AP Chemistry Questions on Standard Enthalpies of Formation

📚 Quick Study Guide

• ⚛️ Standard Enthalpy of Formation ($\Delta H_f^\circ$): 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).
• 🌡️ Standard State: The most stable form of a substance at 298 K and 1 atm. For example, for oxygen, it's $O_2(g)$.
• 📝 Formula for Enthalpy Change of Reaction: $\Delta H_{rxn}^\circ = \sum n \Delta H_f^\circ (products) - \sum n \Delta H_f^\circ (reactants)$, where $n$ represents the stoichiometric coefficients.
• 🔥 Elements in their Standard States: By definition, the standard enthalpy of formation ($\Delta H_f^\circ$) of an element in its standard state is zero.
• 🔢 Hess's Law: The enthalpy change of a reaction is the same whether it occurs in one step or in a series of steps.
• 💡 Tips: Always pay attention to the states of matter (solid, liquid, gas, aqueous) as they affect the enthalpy of formation. Double-check the stoichiometric coefficients in the balanced equation!

🧪 Practice Quiz

1. Question 1: What is the standard enthalpy of formation of an element in its standard state?
   1. A) Always positive
   2. B) Always negative
   3. C) Zero
   4. D) Depends on the element
2. Question 2: Given the following reaction: $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$, which of the following is needed to calculate $\Delta H_{rxn}^\circ$?
   1. A) $\Delta H_f^\circ [H_2(g)]$ only
   2. B) $\Delta H_f^\circ [O_2(g)]$ only
   3. C) $\Delta H_f^\circ [H_2O(l)]$ only
   4. D) $\Delta H_f^\circ [H_2(g)]$, $\Delta H_f^\circ [O_2(g)]$, and $\Delta H_f^\circ [H_2O(l)]$
3. Question 3: Which of the following has $\Delta H_f^\circ = 0$?
   1. A) $O(g)$
   2. B) $O_3(g)$
   3. C) $O_2(g)$
   4. D) $H_2O(g)$
4. Question 4: Consider the reaction: $C(s) + O_2(g) \rightarrow CO_2(g)$. If $\Delta H_f^\circ [CO_2(g)] = -393.5 kJ/mol$, what is the enthalpy change for this reaction?
   1. A) 0 kJ/mol
   2. B) -393.5 kJ/mol
   3. C) 393.5 kJ/mol
   4. D) Cannot be determined
5. Question 5: Given $\Delta H_f^\circ [NH_3(g)] = -46 kJ/mol$, what is the enthalpy change for the reaction: $N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$?
   1. A) -46 kJ/mol
   2. B) -92 kJ/mol
   3. C) 46 kJ/mol
   4. D) 92 kJ/mol
6. Question 6: For the reaction: $2CO(g) + O_2(g) \rightarrow 2CO_2(g)$, given $\Delta H_f^\circ [CO(g)] = -110.5 kJ/mol$ and $\Delta H_f^\circ [CO_2(g)] = -393.5 kJ/mol$, calculate $\Delta H_{rxn}^\circ$.
   1. A) -283.0 kJ/mol
   2. B) -566.0 kJ/mol
   3. C) -806.0 kJ/mol
   4. D) -101.5 kJ/mol
7. Question 7: Which of the following is NOT required to calculate the standard enthalpy change of a reaction?
   1. A) Balanced chemical equation
   2. B) Standard enthalpies of formation of reactants
   3. C) Standard enthalpies of formation of products
   4. D) The rate of the reaction