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Thermodynamics

Thermodynamics. Science of heat and energy flow in a chemical reaction. Specific Heat (C) heat required to produce a given temperature change per gram of material J / (g * ºC) or J/ (g * ºK). Enthalphy. Δ H = Hproducts -Hreactants heat transferred into a system at constant pressure.

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Thermodynamics

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  1. Thermodynamics • Science of heat and energy flow in a chemical reaction • Specific Heat (C) • heat required to produce a given temperature change per gram of material • J / (g * ºC) or J/ (g * ºK)

  2. Enthalphy • ΔH = Hproducts -Hreactants • heat transferred into a system at constant pressure

  3. Phase Diagrams • q(slant) = mCΔ t • q(plat) = m Δ H • Heat of fusion - heat needed to melt • Heat of Vaporization - heat needed to vaporize

  4. Phase Diagram Example • Csol = 2.1 Cliq = 4.2 Csteam = 2.0 (all in J/g*K) • ΔHfusion = 333 J/g ΔHvaporization = 2256 J/g • Find the energy is required to turn 500g of ice at –50 deg C to steam at 200 deg C • SOLUTION: q(warm ice to 0 deg) = (500)(2.1)(50) = 5.3 x 10^4 J q(melt ice) = (500)(333) = 1.67 x 10^5 J q(warm water to 100 deg) = (500)(4.2)(100) = 2.1 x 10^5 J q(vaporize water) = (500)(2256) = 1.13 x 10^6 J + q(warm steam to 200 deg) = (500)(2.0)(100) = 1.0 x 10^5 J TOTAL ENERGY REQUIRED = 1.7x 10^6 J

  5. Hess’s Law • If a reaction is the sum of two or more reactions, then the ΔH for overall process is sum of ΔH’s for consituent reactions • Example: Find ΔH for the reaction H20(l) --> H2(g) + O2(g) when given… 1) H20(l) --> H20(g) ΔH1 = 44 kJ + 2) H20(g) --> H2(g) + O2(g)ΔH2 = 241.8 kJ NET: H20(l) --> H2(g) + O2(g) ΔHnet= 285.8 kJ

  6. Calorimetry • qbomb = CbombΔt • Example: 1.00 g of Octane (C8H18) is burned in a calormeter which has 1.20kg water. The temperature of the bomb and water rises from 25 to 33.2 deg. C. If C(bomb)=837 J/K,calculate the heat transferred to 1 mol of octane. • -q lost = q gained • Solution: q(water) + q(bomb) = -q(octane) ***watch out for - sign!!!!! (1200)(4.18)(8.2) + 837(8.2) = -q(octane) -48.1 kJ = q (octane) <----This is for 1g of octane Do factor label to get energy per mole of octane- Heat transferred /mol = (-48.1 kJ/mol)(114.2 g/mol) = -5.49 x 10^3 kj/mol

  7. Bond Energy • ΔH (reaction) = sum of BE’s of REACTANTS - sum of BE’s of PRODUCTS EnTrOpY • (S) - measure of randomness • Progression of randomness: • solid-->aqueus-->liquid-->gas (most random)

  8. Gibbs Free Energy • ΔG = ΔH - TΔS NOTE:T in K, watch energy units (kJ or J) • +ΔG = nonspontaneous , -ΔG = spontaneous • Spontaneous only means PRODUCT FAVORED • Transition temperature of spontaneity: • T = ΔH / ΔS • ΔG = -RTlnK ----relates w/equilibrium constant

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