How can chemists predict whether a chemical reaction will explode violently or not react at all just by calculating a single value called Gibbs Free Energy ($\Delta G$)? Enthalpy and Entropy govern thermodynamic spontaneity.
Why This Chapter Matters
In Class 11 Chemistry, "Thermodynamics" provides an authoritative, curriculum-verified master resource aligned with the 2026–27 NCERT syllabus.
Before You Begin (Prerequisites)
Exothermic and endothermic reactions from Class 10.
First Law of Thermodynamics from Physics.
Work done.
What You Will Learn (Core Objectives)
Distinguish between System (Open, Closed, Isolated) and Surroundings; State Functions vs Path Functions.
State First Law of Thermodynamics: $\Delta U = q + w$ and define Enthalpy ($H = U + PV, \Delta H = q_p$).
State Hess's Law of Constant Heat Summation.
Define Entropy ($S = \frac{q_{rev}}{T}$) and the Second Law of Thermodynamics ($\Delta S_{\text{total}} > 0$).
Apply Gibbs Free Energy: $\Delta G = \Delta H - T\Delta S$; evaluate Spontaneity ($\Delta G < 0$ for spontaneous process).
Chapter Roadmap & Progression
11. Internal Energy & Enthalpy
22. Entropy & The Second Law
33. Gibbs Free Energy ($\Delta G$) &...
Complete Concept Guide (100% Curriculum Coverage)
1. Internal Energy & Enthalpy
First Law: $\mathbf{\Delta U = q + w}$. Internal energy $U$ is a state function.
Enthalpy ($H$): Heat content at constant pressure: $$\mathbf{\Delta H = \Delta U + \Delta n_g R T} \quad (\Delta n_g = \text{gaseous product moles} - \text{reactant moles})$$ Exothermic: $\Delta H < 0$; Endothermic: $\Delta H > 0$.
Hess's Law: Total enthalpy change is identical whether a reaction occurs in one step or multiple steps!
2. Entropy & The Second Law
Entropy ($S$): A thermodynamic measure of atomic randomness or disorder: $$\mathbf{\Delta S = \frac{q_{\text{rev}}}{T}} \quad (\text{J/K}\cdot\text{mol})$$ Second Law of Thermodynamics: In any spontaneous process, the total entropy of the universe increases: $\mathbf{\Delta S_{\text{univ}} = \Delta S_{\text{sys}} + \Delta S_{\text{surr}} > 0}$.
3. Gibbs Free Energy ($\Delta G$) & Spontaneity
The master criterion of chemical feasibility: $$\mathbf{\Delta G = \Delta H - T\Delta S}$$ • $\Delta G < 0$ (Negative): Reaction is strictly Spontaneous. • $\Delta G = 0$: Reaction is at Dynamic Equilibrium. • $\Delta G > 0$ (Positive): Non-spontaneous in forward direction.
Thermodynamics - Key Conceptual Architecture & Molecular Model
Chapter Summary & 10 Key Takeaways
Takeaway 1
Enthalpy Relation: $\Delta H = \Delta U + \Delta n_g RT$ linking isobaric and isochoric heats.
Takeaway 2
Hess's Law: Enthalpy change is strictly path-independent (state function).
Takeaway 3
Entropy ($S$): Fundamental metric quantifying microscopic chaos and atomic disorder.
Takeaway 4
Gibbs Free Energy: $\Delta G = \Delta H - T\Delta S$ establishing universal chemical spontaneity.
Takeaway 5
Equilibrium Relation: $\Delta G^\circ = -2.303 RT \log_{10} K$ connecting free energy to equilibrium constant.
Check Your Understanding (Diagnostic Practice Questions)
Diagnostic questions testing core conceptual clarity. Answers are hidden initially — solve each problem first, then click to reveal the step-by-step verified solution.
1
State Hess's Law of Constant Heat Summation.
Reveal Answer & Explanation
Answer: Hess's Law states that if a chemical reaction can take place in several steps, the overall enthalpy change for the reaction is the same regardless of the pathway taken, because enthalpy is a state function. Total enthalpy change is independent of pathway.
2
For a reaction, $\Delta H = -110\text{ kJ/mol}$ and $\Delta S = -40\text{ J/K}\cdot\text{mol}$. At what temperature range will the reaction be spontaneous?
Reveal Answer & Explanation
Answer: For spontaneity: $\Delta G = \Delta H - T\Delta S < 0 \implies -110,000 - T(-40) < 0 \implies 40T < 110,000 \implies T < \frac{110,000}{40} = 2750\text{ K}$. The reaction is spontaneous below 2750 K. Spontaneous below 2750 K.
3
Differentiate between an Extensive property and an Intensive property with examples.
Reveal Answer & Explanation
Answer: An Extensive property depends on the amount of matter in the system (e.g. Mass, Volume, Enthalpy, Heat Capacity); an Intensive property is independent of the amount of substance (e.g. Temperature, Density, Pressure, Molar Heat Capacity). Dependent on mass (Extensive) vs independent of mass (Intensive).
4
Calculate $\Delta U$ when a system absorbs $700\text{ J}$ of heat and performs $300\text{ J}$ of work on surroundings.
Reveal Answer & Explanation
Answer: By First Law: $\Delta U = q + w$. Heat absorbed $q = +700\text{ J}$. Work done by system $w = -300\text{ J}$. $\Delta U = 700 + (-300) = +400\text{ Joules}$. ΔU = +400 J.
5
Why is the standard entropy of a pure crystalline solid zero at absolute zero ($0\text{ K}$)?
Reveal Answer & Explanation
Answer: By the Third Law of Thermodynamics, at absolute zero ($0\text{ K}$), all thermal atomic vibrations cease completely and a perfectly pure crystalline substance attains perfect atomic order, so its entropy is zero ($S = 0$). Third Law: perfect crystalline order at absolute zero.
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