A simple way of looking at the energy associated with the combustion of methane

Question

A simple way of looking at the energy associated with the combustion of methane [as shown in equation (1.15)] is to view the oxidation of the carbon by equation (1.13) and the oxidation of hydrogen by equation (1.19) C + O2 CO2 +32.8 MJ/kg CH, +202 CO2 +2H2+55.5 MJ/kg (1.15) 2H2 + O2 2H2O +142 MJ/kg Based on the energies involved in these processes:s (a) Calculate the energy given off per mole by combustion one mole of carbon (C) and four moles of hydrogen (2H). (b) Calculate the energy givern off by combustion of one mole of methane (CH). Note that the difference between these two values is the heat of formation of methane from carbon and hydrogen.) c a. (a) 174.8 M/mol (b) 5o M/mol C b. (o) 0.80 MJ mol (b) 0.96 MJ/mol C c. (a) 0.68 MJ/mol (b) 0.80 MJ/mol C d. (a) 50 MJ/mol (b) 174.8 MJ/mol C e, (a) 0.96 M/mol (b) 0.80 Mrmol

Explanation / Answer

Part a

Energy given off by combustion of one mole of C

C + O2 = CO2 + 32.8 MJ/kg

E1 = - (32.8 MJ/kg) x (12kg/kmol) x (1kmol/1000mol)

= - 0.4 MJ/mol

Energy given off by combustion of four moles of hydrogen

2H2 + O2 = 2H2O + 142 MJ/kg

E2 = - (142 MJ/kg) x (2kg/kmol) x (1kmol/1000mol)

= - 0.28 MJ/mol

Total energy given off

= - 0.4 - 0.28

= - 0.68 MJ/mol

Negative sign shows that the heat is given off

Part b

Energy given off by combustion of one mole of CH4

CH4 + 2O2 = CO2 + 2H2O + 55.5 MJ/kg

E = - (55.5 MJ/kg) x (16kg/kmol) x (1kmol/1000mol)

= - 0.80 MJ/mol

Option C is the correct answer

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