Question

Determine the amount of heat produced if 100.00 grams of Cystein (C3H7NO2S) was burned in excess oxygen to produce carbon dioxide, water, and nitrogen gas. Include the molecular formula and balanced chemical equation for the combustion reaction in your answer.

Answer 1

The balanced combustion equation is given by  

C3H7NO2S +   5.75 O2   ---> 3CO2 + 3.5 H2O + NO2 + SO2

( N gets oxidised to NO2 , S to SO2 while C to CO2)

now dH reaction = dH products - dH reactants

          = dH (SO2) + dH ( NO2) + 3dH ( CO2) +3.5 dH H2O - dH ( C3H7NO2S) - 5.75 dH ( O2)

         = -296.84 + 33.2 + 3( -393.5) + 3.5 ( -241.8) - (-534) -5.75 ( 0)       ( units are KJ/mol for each)

           = -1756.44 KJ/mol

Per 1 moles compound heat released = 1756.44 KJ

we had 100 g , we find moles

Moles = masss /molar mass of compound

       = 100 / 121.16 g/mol

        = 0.825355

Hence heat released = dH combustion x moles of cystein

          = 1756.4 KJ/mol x 0.825355 moles

        = 1450 KJ

( Note : had taken enthalpies of formation of each compound from web, if table is provided we need to take from table , dHo of compound)