Question

Standard molar enthalpy of formation
of which of the following substances is
not zero ?
(1) C (diamond)
(2) C (graphite)
(3) H, (gas)
(4) Brą (liquid)​

Answer 1

Answer:

The Standard Enthalpy of formation is defined as the change in enthalpy when one mole of a substance in the standard state (1 atm of pressure and 298.15 K) is formed from its pure elements under the same conditions.

For carbon, we take the graphite to be most stable form. we calculate standard enthalpies of carbon compounds with reference to graphite. So standard enthalpy is zero for graphite. for others, it is non-zero.

(2) C (graphite) is the correct answer.