Definition laws Henry law solubility product constant and example problems
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
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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 in Dissolution of Solids and Gases in Chemistry
1. What is equilibrium involving dissolution of solids or gases?
Equilibrium involving dissolution of solids or gases is a dynamic equilibrium established between a dissolved substance and its undissolved solid or gaseous phase in a closed system. At this point, the rate of dissolution equals the rate of precipitation (for solids) or the rate of dissolution equals the rate of escape (for gases).
- For solids: CaCO3(s) ⇌ Ca2+(aq) + CO32-(aq)
- For gases: CO2(g) ⇌ CO2(aq)
- The concentrations remain constant, but molecular processes continue.
2. What is solubility product constant (Ksp)?
The solubility product constant (Ksp) is the equilibrium constant for the dissolution of a sparingly soluble salt in water. It is written as the product of the molar concentrations of the dissolved ions, each raised to the power of their stoichiometric coefficients.
- Example: AgCl(s) ⇌ Ag+(aq) + Cl-(aq)
- Ksp = [Ag+][Cl-]
- The solid is not included in the expression because its concentration is constant.
3. How do you write the Ksp expression for a salt?
To write a Ksp expression, first write the balanced dissolution equation and then multiply the equilibrium concentrations of the ions, each raised to their coefficients.
- Step 1: Write dissolution equation: CaF2(s) ⇌ Ca2+(aq) + 2F-(aq)
- Step 2: Write expression: Ksp = [Ca2+][F-]2
- Do not include the solid in the expression.
4. How do you calculate solubility from Ksp?
To calculate solubility from Ksp, express ion concentrations in terms of molar solubility (s) and substitute into the Ksp expression.
- Example: AgCl(s) ⇌ Ag+(aq) + Cl-(aq)
- If solubility = s mol L-1, then [Ag+] = s and [Cl-] = s
- Ksp = s × s = s2
- So, s = √Ksp
5. What is the common ion effect in solubility equilibrium?
The common ion effect is the decrease in solubility of a sparingly soluble salt when a common ion is added to the solution. According to Le Châtelier’s principle, adding a common ion shifts the equilibrium to the left, reducing dissolution.
- Example: AgCl(s) ⇌ Ag+(aq) + Cl-(aq)
- Adding NaCl increases [Cl-]
- Equilibrium shifts left, forming more solid AgCl
6. What is Henry’s law for gases dissolved in liquids?
Henry’s law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid at constant temperature. It is expressed as p = kHx or C = kP.
- p = partial pressure of gas
- x = mole fraction of dissolved gas
- kH = Henry’s law constant
7. How does pressure affect the solubility of gases?
Increasing pressure increases the solubility of a gas in a liquid according to Henry’s law. When pressure rises, more gas molecules dissolve until a new equilibrium is reached.
- Example: CO2(g) ⇌ CO2(aq)
- Higher CO2 pressure in sealed bottles increases dissolved CO2
- Opening the bottle lowers pressure and gas escapes
8. How does temperature affect solubility of solids and gases?
Temperature increases the solubility of most solids but decreases the solubility of most gases in liquids.
- For solids: If dissolution is endothermic, higher temperature increases solubility.
- For gases: Dissolution is usually exothermic, so increasing temperature shifts equilibrium to release gas.
- Example: Warm soda loses CO2 faster than cold soda.
9. What is the difference between solubility and Ksp?
Solubility is the amount of substance that dissolves in a given volume of solvent, while Ksp is the equilibrium constant for its dissolution reaction.
- Solubility is expressed in mol L-1 or g L-1
- Ksp is a constant at a given temperature
- Different salts can have similar solubility but different Ksp values due to stoichiometry
10. How do you predict whether a precipitate will form using Ksp?
To predict precipitation, compare the ionic product (Q) with the Ksp value.
- Step 1: Calculate Q using current ion concentrations.
- Step 2: Compare with Ksp.
- If Q > Ksp, a precipitate forms.
- If Q = Ksp, the system is at equilibrium.
- If Q < Ksp, no precipitate forms.
