3.4: Calculate the Molar Enthalpy Change (△H) from the Heat Energy Change, Q

MOLAR ENTHALPY CHANGE:

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Data from a Calorimetry Experiment can be used to calculate the molar enthalpy change of reaction:

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EQUATION:

* m = Mass of Water or Volume of Water

*If Water is not used, this is replaced by the mass or volume of other solutions

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Example:

Energy from burning 0.5 g of Propane was transferred to 100 cm3 of Water to raise its temperature by 20°C. Calculate the molar enthalpy change (in KJ / Mol)

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Mass of Water =     100 cm3 Heat Capacity of Water =     4.2 j / g Temperature Rise     =     20°C Energy Transferred     =     100    x    4.2    x    20    =    8400 J *1000 J = 1 KJ So 8400 J   =  8.4 KJ Mr of Propane = 44 Moles of Propane Burned     =     0.5     ÷     44     =     0.01136 Molar Enthalpy Change     =     8.4     ÷     0.01136     =     739 Reaction is exothermic so enthalpy change needs to be Negative                                                                     Molar Enthalpy Change = - 739 KJ / Mol