6.6 Enthalpy of Formation

• Arrange the standard state conditions for the standard enthalpy of formation in order of importance:

  1️⃣ Temperature: 298 K

  2️⃣ Pressure: 1 atm

  3️⃣ State of Elements: Most stable form at 298 K and 1 atm
• What is Hess's Law based on?

  Enthalpy change invariance
• The enthalpy change ($\Delta H$) represents the heat absorbed or released during a reaction
• What are the standard state conditions for temperature and pressure?

  298 K and 1 atm
• Steps to use Hess's Law

  1️⃣ Arrange the intermediate reactions to match the target reaction

  2️⃣ Multiply $\Delta H$ values by stoichiometric coefficients

  3️⃣ Reverse reactions if necessary and change the sign of $\Delta H$

  4️⃣ Sum the enthalpy changes to find the overall $\Delta H$
• The total enthalpy change for the formation of carbon dioxide is -393.5 kJ/mol.
• The enthalpy change for the formation of carbon dioxide using Hess's Law is -393.5 kJ/mol.
• The standard enthalpy of formation for CO₂(g) is -393.5 kJ/mol.
• The formula to calculate $\Delta H_{rxn}^\circ$ using standard enthalpies of formation is \Delta H_{rxn}^\circ
• What is the key difference between standard enthalpy of formation and enthalpy of reaction?

  Formation applies only to compounds from elements
• What is the formula to calculate $\Delta H_{rxn}^\circ$ using standard enthalpies of formation?


  $\Delta H_{rxn}^\circ =$$\sum \Delta H_{f}^\circ (\text{products}) - \sum \Delta H_{f}^\circ (\text{reactants})$
• Match the key concept with its description:

  Standard Enthalpy of Formation ↔️ Change in enthalpy when one mole of a substance is formed from its elements

  Standard State ↔️ 298 K and 1 atm

  $\Delta H_{f}^\circ$ ↔️ Symbol for standard enthalpy of formation
• Match the key concept with its definition:

  Standard Enthalpy of Formation ↔️ Enthalpy change when one mole of a substance is formed from its elements

  Standard States ↔️ Elements in their most stable form at 298 K and 1 atm

  Standard Conditions ↔️ 298 K and 1 atm
• In the example using Hess's Law to calculate the enthalpy change for the formation of carbon dioxide, the overall $\Delta H$ is -393.5 kJ/mol
• What does Hess's Law state about the enthalpy change of a reaction?

  It is path-independent
• The standard enthalpy of formation ($\Delta H_{f}^\circ$) is defined under standard conditions of 298 K and 1 atm
• Elements in their standard state refer to their most stable form at 298 K and 1 atm.


  True
• Steps to use Hess's Law

  1️⃣ Arrange intermediate reactions to match the target reaction

  2️⃣ Multiply $\Delta H$ values by stoichiometric coefficients

  3️⃣ Reverse reactions if necessary and change $\Delta H$ sign

  4️⃣ Sum the enthalpy changes
• What is the target reaction in the example for Hess's Law?

  C(s) + O₂(g) → CO₂(g)
• What is the standard enthalpy of formation for O₂(g)?

  0 kJ/mol
• Key steps to calculate $\Delta H_{rxn}^\circ$ using standard enthalpies of formation


  1️⃣ Identify reactants and products

  2️⃣ Plug values into the formula

  3️⃣ Multiply enthalpies by stoichiometric coefficients

  4️⃣ Sum the enthalpy changes
• Steps to calculate $\Delta H_{rxn}^\circ$ using standard enthalpies of formation


  1️⃣ Identify reactants and products and their corresponding standard enthalpies of formation

  2️⃣ Plug the values into the formula, multiplying by stoichiometric coefficients

  3️⃣ Subtract the sum of reactant enthalpies from the sum of product enthalpies
• The standard enthalpy of formation is the enthalpy change when one mole of a substance is formed from its elements
• The standard enthalpy of formation is used to calculate enthalpy changes using Hess's Law.

  True
• The standard enthalpy of formation for water is -285.8 kJ/mol.

  True
• Hess's Law states that the enthalpy change for a reaction is the same regardless of the reaction path.


  True
• When reversing a reaction in Hess's Law, the sign of $\Delta H$ must be changed.


  True
• The standard enthalpy of formation for an element in its standard state is zero.


  True
• What is the value of $\Delta H₂$ for the reaction $CO(g) +$$\frac{1}{2} O₂(g) → CO₂(g)$?


  -283.0 kJ/mol
• Hess's Law states that the enthalpy change for a reaction is the same regardless of the reaction path


  True
• The enthalpy change ($\Delta H$) represents the heat absorbed or released during a reaction.
• The standard enthalpy of formation for an element in its standard state is always zero.


  True
• What is the calculated value of $\Delta H_{rxn}^\circ$ for the reaction $CH₄(g) +$$2O₂(g) → CO₂(g) +$$2H₂O(g)$?


  -877.1 kJ/mol
• The standard enthalpy of reaction for the combustion of methane is calculated by subtracting the sum of the reactant enthalpies from the sum of the product enthalpies
• Match the enthalpy change with its definition:

  Standard Enthalpy of Formation ↔️ Enthalpy change to form one mole of a compound from its elements in their standard states

  Enthalpy of Reaction ↔️ Enthalpy change for any chemical reaction

  Bond Enthalpy ↔️ Enthalpy required to break one mole of a specific bond in the gas phase
• The standard enthalpy of formation is defined under standard conditions, which include a temperature of 298 K and a pressure of 1 atm
• Standard state conditions provide a consistent reference point for calculating enthalpy changes using Hess's Law
• Arrange the steps for using Hess's Law in the correct order:

  1️⃣ Arrange the intermediate reactions so they add up to the target reaction

  2️⃣ Multiply the $\Delta H$ values by their stoichiometric coefficients

  3️⃣ Reverse any reactions if necessary and change the sign of $\Delta H$

  4️⃣ Sum the enthalpy changes of the adjusted reactions
• To use Hess's Law, first arrange the intermediate reactions so they add up to the target
• What formula is used to calculate $\Delta H_{rxn}^\circ$ using standard enthalpies of formation?


  \Delta H_{rxn}^\circ = \sum \Delta H_{f}^\circ (\text{products}) - \sum \Delta H_{f}^\circ (\text{reactants})</latex>