Thermochemistry Basics

Thermochemistry

• Thermochemistry is the study of heat changes in chemical reactions.
• An exothermic process releases heat, transferring thermal energy from the system to the surroundings.
• An endothermic process absorbs heat, requiring heat transfer from the surroundings to the system.

Enthalpy

• Enthalpy (H) quantifies heat flow in a constant-pressure process.
• Change in enthalpy (ΔH) = H(products) - H(reactants)
• ΔH represents the heat absorbed or released during a reaction at constant pressure.
• A negative ΔH indicates an exothermic process (heat released).
• A positive ΔH indicates an endothermic process (heat absorbed).
• H_products < H_reactants, ΔH < 0 (exothermic)
• H_products > H_reactants, ΔH > 0 (endothermic)

Thermochemical Equations

• Stoichiometric coefficients represent the number of moles of a substance.
• Reversing a reaction reverses the sign of ΔH.
• Multiplying a reaction by a factor multiplies the ΔH by the same factor.
• Physical states (e.g., solid, liquid, gas) must be specified for reactants and products in thermochemical equations.

Specific Heat and Heat Capacity

• Specific heat (c_m) is the heat required to raise the temperature of 1 gram of a substance by 1°C (or 1K).
• Heat capacity (C) is the heat required to raise the temperature of a given mass (m) of a substance by 1°C (or 1K).
• 𝑞 = 𝑚𝑐_𝑚Δ𝑇

Molar Heat Capacity

• Molar heat capacity is the amount of heat required to raise the temperature of one mole of a substance by 1°C (or 1K).

Standard Conditions

• Standard state: Pure gas at 1 atm pressure; pure solid or liquid in its most stable form at 1 bar; substance in solution with 1 M concentration; typically 25°C
• Standard enthalpy change (ΔH°) is the enthalpy change when all reactants and products are in their standard states.
• Standard enthalpy of formation (ΔH_f°) is the enthalpy change for the reaction forming 1 mole of a pure compound from its constituent elements in standard states. The standard enthalpy change for a pure element in its standard state is zero.

Methods of Determining ΔH

• Standard Enthalpy of Formation (direct method): ΔH^°_rxn = Σ n ΔH^°_f(products) - Σ n ΔH^°_f(reactants) (where n is the coefficient)
• Hess's Law (indirect method): The enthalpy change for a reaction is independent of the pathway.
• Bond Energies: Using bond energies to calculate enthalpy changes and/or energy
• Calorimetry (experimental): Measuring heat changes in a controlled environment.