Chapter 17, Problem 75A

Interpretation Introduction

Interpretation: The exception among a group of elements is to be found out.

Concept introduction: The standard enthalpy of formation is the amount of energy released or consumed when one mole of a substance is formed under standard conditions from its pure elements.

Standard conditions of temperature and pressure are $\text{273 K}$ and $\text{1 atm}$ .

Answer to Problem 75A

The exception among the given elements is $\text{CO(g)}$ .

Explanation of Solution

The standard enthalpy of formation for any free element in its standard state for that matter is zero as no energy is involved when they are formed from themselves. Moreover, all such free elements are stable and occur naturally.

  $\text{Na(s)}$ , ${\text{O}}_2\text{(g)}$ , ${\text{Br}}_2\text{(l)}$ , $\text{Fe(s)}$ and $\text{He(g)}$ all are stable and occur naturally and thus heat of formation for all of them is zero, where as $\text{CO(g)}$ is formed from its elements carbon and oxygen and thus standard heat of formation for $\text{CO(g)}$ will not be zero.

Conclusion

Thus, the exception is $\text{CO(g)}$ .