Thermochemistry Basics

Study Notes

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 (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)

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

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.

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.