FAD1018 W15 — Thermochemistry

Week 15 lecture covering thermochemistry. Source file: W15.pdf from lecture notes folders.

Summary

Study of energy changes in chemical reactions, enthalpy, calorimetry, Hess's Law, and thermodynamic calculations.

Key Concepts

  • Thermochemistry — Energy changes in chemical reactions
  • Enthalpy (H) — Heat content at constant pressure
  • Exothermic Reactions — Release heat (ΔH < 0)
  • Endothermic Reactions — Absorb heat (ΔH > 0)
  • Hess's Law — Enthalpy is a state function
  • Standard Enthalpy — Measured at standard conditions
  • Bond Enthalpy — Energy to break a bond

Lecture Coverage

1. Basic Concepts

System and Surroundings

  • Open, closed, isolated systems
  • Energy exchange mechanisms

State Functions

  • Path independence
  • Examples: Enthalpy, entropy, Gibbs free energy

First Law of Thermodynamics

  • Conservation of energy
  • ΔU = q + w

2. Enthalpy Changes

Types of Enthalpy Changes

  • ΔH°f: Standard enthalpy of formation
  • ΔH°c: Standard enthalpy of combustion
  • ΔH°neut: Enthalpy of neutralization
  • ΔH°sol: Enthalpy of solution
  • ΔH°vap: Enthalpy of vaporization
  • ΔH°fus: Enthalpy of fusion
  • ΔH°sub: Enthalpy of sublimation
  • ΔH°at: Enthalpy of atomization

3. Calorimetry

Bomb Calorimetry (Constant Volume)

  • qv = Cv × ΔT
  • Measures ΔU

Coffee-Cup Calorimetry (Constant Pressure)

  • qp = C × ΔT = ΔH
  • Measures ΔH directly

Heat Capacity

  • Specific heat capacity
  • Molar heat capacity
  • q = mcΔT

4. Hess's Law

Statement

  • The total enthalpy change is independent of the pathway

Applications

  • Calculating ΔH from formation data: ΔH°rxn = ΣΔH°f(products) - ΣΔH°f(reactants)
  • Calculating ΔH from combustion data
  • Constructing energy cycle diagrams
  • Born-Haber cycles for ionic compounds

5. Bond Enthalpies

Definition

  • Energy required to break one mole of bonds in gaseous state

Calculations

  • ΔH°rxn = Σ(Bonds broken) - Σ(Bonds formed)
  • Bond breaking: Endothermic (+)
  • Bond forming: Exothermic (-)

Average Bond Enthalpies

  • C-C: 347 kJ/mol
  • C=C: 614 kJ/mol
  • C≡C: 839 kJ/mol
  • C-H: 413 kJ/mol
  • O-H: 464 kJ/mol

6. Lattice Energy

Definition

  • Energy required to separate one mole of solid ionic compound into gaseous ions

Born-Haber Cycle

  • Hess's law application for ionic compounds
  • Components: Atomization, ionization, electron affinity, lattice energy

Factors Affecting Lattice Energy

  • Ionic charge (higher charge → higher LE)
  • Ionic radius (smaller ions → higher LE)

7. Spontaneity and Gibbs Free Energy

Gibbs Free Energy Equation

  • ΔG = ΔH - TΔS

Spontaneity Criteria

  • ΔG < 0: Spontaneous
  • ΔG = 0: Equilibrium
  • ΔG > 0: Non-spontaneous

Temperature Dependence

  • Exothermic + ΔS > 0: Always spontaneous
  • Exothermic + ΔS < 0: Spontaneous at low T
  • Endothermic + ΔS > 0: Spontaneous at high T
  • Endothermic + ΔS < 0: Never spontaneous

Related Topics

Study Notes

[!important] Physical chemistry core Thermochemistry appears in 4/5 papers with ~7-9% mark weight. Master Hess's Law and calorimetry calculations.

Related Course Page

  • FAD1018 - Basic Chemistry II