Use Hess’s Law to calculate the heat of combustion of dodecane using heat of for

Question

Use Hess’s Law to calculate the heat of combustion of dodecane using heat of formation data. For the calculation you will need the following:

(delta)fH of formation of carbon dioxide gas is –393.5 kJ/mol

water (liquid) is –285.83 kJ/mol.

For the heat of formation for dodecane, it would be useful to consult an online database of thermochemical data from the National Institute of Standards and Technology (NIST), which is a widely recognized database. http://webbook.nist.gov/chemistry/. Look up the heat of formation for dodecane using the search by name function and click on condensed phase thermochemistry data. Record the first fH that is listed and also record the molecular weight of dodecane for use in the laboratory.

Explanation / Answer

C12H26(l) + 18.5 O2(g) 12 CO2(g) + 13 H2O(l)   H = –7513 kJ

so heat of combustion of dodecame= heat of formation of the products- heat of formaiton of the reactants

= -12*393.5 - 13*285.83 - (-349.3)

=-8088.49 kJ/mol

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