What this calculator does
The reaction quotient has the same form as the equilibrium constant but uses current concentrations rather than equilibrium ones. Comparing Q with K tells you which direction the reaction must move.
Three cases cover it. If Q is less than K there is too much reactant and the reaction proceeds forward. If Q exceeds K there is too much product and it runs backward. If they are equal the system is already at equilibrium and nothing net happens.
The formula
Product concentrations are raised to their stoichiometric coefficients and multiplied, then divided by the same construction for the reactants. The coefficients come from the balanced equation and act as exponents, which is why they matter far more than their size suggests: a coefficient of 3 cubes that concentration.
| Term | Meaning |
|---|---|
| Q | The reaction quotient, using current concentrations. |
| K | The equilibrium constant, the value Q takes at equilibrium. |
| Le Chatelier’s principle | The qualitative version of the same comparison. |
| Stoichiometric coefficient | The number in the balanced equation, used here as an exponent. |
The inputs explained
| Field | What to enter |
|---|---|
| [A] concentration (M) | [A] concentration. |
| a (coeff. of A) | Coefficient of A from the balanced equation, used as an exponent. |
| [B] concentration (M) | [B] concentration. |
| b (coeff. of B) | Coefficient of B. |
| [C] concentration (M) | [C] concentration. |
| c (coeff. of C) | Coefficient of C. |
| [D] concentration (M) | [D] concentration. |
| d (coeff. of D) | Coefficient of D. |
When to use it
Predicting reaction direction
Comparing Q against a known K says immediately which way the system will move.
Checking whether equilibrium is reached
Q equal to K means the system has arrived.
Assessing a disturbance
Adding a reactant changes Q without changing K, and the comparison shows the response.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much do coefficients change Q?
The same concentrations with different coefficients on A.
| Coefficient of A | Reaction quotient Q | Numerator ([C]^c[D]^d) | Denominator ([A]^a[B]^b) |
|---|---|---|---|
| a = 1 | 0.5 | 1 | 2 |
| a = 2 | 0.25 | 1 | 4 |
| a = 3 | 0.125 | 1 | 8 |
Questions
What does it mean if Q is less than K?
There is too much reactant relative to equilibrium, so the reaction proceeds forward, consuming reactants and forming products until Q rises to meet K. The larger the gap, the further the system is from equilibrium.
What is the difference between Q and K?
They have the identical mathematical form. K uses equilibrium concentrations and is a fixed value at a given temperature; Q uses whatever the concentrations are right now and changes as the reaction proceeds. Q equals K precisely at equilibrium.
Do solids and liquids appear in Q?
No. Pure solids and pure liquids are omitted, because their activities are defined as 1 and do not change with amount. Only gases and dissolved species appear. Including a solid is a common error that makes Q depend on how much of it is present.
How does this relate to Le Chatelier’s principle?
It is the quantitative version. Le Chatelier says a system responds to a disturbance by shifting to counteract it; comparing Q with K says precisely which direction and how far from equilibrium the system currently sits.
For converting between Kp and Kc, see the Kp and Kc conversion calculator. For partial pressures in a gas equilibrium, see the partial pressure calculator.