For which of the following reactions is the enthalpy change for the reaction equal to the standard enthalpy of formation (ΔHf°) of the product?

• A. CO (g) + ½ O2 (g) → CO2 (g)
• B. 2 H (g) → H2 (g)
• C. N2 (g) + 3 H2 (g) → 2 NH3 (g)
• D. H2 (g) + Cl2 (g) → 2 HCl (g)

Answer: (A)

Verified step by step guidance

1. Understand the concept of standard enthalpy of formation (ΔHf°): It is the enthalpy change when one mole of a compound is formed from its elements in their standard states.
2. Identify the reaction that forms one mole of a compound from its elements in their standard states. This is crucial because ΔHf° is defined for the formation of one mole of a compound.
3. Examine each reaction: - CO (g) + ½ O2 (g) → CO2 (g) forms one mole of CO2 from its elements, carbon monoxide and oxygen. - 2 H (g) → H2 (g) forms hydrogen gas from hydrogen atoms, not from elements in their standard states. - N2 (g) + 3 H2 (g) → 2 NH3 (g) forms two moles of ammonia, not one. - H2 (g) + Cl2 (g) → 2 HCl (g) forms two moles of hydrogen chloride, not one.
4. Recognize that the reaction CO (g) + ½ O2 (g) → CO2 (g) fits the definition of standard enthalpy of formation because it forms one mole of CO2 from its elements in their standard states.
5. Conclude that the enthalpy change for the reaction CO (g) + ½ O2 (g) → CO2 (g) is equal to the standard enthalpy of formation (ΔHf°) of CO2.