What is the standard enthalpy of formation (ΔH_f^ext{°}) value for CO_2(g) in kJ/mol?

• A. 0 kJ/mol
• B. -110.5 kJ/mol
• C. -393.5 kJ/mol
• D. +393.5 kJ/mol

Answer: (C)

Verified step by step guidance

1. Understand that the standard enthalpy of formation (\( \Delta H_f^{\circ} \)) of a compound is defined as the enthalpy change when one mole of the compound is formed from its elements in their standard states under standard conditions (usually 1 atm and 25°C).
2. Recognize that for carbon dioxide gas (\( \text{CO}_2(g) \)), the formation reaction from its elements in their standard states is: \n\n\( \text{C (graphite)} + \text{O}_2(g) \rightarrow \text{CO}_2(g) \)
3. Recall that the standard enthalpy of formation of an element in its standard state (like graphite for carbon or \( \text{O}_2 \) gas for oxygen) is defined as zero. This is why the enthalpy of formation for \( \text{CO}_2(g) \) is not zero.
4. Use standard thermodynamic tables or reliable data sources to find the value of \( \Delta H_f^{\circ} \) for \( \text{CO}_2(g) \), which is typically given in kJ/mol.
5. Interpret the sign and magnitude of the value: a negative value indicates that the formation of \( \text{CO}_2(g) \) from its elements releases energy (exothermic process), which is consistent with combustion reactions.