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Heat of Fusion and Vaporization

Heat of Fusion and Vaporization. Water H f = 334 J/g Water H v = 2,270 J/g The specific heat of a substance depends on its state. Solid water Cp = 2.108 J/g o C Liquid water Cp = 4.186 J/g o C Water vapor Cp = 1.996 J/g o C

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Heat of Fusion and Vaporization

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  1. Heat of Fusion and Vaporization • Water Hf = 334 J/g • Water Hv = 2,270 J/g • The specific heat of a substance depends on its state. • Solid water Cp = 2.108 J/g oC • Liquid water Cp = 4.186 J/g oC • Water vapor Cp = 1.996 J/g oC • So how much energy is required to heat 24.0 g of water from ice at -10 oC to vapor at 120 oC?

  2. E Climb the stairs DQ=mC(Tf-T0) DQ=mHv DQ=mC(Tf-T0) DQ=mC(Tf-T0) DQ=mHf

  3. Heat water from Ice to Vapor (A) First heat to melting point, 24.0 x 2.108 x (0- -10) = 506 (B) Then melt the ice, 24.0 x 334 = 8016 (C) Then heat to boiling, 24.0 x 4.186 x (100-0) =10046 (D) Then boil the water, 24.0 x 2270 =54480 (E) Then heat the vapor to 120o24.0 x 1.996 x (120-100)= 958 =74006 J or 74 kJ Steps m x C x (Tf-T0) = DQ m x Hf = DQ m x C x (Tf-T0) = DQ m x Hv = DQ m x C x (Tf-T0) = DQ

  4. Substance Heat of formation (kJ/mol) C2H4 52.30 O2 0 CO2 -393.51 H2O -285.85 With these values we can determine the heat released in the reaction: C2H4 + 3O2→ 2CO2 + 2H2O Reactants Products 1 x 52.30 +3 x 0 → 2 x (-393.51) + 2 x (-285.85) +52.30 + 0 → -787.02 -571.70 +52.30(reactants) → -1358.72 (products) -52.30 (subtract reactant’s DH) -1411.02 kJ (total for reaction) DH of Reaction

  5. Entropy • Entropy is a measure of the disorder of a system. Matter tends to disorder. • Reactions that release energy (exothermic, -DH reaction) and produce more total mols of products than existed as reactants are said to be increasing entropy. Such reactions can be spontaneous. • NaOH (s) Na+ (aq) + OH- (aq) + heat • There are twice the number of moles on the product side and it releases heat (feels warm). Entropy increases.

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