Reaction Quotient (Q)

The  Reaction Quotient calculator computes the reaction quotient based on the concentration of two substances and two exponential coefficients.

INSTRUCTIONS: Choose units and enter the following:

• [P] Concentration of product P
• (y) Coefficient of product P
• [R] Concentration of reactant R
• (x) Coefficient of reactant R

Reaction Quotient (Q):  The calculator returns the quotient as a real number.

The Math / Science

The Reaction Quotient ^[1] formula, Q = [P]^y / [R]^x, is used to calculate the reaction quotient when a reaction is at any state other than equilibrium. Calculating reaction quotients can help determine the composition of a system as well as determine which way the reaction will proceed. A typical equation, xR = yP, can be used to represent a chemical reaction formula because [products] are always in the numerator, with [reactants] in the denominator. Therefore, the product "P" belongs in the numerator with reactant "R" in the denominator. Each variable is raised to its respective equation coefficient, y and x. The input variables are coefficients x and y (unit-less) and the concentrations of [P] and [R] in mols/L. 

The formula for the Reaction Quotient is:

                  Q = [P]^y / [R]^x

where:

• Q is the Reaction Quotient
• [P] is the concentration of product P
• [R] is the concentration of reactant R
• y is the coefficient of product P
• x is the coefficient of reactant R

Setting Q against K can provide useful information about which direction a reaction will proceed. It gives five possibilities:

• Q = K      The system is at equilibrium and does not shift left or right
• Q = 0       The system will shift forward/to the right (towards products)
• Q < K       There are more reactants than products, thus the system will shift to the right (towards products)
• Q > K      There are more products than reactants, thus the system will shift to the left (towards reactants)
• Q = infinity (Q >>> K) The system will shift left (towards reactants)

Example:

aA + bB = dD + eE

Q= [products] / [reactants]

Q = [E]^e [D]^d / [B]^b [A]^a

References

[1] For the Spanish site click here

Whitten, et al. 10th Edition. Pp. 674