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Thermodynamics. By Alex Weber and Lee Cheung. Standard Thermodynamic Conditions. 25 C°, and 1 barr = 100kPa ≈ 1 atm. 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).

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Thermodynamics

Thermodynamics

By Alex Weber and Lee Cheung


Standard thermodynamic conditions
Standard Thermodynamic Conditions

  • 25 C°, and 1 barr = 100kPa ≈ 1 atm


Specific heat c
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)


Heat transferred q
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


Bond energies
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


Enthalpy h
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)


Hess s law
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


Entropy s
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


Calorimetry
Calorimetry

  • Mmetal cΔT = mwater cΔT

  • Mmetal c(Tf-Ti) = mwater c(Tf-Ti)

  • q reaction = - (q water + q bomb)


Changes in state
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


Gibbs free energy
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


Gibbs free energy cont
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)


Spontaneity
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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