Combustion of graphite to form CO2 can be done by two ways. Reactions are given as follows: 1.C + O2 -> CO2 ∆H = -393.7 kJ mol-12.C + ½O2 -> CO ∆H = ?3.CO + ½O2 -> CO2 ∆H = -283 kJ mol-1What will be the enthalpy of the formation of CO?
Correct answer: B. -110 kJ mol-1
- A. -676 kJ mol-1
- B. -110 kJ mol-1
- C. 110 kJ mol-1
- D. 676 kJ mol-1
Explanation
Writing equation 1 as it is and flipping equation 3: 1.C+02--> CO2 H=-393.7 kJ 3.CO2 --> CO +1/2O2 H=+283 kJ Adding these two equations gives equation 2, hence the enthalpy of these two equations is also equal to the enthalpy of equation 2. -393.7+283-110kJ As it is numerical, it can have only one possible answer.
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About Hess's Law
Hess's law states that the enthalpy change of a reaction is independent of the route because enthalpy is a state function. Calculations involve reversing and adding thermochemical equations, using standard enthalpies of formation or combustion, and finding an unknown reaction enthalpy without confusing enthalpy change with reaction rate.
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