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Reaction Quotient Calculator

Enter the product and reactant terms and K to get Q and the direction.

How it is calculated

You enter two numbers that you have already worked out from the balanced equation: the product term (the product concentrations, each raised to its coefficient and multiplied together) and the reactant term (the same for the reactants). The tool then divides: Q = product term / reactant term, shown with four decimals. For aA + bB ⇌ cC + dD the terms are [C]c[D]d and [A]a[B]b.

If you also enter an equilibrium constant K above zero, the tool compares: Q < K means the reaction proceeds forward, Q > K means it proceeds in reverse, and Q exactly equal to K means it is at equilibrium. Without K, no direction is shown. The reactant term must be above zero and the product term cannot be negative. The tool does not raise concentrations to powers itself, and Q and K must be written in the same form and at the same temperature.

Worked example

Suppose the product term is 0.40 and the reactant term is 0.25, and the reaction has K = 4.5 at the working temperature. Q = 0.40 / 0.25 = 1.6000. Since 1.6 is below 4.5, there is too little product compared with equilibrium, so the reaction proceeds forward. With a product term of 1.5, Q would be 6.0000, above K, and the direction would be reverse.

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Common mistakes

  • Typing plain concentrations and ignoring coefficients. For 2NO₂ ⇌ N₂O₄ with [NO₂] = 0.20, the reactant term is 0.20², which is 0.04, not 0.20, and the Q would be off by a factor of five.
  • Putting reactants in the product box. Swapping them gives 1/Q, which flips the predicted direction.
  • Including pure solids and liquids in the terms. Their activity is taken as 1, so they are left out of both Q and K.
  • Comparing Q with a K measured at another temperature. K changes with temperature, so the comparison is only valid at the K’s own temperature.
  • Reading the direction as a speed. It says which way the mixture shifts, not how fast the reaction runs.

About this calculator

The reaction quotient Q has the same algebraic form as the equilibrium constant K — products' concentrations raised to their stoichiometric coefficients, divided by reactants' concentrations raised to theirs — but Q can be calculated at any point in a reaction, not just at equilibrium. Comparing Q to K tells you which direction the reaction will shift to reach equilibrium: Q < K means it proceeds forward (more product forms), Q > K means it shifts backward (reverts toward reactants), and Q = K means the system is already at equilibrium.

Chemistry students and chemical engineers use this comparison to predict reaction direction before running an experiment or designing a reactor, and it's the same logic behind Le Chatelier's principle — adding or removing a reactant or product changes Q relative to the fixed K, telling you exactly which way the system will respond.

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Comments (1)

  • Editorial team

    Before typing the two terms, decide which species belong in them. Pure solids and pure liquids are left out of Q and K, so if one side then has nothing left, its term is 1. For gases, check whether your K is written with concentrations or partial pressures and use the same form for Q. The direction only appears when you also enter K. Which reaction are you working on, and is K given at your temperature?

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