C(graphite) + O2 yields CO2gas Enthalpychange -393.5 kj/mol H2gas + 1/2O2 gas yi

Question

C(graphite) + O2 yields CO2gas     Enthalpychange -393.5 kj/mol
H2gas + 1/2O2 gas yields H2O liquid   Enthalpy change-285.8 kj/mol
2C2H6 gas + 7O2gas yields 4CO2 gas + 6H2O l   Enthalpychange -3119.6 kj/mol

using this calculate the enthalpy change for
2C(graphite) + 3H2 yields C2H6 gas

Explanation / Answer

Double the 1st equation: 2C + 2O2 ->2CO2               H = 2(-393.5)= -787 kj/mol Triple the 2nd equation: 3H2 + 3/2O2 ->3H2O           H = 3(-285.8)= -857.4 kj/mol Flip & take half of the 3rd eq: 2CO2 + 3H2O -> C2H6 +7/2O2     H = (-1/2)(-3119.6) = 1559.8kj/mol noticed if you add up all the equations above. Final: 2C + 3H2 -> C2H6 Add up all the enthalpy: -787 + -857.4+1559.8 = -84.6 kj/mol

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