Thermodynamics

The study of energy flow, heat, and work. This chapter is mathematically rigorous. The key to survival here is mastering the Sign Conventions immediately—mistaking a + for a - is the most common error.

🔥

⚠️ Prerequisites

Gaseous State: Ideal Gas Equation ($PV=nRT$) is used constantly.
Logarithms: Essential for entropy and Gibb's energy calculations.
Graphs: Understanding area under the curve (Work done).

🧠 Study Approach

Define your System: In every problem, ask "What is the system?" and "Is work done ON it or BY it?". Focus on the state functions ($\Delta H, \Delta S, \Delta G$) as they depend only on start and end points.

Study Sequence

1. Basic Terminology (Don't Skip)

System vs Surroundings. Types of Systems (Open, Closed, Isolated). State Functions (Pressure, Temp, Volume) vs Path Functions (Heat, Work).

2. First Law & Internal Energy

Law of Conservation of Energy. $\Delta U = q + w$.
Sign Convention (IUPAC): Work done on system = +ve. Work done by system = -ve. Heat absorbed = +ve.

3. Work, Enthalpy, & Heat Capacity

PV Work ($w = -P_{ext}\Delta V$). Reversible vs Irreversible work.
Enthalpy ($H$): Heat change at constant pressure. Relationship: $\Delta H = \Delta U + \Delta n_g RT$.
$C_p$ vs $C_v$ relation ($C_p - C_v = R$).

4. Thermochemistry (Hess's Law)

Hess's Law of constant heat summation.
Calculating $\Delta H_{reaction}$ using Enthalpies of Formation, Combustion, and Bond Dissociation Energy.

5. Entropy & Second Law

Concept of Disorder ($\Delta S$). Spontaneous vs Non-spontaneous processes.
Second Law: For a spontaneous process, $\Delta S_{total} > 0$.

6. Gibbs Free Energy (CRITICAL)

The ultimate criteria for spontaneity at constant T and P.
$\Delta G = \Delta H - T\Delta S$.
$\Delta G < 0$ (Spontaneous), $\Delta G = 0$ (Equilibrium). Relationship with Equilibrium Constant ($K$).

🎯 How to Practice

The 'Sign' Drill: Spend 30 mins just determining the signs of $q, w, \Delta H, \Delta S$ for various processes (expansion, compression, melting, boiling).

Hess's Law Puzzles: Solve problems where you must flip and multiply equations to get the target reaction. Remember to flip the sign of $\Delta H$ when you flip the reaction.

Unit Traps: $\Delta H$ is usually in kJ/mol, but $\Delta S$ is in J/K·mol. ALWAYS convert them to the same unit (usually J) before using $\Delta G = \Delta H - T\Delta S$.

📝 Quick Revision Formulas

First Law $$\Delta U = q + w$$ $$w = -P_{ext}\Delta V \quad (\text{Irreversible})$$ $$w = -2.303 nRT \log \frac{V_2}{V_1} \quad (\text{Reversible})$$

Enthalpy Relationship $$\Delta H = \Delta U + \Delta n_g RT$$

$\Delta n_g$: (moles gaseous products) - (moles gaseous reactants)

Gibbs Free Energy $$\Delta G = \Delta H - T\Delta S$$ $$\Delta G^\circ = -2.303 RT \log K$$

Search This Blog

Lesson Plan: Thermodynamics