Write the formation equation for CH₄ from the standard states of its elements and give its standard enthalpy of formation

Oxygen gas, O₂, is very reactive while argon gas is inert. But why do both oxygen gas and argon gas have zero standard enthalpies of formation?

Which of the following substances are in their standard state?

a) I₂(g) at 2 atm

b) C(l)

c) Br₂(g) at 25 °C

d) Fe(s)

e) CH₄(g) at 1 atm and 298 K

Provide the balanced formation equation for Na₂SiO₃(s) using elements in their standard states.