Solubility of a solid equilibria

In summary, it is explained that solids, pure liquids and solvents are not taken into account when equilibrium is described, and that concentrations of these substances are almost constant.
  • #1
EdTheHead
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I'm having some trouble understanding heterogenous equilibria. I read that solids, pure liquids and solvents are omitted from the equilibrium expression. Let's say I have some AgCl in a litre of water. The tiny amount of AgCl that dissociates and dissolved is given by its Ksp. I understand that once a sufficient amount of solid AgCl is present it will give a constant amount of dissociated ions regardless of how much more AgCl is added. What I don't understand is how the solvent (water in this case) is omitted. If I double the volume of water wouldn't I be doubling the amount of dissolved Ag+ and Cl- ions because I would be doubling the amount of solvent for them to dissolve in?
 
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  • #2
In AgCl solution solvent is not taking place in the equilibrium - whatever happens is described by Ksp. However, system is described not only by Ksp, but also by mass balances. Amount of AgCl put into the system is constant (that's just mass conservation). That means if you start with 1L of saturated solution in equilibrium with some small amount of solid AgCl, and you add more water, it may happen that all solid dissolves.

In some other reactions - like hydrolysis (A- + H2O <-> HA + OH-) water is one of the reactants - that means it is part of the equilibrium. In general equilibrium constant is given by

[tex]K = \frac {[HA][OH^-]} {[A^-][H_2O]}[/tex]

however, concentration of water is (almost) constant - it is not changing, as there is a huge excess of water. That means we can multiply both sides by water concentration (something like 55.56 M) and write

[tex]K' = \frac {[HA][OH^-]} {[A^-]}[/tex]

where K' = 55.56K

We can also assume water activity equals 1 - as it doesn't change, it doesn't matter what the value will be. It works for not too concentrated solutions, sometimes it may lead to troubles. But these are rare situations.

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  • #3
That explains everything. Thanks! This forum should really have a thanks button + reputation system so people can at least give reputation in return for help.
 

Related to Solubility of a solid equilibria

1. What is the definition of solubility?

Solubility refers to the ability of a substance, known as the solute, to dissolve in another substance, called the solvent, to form a homogenous mixture.

2. How do temperature and pressure affect the solubility of a solid?

In general, the solubility of a solid increases with an increase in temperature. This is because higher temperatures provide more kinetic energy for the solute molecules to break apart and mix with the solvent. Pressure, on the other hand, has little to no effect on the solubility of solids.

3. What is the difference between solubility and solubility equilibrium?

Solubility refers to the maximum amount of solute that can dissolve in a given amount of solvent. Solubility equilibrium, on the other hand, refers to the state in which the rate of dissolution of the solute is equal to the rate of precipitation, resulting in a constant concentration of the solute in the solution.

4. How is the solubility of a solid equilibria determined?

The solubility of a solid equilibria can be determined experimentally by measuring the concentration of the solute in a solution at different temperatures and using this data to plot a solubility curve. The point at which the curve intersects with the saturation point represents the solubility of the solid at that temperature.

5. How can the solubility of a solid be increased?

The solubility of a solid can be increased by increasing the temperature of the solution, using a solvent with a higher polarity, or by increasing the surface area of the solid through crushing or grinding. Additionally, the use of solubility enhancers, such as surfactants, can also increase the solubility of a solid.

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