Constants in Q Equation

[En-Ya Zhang 3K]

How can we determine when a term in the equation for Q would be equal to the value of 1?

[Esmeralda Polanco]

Hi, this is the guide I use to determine when a term in the equation for the reaction quotient (Q) would be equal to 1:

For gaseous reactions or reactions in aqueous solutions:
If the concentration of a product is equal to the concentration of the corresponding reactant, the term for that species in the reaction quotient equation is 1.

For heterogeneous reactions involving solids or pure liquids:
Terms involving concentrations of solids or pure liquids are treated as constants and are equal to 1.

At equilibrium:
All terms in the reaction quotient equation (except for solids and pure liquids) are equal to their equilibrium concentrations or pressures, resulting in Q = K (the equilibrium constant).

During the reaction:
If the reaction is not at equilibrium, terms in the reaction quotient equation differ from 1, indicating that the reaction has not yet reached equilibrium.

[Soobin Lim 2B]

A term in the equation for the reaction quotient Q will equal 1 when:
gases: The partial pressure of that gas is 1 atm
solids/liquids: The activity of that substance is 1
solutions (aqueous): The molar concentration of that species is 1 M

So any pure solids, pure liquids, or solutions at 1 M concentration will have their term in Q equal to 1. Gases will have their term = 1 if their partial pressure is 1 atm.