What does the active mass depend on?

The active mass primarily depends on the density and molecular mass of a substance.

The concept of active mass is crucial in understanding chemical kinetics and equilibrium. It essentially represents the effective concentration of a reactant or product in a chemical reaction. This effective concentration isn't just the number of moles per liter; it's influenced by factors that impact the substance's ability to participate in the reaction.

Here's a more detailed breakdown:

• For Gases: Active mass is directly proportional to the partial pressure. A higher partial pressure indicates a greater concentration of the gas molecules, leading to a higher active mass and a potentially faster reaction rate.

• For Solutions: Active mass is proportional to the molar concentration (moles per liter). A higher concentration means more molecules are available to react, increasing the active mass.

• For Pure Solids and Liquids: The active mass is usually taken as unity (1). This is because their concentrations remain practically constant throughout the reaction. As the reference indicates, the density and molecular mass of pure solids and liquids are generally constant. Even if the amount of the solid or liquid changes, the concentration (mass per unit volume) remains nearly the same. Therefore, their contribution to the reaction rate is considered constant and doesn't significantly change the equilibrium. In effect, the active mass is already incorporated into the rate constant of the reaction.

Here's a table summarizing the active mass dependence for different phases:

In summary, the active mass represents the effective concentration of a substance and depends on its physical state and concentration. While it directly correlates with partial pressure for gases and molarity for solutions, it is considered constant (unity) for pure solids and liquids because their densities and molar masses typically remain unchanged during a reaction.