Solubility & Henry's Law
Module 2 | CBSE Class 12 Chemistry | Solutions Chapter
1. Concept of Solubility
It depends upon the nature of solute and solvent as well as temperature and pressure. We will explore the effect of these factors in two distinct cases: solid in liquid and gas in liquid.
2. Solubility of a Solid in a Liquid
Every solid does not dissolve in a given liquid. For instance, sodium chloride and sugar dissolve readily in water, whereas naphthalene and anthracene do not. Conversely, naphthalene and anthracene dissolve readily in benzene, but sodium chloride and sugar do not.
This observation leads to the fundamental rule of solubility: "Like dissolves like". Polar solutes dissolve in polar solvents (e.g., NaCl in water), and non-polar solutes dissolve in non-polar solvents (e.g., Anthracene in benzene).
Key Terms to Understand:
- Dissolution: When a solid solute is added to a solvent, some solute dissolves and its concentration increases in solution.
- Crystallization: Some solute particles in solution collide with the solid solute particles and get separated out of solution.
- Dynamic Equilibrium: A stage is reached when the rate of dissolution equals the rate of crystallization. At this stage, the concentration of solute remains constant.
Solute + Solvent ⇋ Solution - Saturated Solution: A solution in which no more solute can be dissolved at the same temperature and pressure.
2.1 Factors Affecting Solubility of a Solid in a Liquid
A. Effect of Temperature
The solubility of a solid in a liquid is significantly affected by temperature changes. This can be understood using Le Chatelier's Principle.
- If the dissolution process is endothermic (ΔsolH > 0), the solubility should increase with a rise in temperature.
- If the dissolution process is exothermic (ΔsolH < 0), the solubility should decrease with a rise in temperature.
B. Effect of Pressure
3. Solubility of a Gas in a Liquid
Many gases dissolve in water. Oxygen dissolves only to a small extent in water, but it is this dissolved oxygen which sustains all aquatic life. On the other hand, hydrogen chloride gas (HCl) is highly soluble in water.
The solubility of gases in liquids is highly affected by temperature and pressure.
3.1 Effect of Pressure: Henry's Law
The most important relationship between pressure and solubility of a gas is given by Henry's Law.
Where:
p = Partial pressure of the gas
x = Mole fraction of the gas in solution
KH = Henry's law constant
3.2 Significance of Henry's Law Constant (KH)
- At a given pressure, the higher the value of KH, the lower is the solubility of the gas in the liquid. (Since x = p / KH).
- The value of KH increases with an increase in temperature indicating that the solubility of gases decreases with increase in temperature.
Due to the decrease in solubility of oxygen with an increase in temperature (higher KH value at higher temperatures), aquatic species are more comfortable in cold water rather than in warm water.
3.3 Real-life Applications of Henry's Law
Henry's Law has crucial applications in biological and industrial phenomena. These are highly tested in CBSE board exams.
- Carbonated Beverages: To increase the solubility of CO2 in soft drinks and soda water, the bottle is sealed under high pressure.
- Scuba Diving (The "Bends"): Scuba divers breathe air at high pressure underwater, increasing the solubility of atmospheric gases in blood. When divers come towards surface, pressure gradually decreases. This releases the dissolved gases and leads to the formation of bubbles of nitrogen in the blood. This blocks capillaries and creates a medical condition known as bends, which are painful and dangerous to life.
Solution: The tanks used by scuba divers are filled with air diluted with Helium (11.7% He, 56.2% N2, 32.1% O2) because Helium has very low solubility in blood. - High Altitudes (Anoxia): At high altitudes, the partial pressure of oxygen is less than that at the ground level. This leads to low concentrations of oxygen in the blood and tissues of people living at high altitudes or climbers. Low blood oxygen causes climbers to become weak and unable to think clearly, symptoms of a condition known as anoxia.
3.4 Effect of Temperature on Gas Solubility
Solubility of gases in liquids decreases with rise in temperature. When dissolved, the gas molecules are present in liquid phase and the process of dissolution can be considered similar to condensation. Therefore, heat is evolved in this process.
4. NCERT Solved Examples (Step-by-Step)
NCERT Example 2.4: If N2 gas is bubbled through water at 293 K, how many millimoles of N2 gas would dissolve in 1 litre of water? Assume that N2 exerts a partial pressure of 0.987 bar. Given that Henry's law constant for N2 at 293 K is 76.48 kbar.
Given:
p(N2) = 0.987 bar
KH = 76.48 kbar = 76,480 bar
Volume of water = 1 L
Step 1: Find Mole fraction of N2 (x)
According to Henry's Law: p = KH × x
x = p / KH = 0.987 / 76480 = 1.29 × 10-5
Step 2: Find Moles of water
Density of water = 1 g/mL. Therefore, 1 L of water = 1000 g.
Moles of water (nH2O) = 1000 / 18 = 55.5 mol
Step 3: Calculate Moles of N2
Let n = number of moles of N2.
Mole fraction (x) = n / (n + 55.5)
Since 'n' is very small in the denominator compared to 55.5, we approximate (n + 55.5) ≈ 55.5.
1.29 × 10-5 = n / 55.5
n = 1.29 × 10-5 × 55.5 = 7.16 × 10-4 mol
Step 4: Convert to millimoles (mmol)
Millimoles = 7.16 × 10-4 × 1000 = 0.716 mmol.
5. Previous Year Questions (PYQs) & Exhaustive Question Bank
Part A: Conceptual & Give Reasons (1-2 Marks)
Q1. Why do aquatic animals feel more comfortable in cold water than in warm water?
Q2. What are 'bends' in relation to scuba divers? How is this prevented?
Prevention: To prevent this, scuba tanks are filled with air diluted with Helium gas (about 11.7%), as Helium is much less soluble in blood even at high pressures.
Q3. State Henry's Law. How does the value of KH depend on the nature of the gas?
The value of KH is a function of the nature of the gas. Different gases have different KH values at the same temperature. Gases with higher KH values are less soluble in a given solvent.
Part B: Assertion-Reason Type (1 Mark)
Q4. Assertion (A): The solubility of a gas in liquid decreases with an increase in temperature.
Reason (R): Dissolution of a gas in liquid is an endothermic process.
The solubility of gas indeed decreases with temperature. However, the dissolution of a gas in a liquid is an exothermic process (heat is evolved), not endothermic.
Q5. Assertion (A): People living at high altitudes have low concentration of oxygen in their blood and tissues.
Reason (R): At high altitudes, the partial pressure of oxygen is less than that at the ground level.
Part C: Numerical Problems (3 Marks)
Q6. H2S, a toxic gas with rotten egg like smell, is used for qualitative analysis. If the solubility of H2S in water at STP is 0.195 m, calculate Henry's law constant.
Step 1: Understand the given data
Solubility = 0.195 m (molal). This means 0.195 moles of H2S are dissolved in 1 kg (1000 g) of water.
Pressure at STP (Standard Temperature and Pressure) = 1 bar (or 0.987 atm, but standard is 1 bar).
Step 2: Calculate Mole Fraction (x)
Moles of H2S (nA) = 0.195 mol
Moles of water (nB) = 1000 g / 18 g mol-1 = 55.55 mol
Mole fraction of H2S (x) = nA / (nA + nB) = 0.195 / (0.195 + 55.55) = 0.195 / 55.745 = 0.0035
Step 3: Calculate KH using Henry's Law
p = KH × x
KH = p / x = 1 bar / 0.0035 = 285.7 bar.
No comments:
Post a Comment