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Understanding Thermodynamics: Concepts, Equations, and Applications

Explore the fundamental principles of thermodynamics, including specific heat, bond energies, enthalpy, Hess's Law, entropy, calorimetry, changes in state, Gibbs free energy, and spontaneity. Gain insights into the laws governing energy transfer and chemical reactions.

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Understanding Thermodynamics: Concepts, Equations, and Applications

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  1. Thermodynamics By Alex Weber and Lee Cheung

  2. Standard Thermodynamic Conditions • 25 C°, and 1 barr = 100kPa ≈ 1 atm

  3. Specific Heat (C) • Energy required to raise a gram of material by 1° C • C is a constant based on material and state of the matter • Units in J/g° C • C = 4.18 J/g° C (For Water)

  4. Heat Transferred (q) • q= mC(ΔT) where C is specific heat, ΔT is the change in temperature (in ° C), and m is mass in grams • Units for q in Joules • +q means heat is transferred to the system • -q means heat is transferred from the system

  5. Bond Energies • q = Σ Reactants Bond Energy – Σ Products Bond Energy (note: reactants – products) • Endothermic reactions need heat/energy added to progress • Breaking bonds requires heat/energy to progress • Exothermic reactions give off heat/energy during the reaction • Forming bonds gives off heat/energy • Potential energy in an elemental state = 0

  6. Enthalpy (ΔH) • ΔH is the heat transferred into a system per mole or per gram • ΔH = q/m= change in potential energy from products to reactants • ΔHrxn = ΣHf products - ΣHf reactants • -ΔH = exothermic • +ΔH = endothermic • Units (J/g or J/mol)

  7. Hess’s Law • If a reaction equals the sum of a series of reactions, then the overall ΔH equals the sum of ΔH from each individual reaction • Reverse reactions = reverse signs • If you change the coefficients of a reaction by a certain factor, then change ΔH by the same factor

  8. Entropy (ΔS) • ΔS is the measure of randomness of molecules • ΔS = the change in S from products to reactants • Gases are the most random and have the highest entropy, solids the lowest • Product favored reactions have higher entropy • Units of J/K*mol

  9. Calorimetry • Mmetal cΔT = mwater cΔT • Mmetal c(Tf-Ti) = mwater c(Tf-Ti) • q reaction = - (q water + q bomb)

  10. Changes in State • q= mHf or q= mHv (Hf = Heat of fusion, Hv = Heat of vaporization) • Heat of fusion = heat required to melt a substance into liquid • Heat of vaporization = heat required to vaporize substance into gas

  11. Gibbs Free Energy • ΔG = ΔH - TΔS (T is in K) • When ΔG is negative reaction is spontaneous and vice versa • Threshold Energy = when ΔG = 0, equation is at equilibrium • Spontaneous reactions favor products

  12. Gibbs Free Energy Cont. • ∆G = ∆G˚ + RT lnQ • ∆G˚ = -RT lnK (at equilibrium), where R=8.314 J/(mol*K) and T = Temperature (K) • K = Thermodynamic Equilibrium Constant • Q= reaction quotient = K (at equilibrium) • When ∆G˚ < 0 and K > 1 Reaction is product favored (spontaneous) • When ∆G˚ = 0 and K = 1 Reaction is at equilibrium • When ∆G˚ > 0 and K < 1 Reaction is reactant favored (non-spontaneous)

  13. Spontaneity • Considering T= ΔH/ΔS when • ΔS < 0 ΔS > 0 • ΔH < 0 Spontaneous at Always • Low Temps Spontaneous • ΔH > 0 Never Spontaneous • Spontaneous at Higher Temps

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