AP Chemistry Flashcards: Enthalpy Of Formation

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This deck focuses on Enthalpy Of Formation, giving you a quick way to review the definitions, rules, and examples that matter most for AP Chemistry.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: What is the sign of enthalpy for an exothermic formation reaction?

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

Flashcard 2: Identify the enthalpy change for the formation of 1 mole of $HCl(g)$ from $H_2(g)$ and $Cl_2(g)$.

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

Flashcard 3: Describe the enthalpy of formation for a compound with high stability.

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

Flashcard 4: 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.

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

Flashcard 5: What is the unit for enthalpy of formation?

Answer: kJ/mol. Energy per mole of substance formed.

Flashcard 6: What is the standard enthalpy of formation ($\text{ΔH}_f^\text{°}$) for an element in its standard state?

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

Flashcard 7: What is the enthalpy of formation for sodium chloride, $NaCl(s)$?

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

Flashcard 8: Define enthalpy of formation.

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

Flashcard 9: What is the enthalpy change when forming 1 mole of water from its elements under standard conditions?

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

Flashcard 10: What is the enthalpy of formation for carbon monoxide, $CO(g)$?

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

Flashcard 11: 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.

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

Flashcard 12: How is enthalpy of formation used in Hess's Law?

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

Flashcard 13: Identify the enthalpy change for forming 1 mole of $CH_3OH(l)$ from its elements.

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

Flashcard 14: What is the enthalpy of formation for hydrochloric acid in aqueous solution, $HCl(aq)$?

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

Flashcard 15: 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.

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

Flashcard 16: What is the standard enthalpy of formation for oxygen gas, $O_2(g)$?

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

Flashcard 17: Identify the standard conditions for measuring enthalpy of formation.

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

Flashcard 18: Identify the enthalpy change for forming 1 mole of $C_2H_5OH(l)$ from its elements.

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

Flashcard 19: Which process typically has a positive enthalpy of formation: formation of bonds or breaking bonds?

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

Flashcard 20: What is the enthalpy of formation for methane, $CH_4(g)$?

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

Flashcard 21: What is the standard enthalpy of formation for glucose, $C_6H_{12}O_6(s)$?

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

Flashcard 22: What is the enthalpy of formation for benzene, $C_6H_6(l)$?

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

Flashcard 23: What is the enthalpy of formation for sulfur dioxide, $SO_2(g)$?

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

Flashcard 24: What is the enthalpy of formation for hydrogen peroxide, $H_2O_2(l)$?

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

Flashcard 25: What is the enthalpy of formation for hydrogen gas, $H_2(g)$?

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

Flashcard 26: Which substance has an enthalpy of formation of 0: $N_2(g)$, $N_2O(g)$, or $NH_3(g)$?

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

Flashcard 27: What is the enthalpy of formation for nitric oxide, $NO(g)$?

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

Flashcard 28: What is the sign of enthalpy for an exothermic formation reaction?

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

Flashcard 29: What is the enthalpy of formation for nitrogen dioxide, $NO_2(g)$?

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

Flashcard 30: State the formula for calculating the enthalpy change of a reaction using formation enthalpies.

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