Question

question 4: for which of the following equations is δh = δhf° for no₂(g)? a) n₂(g) + 2o₂(g) → 2no₂(g) b) no(g) + ½o₂(g) → no₂(g) c) ½n₂o(g) + 3/4o₂(g) → no₂(g) d) ½n₂(g) + o₂(g) → no₂(g) e) n(g) + 2o(g) → no₂(g) multiple choice

Explanation:

The standard enthalpy of formation ($\Delta H_f^{\circ}$) is defined as the enthalpy change when one mole of a compound is formed from its elements in their standard states.

• In option A, 2 moles of $\ce{NO2}$ are formed, so $\Delta H$ is not $\Delta H_f^{\circ}$ for one mole of $\ce{NO2}$.
• In option B, $\ce{NO}$ is a compound, not an element in its standard state.
• In option C, $\ce{N2O}$ is a compound, not an element in its standard state.
• In option D, $\ce{N2}$ and $\ce{O2}$ are elements in their standard states ($\ce{N2}$ is diatomic gas, $\ce{O2}$ is diatomic gas), and one mole of $\ce{NO2}$ is formed.
• In option E, $\ce{N}$ and $\ce{O}$ are not in their standard states (standard state of nitrogen is $\ce{N2}$, of oxygen is $\ce{O2}$).

Answer:

D. $\frac{1}{2}\ce{N2}(g)+\ce{O2}(g)\to\ce{NO2}(g)$