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Enthalpy The Meaning of Enthalpy. 1. Enthalpy is a state function with the symbol H. H = E + PV E is the internal energy of the system, P is the pressure of the system, and V is the volume of the system.

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Heat at Constant Pressure The change in enthalpy of a system has no easily interpreted meaning except at constant pressure, where H = heat. H = H products - H reactants. At constant pressure, exothermic means H is negative; endothermic means H is positive.

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Enthalpy When 1 mole of methane is burned at constant pressure, 890 kJ of energy is released as heat. Calculate H for a process in which a 5.8-g sample of methane is burned at constant pressure.

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Calorimetry The device used experimentally to determine the heat associated with a chemical reaction is called a calorimeter. Calorimetry is the science of measuring heat. Heat capacity is the amount of heat required to raise an object’s temperature one degree. * C = heat absorbed. increase in temperature

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Heat Capacity Specific heat capacity is the energy required to raise the temperature of one gram of a substance by one degree Celsius. units are J/ o C. g or J/K. g. Molar heat capacity is the energy required to raise the temperature of one mole of a substance one degree Celsius. units are J/ o C. mol or J/K. mol.

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Heat Capacity Some common values are found in Table 6.1, p 251. * The higher the value, the longer it takes to heat an object and the longer it takes for that same object to cool.

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If the pressure remains constant, the process is called constant-pressure calorimetry. Calorimetry experiments can also be carried out under conditions of constant volume. If V = 0, P V = 0, and E = q v

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Constant-Pressure Calorimetry I When 1.00 L of 1.00 M Ba(NO 3 ) 2 solution at 25.0 o C is mixed with 1.00 L of 1.00 M Na 2 SO 4 solution at 25 o C in a calorimeter, the white solid BaSO 4 forms and the temperature of the mixture increases to 28.1 C. Assuming that the calorimeter absorbs only a negligible quantity of heat, that the specific heat capacity of the solution is 4.18 J/ o C g, and that the density of the final solution is 1.0 g/mL, calculate the enthalpy change per mole of BaSO 4 formed.

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Constant-Pressure Calorimetry II One piece of copper jewelry is placed at 100 o C has exactly twice the mass of another piece, which is at 40 o C. They are placed inside a calorimeter whose heat capacity is negligible. What is the final temperature inside the calorimeter? (C of copper = 0.387 J/g K).

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Constant-Pressure Calorimetry III A 505-g piece of copper tubing is heated to 99.9 o C and placed in a 59.8 g insulated vessel containing water at 24.8 o C. If the vessel has a constant heat capacity of 10.0 J/K, what is the final temperature of the system? (C of copper = 0.387 J/g K)

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