When we talk about pure solids or pure liquids, we refer to substances that are homogeneous in nature. Homogeneous substances have a uniform composition throughout and do not have any impurities. In other words, they are chemical species made up of a single component, which we can consider as the solvent. For example, when we talk about pure water, we refer to a substance that is made up of only water and has no other impurities. Similarly, pure iron would refer to a substance made solely of iron without any other metals or elements.
Why is active mass of a pure solid or liquid always taken as unity?
The active mass of a substance is a measure of its concentration in a system. In the case of pure solids and pure liquids, their concentration remains constant throughout the reaction, and thus their active mass is taken as unity (i.e., 1). Why? Because we cannot change the amount of these substances without physically adding or removing them from the reaction. For example, we cannot add more pure water to the water already present in the reaction to increase its concentration. Thus, we take their activity as unity to calculate changes in the concentration of other species present in the reaction.
Example
Let’s consider the reaction between calcium carbonate (CaCO3) and hydrochloric acid (HCl):
CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
In this reaction, CaCO3 is the pure solid and H2O is the pure liquid. We take their activity as unity because they do not change throughout the reaction. To calculate the changes in concentration of other species, we must use their activity coefficients, which depend upon their concentrations and the temperature of the reaction.
Reference states for activities
When referring to activities in equilibrium expressions, we use reference states to standardize them. The reference state defines the activity of a substance as 1, which simplifies the expression and makes it easier to compare with experimental data. The activity coefficient is then expressed relative to the reference state. For pure solids and pure liquids, their activity is taken as unity in the reference state.
Example
Let’s consider the equilibrium expression for the reaction between nitric oxide (NO) and nitrogen dioxide (NO2):
2NO(g) + O2(g) ⇄ 2NO2(g)
The equilibrium constant expression for this reaction is
Kc = [NO2]2 / [NO]2[O2]
We use activity coefficients for the gases in the reaction and take the activity of the pure solid nitrogen dioxide and the pure liquid water as unity in the reference state. This simplifies the expression to
This equilibrium constant expression reflects the concentrations of the gases relative to the activity of the pure solids and pure liquids in the reaction.
Conclusion
Pure solids and pure liquids are homogeneous chemical species made up of only one component. Their activity is taken as unity in the reference state because they do not change throughout the reaction. This simplifies the expression for equilibrium constants and helps us calculate changes in concentration of other species. Understanding the concept of pure solids and pure liquids is essential in the study of equilibrium reactions and their applications in chemistry.
What is Meant By Pure Solids Or Pure Liquids?
When we talk about pure solids or pure liquids, we refer to substances that are homogeneous in nature. Homogeneous substances have a uniform composition throughout and do not have any impurities. In other words, they are chemical species made up of a single component, which we can consider as the solvent. For example, when we talk about pure water, we refer to a substance that is made up of only water and has no other impurities. Similarly, pure iron would refer to a substance made solely of iron without any other metals or elements.
Why is active mass of a pure solid or liquid always taken as unity?
The active mass of a substance is a measure of its concentration in a system. In the case of pure solids and pure liquids, their concentration remains constant throughout the reaction, and thus their active mass is taken as unity (i.e., 1). Why? Because we cannot change the amount of these substances without physically adding or removing them from the reaction. For example, we cannot add more pure water to the water already present in the reaction to increase its concentration. Thus, we take their activity as unity to calculate changes in the concentration of other species present in the reaction.
Example
Let’s consider the reaction between calcium carbonate (CaCO3) and hydrochloric acid (HCl):
In this reaction, CaCO3 is the pure solid and H2O is the pure liquid. We take their activity as unity because they do not change throughout the reaction. To calculate the changes in concentration of other species, we must use their activity coefficients, which depend upon their concentrations and the temperature of the reaction.
Reference states for activities
When referring to activities in equilibrium expressions, we use reference states to standardize them. The reference state defines the activity of a substance as 1, which simplifies the expression and makes it easier to compare with experimental data. The activity coefficient is then expressed relative to the reference state. For pure solids and pure liquids, their activity is taken as unity in the reference state.
Example
Let’s consider the equilibrium expression for the reaction between nitric oxide (NO) and nitrogen dioxide (NO2):
The equilibrium constant expression for this reaction is
We use activity coefficients for the gases in the reaction and take the activity of the pure solid nitrogen dioxide and the pure liquid water as unity in the reference state. This simplifies the expression to
This equilibrium constant expression reflects the concentrations of the gases relative to the activity of the pure solids and pure liquids in the reaction.
Conclusion
Pure solids and pure liquids are homogeneous chemical species made up of only one component. Their activity is taken as unity in the reference state because they do not change throughout the reaction. This simplifies the expression for equilibrium constants and helps us calculate changes in concentration of other species. Understanding the concept of pure solids and pure liquids is essential in the study of equilibrium reactions and their applications in chemistry.