Standard enthalpy of formation examples

📚 Quick Study Guide

• 🌡️ The standard enthalpy of formation ($\Delta H_f^\circ$) is the change in enthalpy when one mole of a substance is formed from its elements in their standard states (usually 298 K and 1 atm).
• ✔️ The standard state of an element is its most stable form under standard conditions (e.g., O₂(g) for oxygen, C(s, graphite) for carbon).
• 🔥 $\Delta H_f^\circ$ is zero for elements in their standard states.
• ⚗️ The enthalpy change for a reaction ($\Delta H_{rxn}^\circ$) can be calculated using the standard enthalpies of formation: $\Delta H_{rxn}^\circ = \sum n \Delta H_f^\circ(products) - \sum n \Delta H_f^\circ(reactants)$, where *n* represents the stoichiometric coefficients.
• 🔢 Units for $\Delta H_f^\circ$ are typically kJ/mol.
• 💡 A negative $\Delta H_f^\circ$ indicates an exothermic formation (releases heat), while a positive $\Delta H_f^\circ$ indicates an endothermic formation (requires heat).

🧪 Practice Quiz

1. What is the standard enthalpy of formation ($\Delta H_f^\circ$) for an element in its standard state?
   1. A. Always positive
   2. B. Always negative
   3. C. Zero
   4. D. Depends on the element
2. Which of the following represents the correct formation reaction for CO₂(g)?
   1. A. C(g) + O₂(g) → CO₂(g)
   2. B. C(s, diamond) + O₂(g) → CO₂(g)
   3. C. C(s, graphite) + O₂(g) → CO₂(g)
   4. D. CO(g) + O(g) → CO₂(g)
3. The standard enthalpy of formation of H₂O(l) is -286 kJ/mol. What does this indicate?
   1. A. The formation of H₂O(l) from its elements is endothermic.
   2. B. The formation of H₂O(l) from its elements is exothermic.
   3. C. The decomposition of H₂O(l) into its elements releases energy.
   4. D. No heat is exchanged during the formation of H₂O(l).
4. Which of the following compounds would you expect to have a $\Delta H_f^\circ$ of zero?
   1. A. O₃(g)
   2. B. Fe(s)
   3. C. H₂O(g)
   4. D. CO(g)
5. Given the reaction: 2H₂(g) + O₂(g) → 2H₂O(g), how can you calculate the enthalpy change of the reaction using standard enthalpies of formation?
   1. A. $\Delta H_{rxn}^\circ$ = 2$\Delta H_f^\circ$(H₂O(g)) - [2$\Delta H_f^\circ$(H₂(g)) + $\Delta H_f^\circ$(O₂(g))]
   2. B. $\Delta H_{rxn}^\circ$ = [2$\Delta H_f^\circ$(H₂(g)) + $\Delta H_f^\circ$(O₂(g))] - 2$\Delta H_f^\circ$(H₂O(g))
   3. C. $\Delta H_{rxn}^\circ$ = $\Delta H_f^\circ$(H₂O(g)) - [$\Delta H_f^\circ$(H₂(g)) + $\Delta H_f^\circ$(O₂(g))]
   4. D. $\Delta H_{rxn}^\circ$ = $\Delta H_f^\circ$(H₂(g)) + $\Delta H_f^\circ$(O₂(g)) - $\Delta H_f^\circ$(H₂O(g))
6. Why is it important to specify the state (e.g., solid, liquid, gas) when reporting standard enthalpies of formation?
   1. A. The state does not affect the enthalpy of formation.
   2. B. Different states have different energy contents, thus affecting $\Delta H_f^\circ$.
   3. C. It only matters for elements, not compounds.
   4. D. It's just a convention, it has no scientific significance.
7. Which of the following is NOT a standard condition for standard enthalpy of formation?
   1. A. 298 K
   2. B. 1 atm
   3. C. 1 M concentration
   4. D. Elements in their standard states