Enthalpy of Formation - AP Chemistry

1. What is the standard enthalpy of formation for oxygen gas, $O_2(g)$?

   0 kJ/mol. Diatomic oxygen is an element in its standard state.

2. Identify the enthalpy change for forming 1 mole of $CH_3OH(l)$ from its elements.

   -238.7 kJ/mol. Standard reference value for methanol formation enthalpy.

3. What is the enthalpy of formation for ammonia, $NH_3(g)$?

   -45.9 kJ/mol. Standard reference value for ammonia formation enthalpy.

4. What is the enthalpy of formation for carbon monoxide, $CO(g)$?

   -110.5 kJ/mol. Standard reference value for CO formation enthalpy.

5. What is the enthalpy of formation for hydrogen gas, $H_2(g)$?

   0 kJ/mol. Diatomic hydrogen is an element in its standard state.

6. What is the enthalpy change when forming 1 mole of water from its elements under standard conditions?

   -285.8 kJ/mol. Standard enthalpy of formation for liquid water.

7. What is the enthalpy of formation for hydrochloric acid gas, $HCl(g)$?

   -92.3 kJ/mol. Standard reference value for gaseous HCl formation enthalpy.

8. What is the enthalpy of formation for hydrogen peroxide, $H_2O_2(l)$?

   -187.8 kJ/mol. Standard reference value for H₂O₂ formation enthalpy.

9. Which process typically has a positive enthalpy of formation: formation of bonds or breaking bonds?

   Breaking bonds. Bond breaking requires energy input (endothermic).

10. What is the sign of enthalpy for an exothermic formation reaction?

    Negative. Exothermic reactions release heat (negative $\Delta H$).

11. What is the unit for enthalpy of formation?

    kJ/mol. Energy per mole of substance formed.

12. What is the enthalpy of formation for liquid water, $H_2O(l)$?

    -285.8 kJ/mol. Standard reference value for liquid water formation enthalpy.

13. Calculate the standard enthalpy change for $C(s) + O_2(g) \rightarrow CO_2(g)$ given $\text{ΔH}_f^\text{°}(CO_2(g)) = -393.5$ kJ/mol.

    -393.5 kJ/mol. Formation reaction equals the formation enthalpy directly.

14. State the formula for calculating the enthalpy change of a reaction using formation enthalpies.

    $\text{ΔH}_\text{reaction} = \text{ΣΔH}_f^\text{°}(\text{products}) - \text{ΣΔH}_f^\text{°}(\text{reactants})$. Sum product enthalpies, subtract reactant enthalpies.

15. Describe the enthalpy of formation for a compound with high stability.

    Typically more negative. Stable compounds release more energy upon formation.

16. How is enthalpy of formation used in Hess's Law?

    To calculate enthalpy changes of reactions using known formation enthalpies. Formation enthalpies enable indirect enthalpy calculations.

17. Identify the enthalpy change for forming 1 mole of $C_2H_5OH(l)$ from its elements.

    -277.38 kJ/mol. Standard reference value for ethanol formation enthalpy.

18. What is the enthalpy of formation for sodium chloride, $NaCl(s)$?

    -411 kJ/mol. Standard reference value for NaCl formation enthalpy.

19. Identify the standard conditions for measuring enthalpy of formation.

    1 atm pressure, 25°C (298 K). Standard temperature and pressure conditions for thermodynamics.

20. Calculate the enthalpy change for $N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)$ given $\text{ΔH}_f^\text{°}(NH_3(g)) = -45.9$ kJ/mol.

    -91.8 kJ/mol. Formation of 2 moles: $2 × (-45.9) = -91.8$ kJ.

21. What is the standard enthalpy of formation ($\text{ΔH}_f^\text{°}$) for an element in its standard state?

    0 kJ/mol. Elements in their standard state are the reference point (zero).

22. Define enthalpy of formation.

    Heat change when 1 mole of a compound forms from elements in standard states. The fundamental definition of $\Delta H_f^°$.

23. Which substance has an enthalpy of formation of 0: $N_2(g)$, $N_2O(g)$, or $NH_3(g)$?

    $N_2(g)$. Nitrogen gas is an element in its standard state.

24. What is the enthalpy of formation for methane, $CH_4(g)$?

    -74.8 kJ/mol. Standard reference value for methane formation enthalpy.

25. Identify the enthalpy change for the formation of 1 mole of $HCl(g)$ from $H_2(g)$ and $Cl_2(g)$.

    -92.3 kJ/mol. Standard reference value for HCl gas formation enthalpy.

26. What is the enthalpy of formation for nitric oxide, $NO(g)$?

    90.3 kJ/mol. Standard reference value for NO formation enthalpy.

27. What is the enthalpy of formation for iron(III) oxide, $Fe_2O_3(s)$?

    -824.2 kJ/mol. Standard reference value for iron oxide formation enthalpy.

28. What is the enthalpy of formation for hydrochloric acid in aqueous solution, $HCl(aq)$?

    -167.2 kJ/mol. Aqueous HCl has different enthalpy than gas phase.

29. What is the standard enthalpy of formation for glucose, $C_6H_{12}O_6(s)$?

    -1273 kJ/mol. Standard reference value for glucose formation enthalpy.

30. What is the enthalpy of formation for benzene, $C_6H_6(l)$?

    49.0 kJ/mol. Standard reference value for benzene formation enthalpy.

31. What is the enthalpy of formation for nitrogen dioxide, $NO_2(g)$?

    33.2 kJ/mol. Standard reference value for NO₂ formation enthalpy.

32. What is the enthalpy of formation for sulfur dioxide, $SO_2(g)$?

    -296.8 kJ/mol. Standard reference value for SO₂ formation enthalpy.

33. What is the enthalpy of formation for carbon dioxide, $CO_2(g)$?

    -393.5 kJ/mol. Standard reference value for CO₂ formation enthalpy.

34. Calculate the enthalpy change for $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$ given $\text{ΔH}_f^\text{°}(\text{H}_2O(l)) = -285.8$ kJ/mol.

    -571.6 kJ/mol. Formation of 2 moles: $2 × (-285.8) = -571.6$ kJ.

35. Identify the enthalpy change for forming 1 mole of $N_2O(g)$ from $N_2(g)$ and $O_2(g)$.

    82.05 kJ/mol. Standard reference value for N₂O formation enthalpy.

Study these flashcards