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NANIK DWI NURHAYATI,S.Si, M.Si

CHEMICAL THERMODYNAMICS. NANIK DWI NURHAYATI,S.Si, M.Si. www.nanikdn.staff.uns.ac.id (0271) 821585, 081556431053. Enthalpy and Enthalpy Change of Chemical Reactions.

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NANIK DWI NURHAYATI,S.Si, M.Si

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  1. CHEMICAL THERMODYNAMICS NANIK DWI NURHAYATI,S.Si, M.Si www.nanikdn.staff.uns.ac.id (0271) 821585, 081556431053

  2. Enthalpy and Enthalpy Change of Chemical Reactions • We have already investigated energy transfers during physical transformations, what about chemical transformations – reactions? H = Hproducts – Hreactants • An endothermic reaction (H > 0) is a reaction in which heat is absorbed from the surroundings. • An exothermic reaction (H < 0) is a reaction in which heat is given off to the surroundings.

  3. Thermochemical Equations • Thermochemical Equation: a balanced chemical equation (including phase labels) with the molar enthalpy of reaction written directly after the equation N2 (g) + 3 H2 (g)  2 NH3 (g); H = -91.8 kJ a) Molar Interpretation: When 1 mol of nitrogen gas reacts with 3 mol of hydrogen gas to form 2 mol of ammonia gas, 91.8 kJ of energy is given off.

  4. Heat Transfer Heat Capacity (C): quantity of energy required to increase the temperature of a sample by one degree C = q/T The magnitude of the heat capacity depends on: • Mass of the sample • Composition of the sample

  5. Calculate the heat capacity of an aluminum block that must absorb 629 J of heat from its surroundings in order for its temperature to rise from 22 C to 145 C. Heat Capacity (C): C =q/T C = 629 J / (145 - 22 C) = 629 J / 123 C = 5.11 J/ C

  6. Heat Capacity When comparing the heat capacities of different substances with different masses, it is more useful to compare specific heat capacities. Specific Heat capacities (c): quantity of energy needed to increase the temperature of one gram of a substance by one degree Celsius • Molar Heat Capacity (cm): related to specific heat, but for one mole of substance

  7. Heat Capacity When dealing with specific heat capacities (c): Given: c = q/mT Derive: q = cmT T = Tfinal – Tinitial = q/cm m = q/cT q = thermal heat c = specific heat m = mass T = change in temperature

  8. What will be the final temperature of a 5.00 g silver ring at 37.0 C that gives off 25.0 J of heat to its surroundings (c = 0.235 J/g C)? T = Tfinal – Tinitial = q/cm Tfinal – 37.0 C = -25.0 J / (0.235 J/g C)(5.00 g) Tfinal – 37.0 C = -21.3 C Tfinal = 37.0 C - 21.3 C Tfinal = 15.7 C

  9. 148 J of heat are transferred to a a piece of glass (c = 0.84 J/gC), raising the temperature from 25.0 C to 49.4 C. What is the mass of the glass? m = q/cT m = (148 J)/(0.84 J/gC)(24.4 C) m = 7.2 g

  10. Phase Changes • We just saw that energy transfers ALWAYS accompany temperature changes. • Energy transfers also accompany physical and chemical changes, even when there is no change in temperature. • eg. Energy is always transferred into or out of a system during a phase change.

  11. Melting/Freezing Heat of Fusion: quantity of thermal energy that must be transferred to a solid as it melts (qfusion = - qfreezing) Water: Heat of fusion = +333 J/g at 0 C. Specific Heat (l) = 1.00 cal/gC Specific Heat (s) depends on T:  0.5 cal/gC near 0 C

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