Thermodynamics Solved Examples (Class 11 Chemistry)
Thermodynamics numericals in chemistry cover enthalpy, entropy, Hess's law, and spontaneity. These problems connect to reaction mechanisms and equilibrium
TL;DR: Thermodynamics numericals in chemistry cover enthalpy, entropy, Hess's law, and spontaneity. These problems connect to reaction mechanisms and equilib…
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Thermodynamics numericals in chemistry cover enthalpy, entropy, Hess's law, and spontaneity. These problems connect to reaction mechanisms and equilibrium
Thermodynamics — Solved Numerical Examples (Step by Step)
Example 1: The enthalpy of combustion of carbon is -394 kJ/mol. Calculate energy released when 6 g of carbon burns. (Atomic mass of C = 12)
Solution: Molar mass of C = 12 g/mol. Moles of C = 6/12 = 0.5 mol. Energy released = 0.5 × 394 = 197 kJ.
Example 2: For reaction A → B, ΔH = -100 kJ/mol. If 2 mol of A reacts, calculate heat released.
Solution: For 1 mol of A, heat released = 100 kJ. For 2 mol of A, heat released = 2 × 100 = 200 kJ.
Example 3: Using Hess's law, find ΔH for: C + O₂ → CO₂. Given: (1) C + 0.5O₂ → CO, ΔH = -111 kJ. (2) CO + 0.5O₂ → CO₂, ΔH = -283 kJ.
Solution: Adding equations (1) and (2): C + 0.5O₂ → CO (ΔH = -111) + CO + 0.5O₂ → CO₂ (ΔH = -283). Result: C + O₂ → CO₂, ΔH = -111 + (-283) = -394 kJ.
Example 4: Calculate the heat of neutralization if 100 mL of 1 M HCl neutralizes 100 mL of 1 M NaOH with temperature rise of 5.76°C. (Assume density = 1 g/mL, specific heat = 4.18 J/g°C)
Solution: Mass of solution = 200 g. Heat absorbed by solution q = m × c × ΔT = 200 × 4.18 × 5.76 = 4815.36 J = 4.815 kJ. Moles of HCl = 0.1 mol. Heat of neutralization = 4.815 / 0.1 = 48.15 kJ/mol.
Example 5: If ΔH = -100 kJ and ΔS = 50 J/K, is the reaction spontaneous at 300 K? (Calculate ΔG)
Solution: Using ΔG = ΔH - T × ΔS. ΔG = -100,000 - 300 × 50 = -100,000 - 15,000 = -115,000 J = -115 kJ. Since ΔG < 0, the reaction is spontaneous.
Tips
- Hess's law: ΔH of a reaction equals sum of ΔH of its steps, independent of path.
- ΔG = ΔH - TΔS. Reaction is spontaneous if ΔG < 0.
- Exothermic reactions (ΔH < 0) release heat; endothermic reactions (ΔH > 0) absorb heat.
Frequently Asked Questions
Why is ΔH negative for exothermic reactions?
Exothermic reactions release energy to surroundings. The system loses energy, so ΔH = Hproducts - Hreactants < 0.
What determines if a reaction is spontaneous?
A reaction is spontaneous if ΔG < 0. Both ΔH and ΔS contribute: ΔG = ΔH - TΔS. Temperature affects spontaneity via the entropy term.
More Chemistry Solved Examples
- Mole Concept and Stoichiometry
- Atomic Structure (Numericals)
- Electrochemistry (Numericals)
- Chemical Equations and Balancing
- Thermodynamics
- Solutions - Previous Year Questions
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