Thermodynamics

Rucete ✏ Chemistry In a Nutshell

1. Energy

• Energy is the capacity to do work.
• Two major forms:
  • Thermal energy: energy due to particle motion (heat).
  • Chemical energy: stored in chemical bonds.
• Unit: kilojoules (kJ)

2. Temperature vs. Heat

• Temperature: average kinetic energy of particles in a substance.
• Heat: thermal energy transferred due to temperature difference.

3. System vs. Surroundings

• System: the part of the universe being studied.
  • Open system: exchanges mass and energy.
  • Closed system: exchanges energy only.
  • Isolated system: no exchange of mass or energy.
• Surroundings: everything outside the system.

4. State Functions

• Properties that depend only on the current state, not the path (e.g., ΔH, ΔS, ΔG).
• Examples: pressure, volume, temperature, enthalpy.

5. Standard State

• Conditions: 1 atm, 1 M concentration, 298 K (25°C).
• Standard enthalpy (ΔH°), entropy (ΔS°), and free energy (ΔG°) values are measured under these.

6. Laws of Thermodynamics

First Law:

• Energy cannot be created or destroyed, only transformed.
• ΔE = q + w (change in internal energy = heat + work)

Second Law:

• In any spontaneous process, entropy (ΔS) increases.
• Systems tend toward disorder.

Third Law:

• Entropy of a perfect crystal at 0 K = 0.

7. Enthalpy (ΔH)

• Measure of heat at constant pressure.
• Exothermic (ΔH < 0): releases heat.
• Endothermic (ΔH > 0): absorbs heat.

8. Spontaneity

• A reaction is spontaneous if it occurs without external input.
• Driven by enthalpy (ΔH) and entropy (ΔS).

9. Hess’s Law

• The total enthalpy change is the same, regardless of how the reaction occurs.
• ΔH (overall) = sum of ΔH for each step.

10. Heat of Formation

• Heat change when one mole of a compound forms from its elements.

11. Specific Heat and Heat Capacity

• Specific heat (c): energy needed to raise 1 g of a substance by 1°C.
• Heat capacity (C): energy needed to raise a given mass by 1°C.
• Formulas:
  • q = mcΔT
  • q = CΔT

12. Calorimetry

• Measurement of heat flow in a system.
• Used to determine:
  • Heat of neutralization
  • Heat of dilution
• q_rxn = - (q_solution + q_calorimeter)

13. Entropy (ΔS)

• Measure of randomness/disorder.
• More gas molecules = higher entropy.
• ΔS increases with temperature, volume, and number of particles.

14. Gibbs Free Energy (ΔG)

• Combines enthalpy and entropy to predict spontaneity.
• ΔG = ΔH - TΔS
• Spontaneity:
  • ΔG < 0 → spontaneous
  • ΔG > 0 → non-spontaneous
  • ΔG = 0 → equilibrium

15. Energy Diagrams

• Activation energy (Ea): barrier that must be overcome for a reaction.
• ΔH is the difference between products and reactants.
  • Exothermic: products lower than reactants.
  • Endothermic: products higher than reactants.