Questions
4 questions per board paper
Difficulty
Medium-Hard
Importance
Core topic — never skip
Overview
Thermodynamics is the study of energy transformations in physical and chemical systems, forming the backbone of physical chemistry. Mastering this topic is critical for exams because it links microscopic energy states to macroscopic spontaneous changes through the laws of thermodynamics. Aspirants must focus on the sign conventions and the state functions, as these drive most numerical calculations.
Enthalpy and Heat Capacity
Enthalpy (H) represents the total heat content of a system at constant pressure. Understanding the relationship between internal energy and enthalpy is crucial for solving problems involving combustion or phase changes.
- H = U + PV
- Delta H = Delta U + P*Delta V (at constant pressure)
- Delta H = Delta U + Delta n(g)RT
- C_p - C_v = R
- Heat capacity is an extensive property
Hess's Law of Constant Heat Summation
Hess's Law states that the total enthalpy change for a reaction is the same regardless of the path taken. This principle is widely tested for calculating reaction enthalpies from standard enthalpies of formation or combustion.
- The overall enthalpy change is independent of the number of steps
- Delta H_reaction = Sum(H_f_products) - Sum(H_f_reactants)
- Works because Enthalpy is a state function
- Valid only at constant pressure
Entropy and Gibbs Free Energy
Entropy is a measure of the randomness or disorder of a system. Gibbs Free Energy (G) is the ultimate criterion for spontaneity, combining enthalpy and entropy to predict whether a process will occur naturally.
- Delta S = q_rev / T
- Delta G = Delta H - T*Delta S
- For spontaneity: Delta G < 0
- At equilibrium: Delta G = 0
- Delta G_standard = -RT*ln(K_eq)
Formula Sheet
Delta H = Delta U + Delta n(g)RT
Delta H_reaction = Sum(Delta H_f_products) - Sum(Delta H_f_reactants)
Delta S_total = Delta S_sys + Delta S_surr > 0 (for spontaneous processes)
Delta G = Delta H - T*Delta S
Delta G_standard = -2.303*RT*log(K_eq)
Delta G = Delta G_standard + RT*ln(Q)
Exam Tip
Always verify that your units for Delta H (usually kJ/mol) and T*Delta S (must be converted to kJ/mol) match before adding or subtracting them in the Gibbs equation.
Common Mistakes
- Confusing the sign of Delta H and Delta G, leading to incorrect predictions about spontaneity.
- Neglecting to use 'Delta n(g)'—the change in moles of gas—instead of total moles of the entire reaction.
- Forgetting to convert temperatures to Kelvin or mismatching energy units (Joules vs Kilojoules).
More Revision Notes
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