Why is active mass of solid is taken as 1?
Active mass is defined as the molar concentration ie. number of Gram-moles per litre.
Molar concentration is directly proportional to density. Since density of solid or liquid always remains constant, the active mass is taken as 1 .
For detailed answer,
The density of a solid or liquid reactant doesn't change over the course of a reaction. The mass and volume do as it is consumed, but the ratio of the two is constant. If the reaction causes a temperature change then there are small changes in density, but that would also alter the equilibrium constant and the concentrations of the species involved. The equilibrium constant is based on activities, which you can think of as the ratio of the concentration (molar density) at standard state to the concentration in the reaction. Since solid/liquid density is constant at a given temperature, activity is too. It takes quite a large change of temperature or pressure to make a substantial change in the density of a substance, assuming no phase changes occur.
But The same cannot be said for gases as gases donot have a fixed volume. So for example even if we start with 10L of O2 for combustion of carbon in a closed vessel. The number of moles of O2 would decrease as the reaction proceeds but the volume of O2 would still be 10L. Hence the molar density of O2 would decrease and so would the activity.
for eg:If you take 5litres of HCL and 50ml of HCL. Their molar density will be same. Because both are same. And active mass is defined as molar density therefore always unity.
Molar concentration is directly proportional to density. Since density of solid or liquid always remains constant, the active mass is taken as 1 .
For detailed answer,
The density of a solid or liquid reactant doesn't change over the course of a reaction. The mass and volume do as it is consumed, but the ratio of the two is constant. If the reaction causes a temperature change then there are small changes in density, but that would also alter the equilibrium constant and the concentrations of the species involved.
The equilibrium constant is based on activities, which you can think of as the ratio of the concentration (molar density) at standard state to the concentration in the reaction. Since solid/liquid density is constant at a given temperature, activity is too. It takes quite a large change of temperature or pressure to make a substantial change in the density of a substance, assuming no phase changes occur.
But The same cannot be said for gases as gases donot have a fixed volume. So for example even if we start with 10L of O2 for combustion of carbon in a closed vessel. The number of moles of O2 would decrease as the reaction proceeds but the volume of O2 would still be 10L. Hence the molar density of O2 would decrease and so would the activity.
