Introduction to Thermochemistry

Introduction to Thermochemistry

• Thermochemistry is the study of heat changes during chemical reactions and phase transitions.
• It examines the relationship between chemical processes and energy transfer, focusing on heat.
• Key concepts include enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG), essential for determining reaction spontaneity and feasibility.

Enthalpy (ΔH)

• Enthalpy (ΔH) measures a system's total heat content.
• It's a state function, meaning its value depends only on the system's current state, not the path.
• Negative ΔH signifies an exothermic reaction (heat released).
• Positive ΔH signifies an endothermic reaction (heat absorbed).
• Standard enthalpy change (ΔH°) represents the enthalpy change at standard conditions (usually 298 K and 1 atm).
• Enthalpy changes are calculated using Hess's Law.

Hess's Law

• Calculates enthalpy changes for complex, unmeasurable reactions.
• The overall enthalpy change is the sum of enthalpy changes for individual steps.
• Reversing or multiplying reaction equations requires adjusting corresponding enthalpy changes.

Entropy (ΔS)

• Entropy (ΔS) measures system disorder or randomness.
• Increased entropy (ΔS > 0) indicates increased disorder.
• The Second Law of Thermodynamics states that the total entropy of the universe increases in any spontaneous process.

Gibbs Free Energy (ΔG)

• Gibbs free energy (ΔG) combines enthalpy and entropy to predict reaction spontaneity.
• ΔG = ΔH - TΔS, where T is the absolute temperature in Kelvin.
• Negative ΔG indicates a spontaneous reaction.
• Positive ΔG indicates a non-spontaneous reaction.
• ΔG = 0 signifies the system is at equilibrium.

Calorimetry

• Calorimetry measures heat changes during chemical or physical changes.
• Constant-pressure calorimetry measures enthalpy changes at constant pressure.
• Constant-volume calorimetry (bomb calorimetry) measures enthalpy changes at constant volume.

Standard Enthalpies of Formation

• Standard enthalpy of formation (ΔH°f) is the enthalpy change when one mole of a substance forms from its elements in their standard states (usually 298 K and 1 atm).
• ΔH°f values are crucial for calculating standard enthalpy changes of complex reactions.
• They are tabulated values.

Bond Energies

• Bond energies are average energies needed to break a specific bond type.
• They help estimate enthalpy changes for reactions involving bond breaking and formation.
• Bond energy calculations are often estimations.

Applications of Thermochemistry

• Understanding and predicting energy flow in reactions.
• Designing efficient chemical processes.
• Assessing reaction pathway feasibility.
• Applying to industrial processes (e.g., combustion, metallurgy).
• Predicting energy requirements.