Q = Keq, the system is at equilibrium.
Worked Examples
Example 1: Comparing Q to Keq
Problem:For N2 + 3H2 -> 2NH3, Keq = 0.500. Current: [N2] = 0.200, [H2] = 0.100, [NH3] = 0.050. Find Q and direction.
Solution:Q = [NH3]^2 / ([N2] * [H2]^3)\nQ = (0.050)^2 / (0.200 * (0.100)^3)\nQ = 0.0025 / 0.0002 = 12.5\nQ (12.5) > Keq (0.500) => Reverse shift
Result:Q = 12.5, Reaction shifts in reverse
Example 2: System at Equilibrium Check
Problem:For H2 + I2 -> 2HI, Keq = 54.3. [H2] = 0.01, [I2] = 0.01, [HI] = 0.0737.
Solution:Q = [HI]^2 / ([H2]*[I2])\nQ = (0.0737)^2 / (0.01 * 0.01)\nQ = 0.005432 / 0.0001 = 54.3\nQ = Keq => At equilibrium
Result:Q = 54.3 = Keq, System is at equilibrium
Frequently Asked Questions
What is the reaction quotient Q?
The reaction quotient Q is a measure of the relative amounts of products and reactants present in a reaction mixture at any point in time. It has the same mathematical form as the equilibrium constant Keq, but uses current concentrations rather than equilibrium concentrations. For the reaction aA + bB -> cC + dD, Q = [C]^c[D]^d / [A]^a[B]^b. Comparing Q to Keq predicts which direction the reaction will shift to reach equilibrium. Q is dimensionless when activities are used but may have units when concentrations are used directly.
How do Q and Keq determine reaction direction?
When Q < Keq, there are relatively too many reactants and too few products compared to equilibrium, so the reaction proceeds forward (toward products). When Q > Keq, there are too many products, so the reaction shifts in reverse (toward reactants). When Q = Keq, the system is at equilibrium and no net reaction occurs. The farther Q is from Keq, the greater the driving force for the reaction to shift. This principle is known as Le Chatelier principle applied quantitatively to concentration changes.
References
Reviewed by Manoj Kumar, Mathematics Educator · Editorial policy