Key Concepts: Dissolution Equilibrium for Solids and Gases
Everything in the universe attempts to achieve stability by lowering its energy. Various chemical and physical processes help to attain this state. All the reactions continue until they reach a point of specific minimum energy that is in accordance with their surroundings. At this stage, the system is at a standstill with no visible change, and it continues to be in the same state unless disturbed. This state is known as equilibrium. In the equilibrium state, the 'rate of forward reaction' equals 'the rate of backward reaction'.
The Solubility of Gases and Solids in Liquids
A substance's solubility refers to the maximum amount of substance that can be dissolved in any given solvent in a specific quantity. The solubility of a substance in a solvent depends upon:
Nature of solute.
Nature of the solvent.
Temperature.
Pressure.
The Solubility of Solids in Liquids
The solubility of solute (solid) in liquids differs with temperature, nature of liquid and solid, and to a lesser degree on the system's pressure.
When a solute is added to a solvent, solid particles dissolve in the solvent, thus increasing its concentration in the solution. This process is called dissolution.
Some solid particles collide with other solid particulates in the solution and are precipitated out. This process is known as crystallization.
The dissolution process continues till the solution achieves a maximum concentration level, beyond which the dissolution of the solute stops. The solution where no excess solute particles can dissolve at the same pressure and temperature is known as a saturated solution.
Equilibrium Involving Dissolution of Solids
Once a solution reaches the saturated stage, a type of equilibrium gets established. The equilibrium reached is between the process of crystallization and dissolution. At this stage, the number of solid particles entering the solution equals the solid particles separating out. This is the state of dynamic equilibrium. In a saturated solution, if the temperature and pressure remain constant, then the concentration of solute particles in the solution will stay the same.
Sugar (solution) = Sugar (solid)
Rate of dissolution of sugar = Rate of crystallization of sugar.
The maximum quantity of solute particles that can dissolve in a solvent (solution) at a specific temperature is known as its solubility. The solubility of solid (solute) in liquids depends on the following factors:
Nature of solvent and solute.
Effect of temperature.
The Solubility of Gases in Liquids
Gases dissolve in liquid to form a homogeneous solution. The solubility of the gas in a liquid (solvent) depends on:
Nature of the solute (gas).
Nature of the solvents.
The temperature of the solution.
Pressure.
Gases like oxygen, helium, hydrogen, nitrogen, etc., dissolve in lesser quantity, whereas gases such as ammonia and hydrogen chloride are highly soluble. The solubility of gases in the liquid is influenced by:
Effect of Temperature
With an increase in temperature, the solubility of gas decreases.
Effect of Pressure
With an increase in pressure, the solubility of gas increases.
Henry’s Law
Henry's law can derive the relation by which the 'quantity of gas' gets dissolved in the liquid. According to the law, the mass of a gas dissolved in the given amount of solvent at any temperature is proportional to the gas's pressure above the solvent. With a rise in temperature, the solubility of gases decreases.
(mass of gas) α (Pressure of gas)
m α p
mm = kHp
(image will be uploaded soon)
Here kH is called proportionality constant. It is also named Henry law constant and is dependent on the temperature. One such example of it is a cold drink bottle. The soft drink bottle is sealed at pressure (of the gas) that is higher than the atmospheric pressure. Here, the solubility of the gas in the solvent is high. When the bottle is opened, it gets exposed to the surrounding atmosphere (it has low pressure than a soft drink). As a result of opening the bottle, some amount of gas CO2 escapes the bottle. This happens to establish a dynamic equilibrium as per the low pressure existing in the atmosphere. If the soft drink is further exposed to the atmosphere, all the dissolved CO2 will escape due to low pressure and the corresponding high temperature. Thus, any cold drink kept at room temperature will lose more gas than the drink taken out of the refrigerator.
Did You Know?
Consuming soft drinks causes belching as your stomach stretches due to the accumulation of CO2. As a result of belching, you may feel heartburn and a sour taste.
FAQs on Equilibrium Involving Dissolution of Solids or Gases Explained
1. What is meant by equilibrium in the dissolution of a solid in a liquid?
Equilibrium in the dissolution of a solid in a liquid is achieved when a solution becomes saturated. At this point, a dynamic equilibrium is established where the rate at which the solid particles dissolve into the solution is exactly equal to the rate at which the dissolved particles crystallise back into the solid state. Although the overall concentration of the solution remains constant, the processes of dissolution and crystallisation continue to occur at an equal pace.
2. How does Henry's Law govern the equilibrium of a gas dissolving in a liquid?
Henry's Law is the principle that governs the solubility of a gas in a liquid. It states that at a constant temperature, the mass or concentration of a gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid. In terms of equilibrium, if the external pressure of the gas increases, the system shifts to dissolve more gas until a new equilibrium is reached, thereby increasing its solubility.
3. What is the difference between a saturated and an unsaturated solution in the context of equilibrium?
The key difference lies in their state of equilibrium. Here's a breakdown:
An unsaturated solution is not in a state of equilibrium because it can still dissolve more solute. The rate of dissolution is greater than the rate of crystallisation (if any).
A saturated solution is in a state of dynamic equilibrium. It contains the maximum amount of solute that can be dissolved at a specific temperature, and the rate of dissolution is precisely equal to the rate of crystallisation.
4. Provide a real-world example of equilibrium involving the dissolution of a gas in a liquid.
A classic example is a sealed bottle of a carbonated beverage like soda. The space above the liquid is filled with carbon dioxide gas under high pressure. This high pressure maintains an equilibrium where a large amount of CO₂ remains dissolved in the liquid. The moment you open the bottle, the pressure is released, disrupting the equilibrium. This causes the dissolved CO₂ to escape from the solution, creating the characteristic fizzing sound and bubbles.
5. Why does the solubility of most gases in liquids decrease as the temperature increases?
The dissolution of a gas in a liquid is typically an exothermic process, meaning it releases heat (Gas + Liquid ⇌ Dissolved Gas + Heat). According to Le Chatelier's principle, if we increase the temperature (add heat) to this system, the equilibrium will shift to the left to counteract the change by absorbing the added heat. This favours the reverse reaction, causing the dissolved gas to come out of the solution and thus decreasing its solubility.
6. How does temperature affect the equilibrium for the dissolution of solids in liquids?
The effect of temperature on the solubility of solids depends on the nature of the dissolution process:
If the dissolution is endothermic (absorbs heat), increasing the temperature will shift the equilibrium to the right, favouring more dissolution. Therefore, solubility increases with temperature. This is the case for most salts, such as sugar or potassium nitrate in water.
If the dissolution is exothermic (releases heat), increasing the temperature will shift the equilibrium to the left, favouring crystallisation. Therefore, solubility decreases with temperature. An example is cerium(III) sulfate, Ce₂(SO₄)₃.
7. What is the distinction between dynamic and static equilibrium in chemical processes?
The primary distinction is based on the state of the reaction at the molecular level. Dynamic equilibrium is a state where the forward and reverse reactions are occurring at the same, non-zero rate, resulting in no net change in reactant and product concentrations. It is characteristic of reversible chemical reactions. In contrast, static equilibrium is a state where the reaction has completely ceased, and the rates of both forward and reverse reactions are zero. This is more common in physical systems (like a book on a table) rather than in chemical dissolution processes.
