which of these equations represents the standard enthalpy of formation of carbon dioxide?

Question

and explain your answer

a. C(s) + O2(g) ---> CO2(g)
b. CO(g) + 1/2O2(g) ---> CO2(g)
c. 2C(s) + 2O2(g) ---> CO2(g)
d. CaCO3(s) + CaO(s) ---> CO2(g)

help me please, i don't understand it.

thank u!

Answer 1

the first one. standard enthalpy of formation is the enthalpy for the reaction between elements in their standard state (298K temperature and 1 atmosphere pressure) to produce the relevant compound.

CO, CaCO3, CaO are not elements (they contain more than one type of atom), so b and d are out. c is not balanced, so it's out too.

Answer 2

a