8.2.1 Application of Hess's Law

• What does Hess's Law state about the enthalpy change of a reaction?

  Depends only on initial and final states
• According to Hess's Law, ΔH₁ = ΔH₂ + ΔH₃ for a direct and indirect pathway, respectively.
• In an indirect pathway, a reaction proceeds through one or more intermediate substances
• What is the relationship between the enthalpy change of a direct pathway (ΔH₁) and an indirect pathway (ΔH₂ + ΔH₃)?

  ΔH₁ = ΔH₂ + ΔH₃
• Identifying direct and indirect pathways is only necessary when the direct pathway's enthalpy change is unknown.
• Arrange the components of an energy cycle in the correct order.

  1️⃣ Reactants at the starting point

  2️⃣ Products as the final state

  3️⃣ Direct Pathway connecting reactants and products with ΔH₁

  4️⃣ Indirect Pathway through intermediate substances with ΔH₂ and ΔH₃
• Match the components of an energy cycle with their descriptions.

  Reactants ↔️ Starting materials of the reaction

  Products ↔️ Substances formed after the reaction

  Direct Pathway ↔️ Straight route from reactants to products

  Indirect Pathway ↔️ Route through intermediates
• To calculate enthalpy changes using energy cycles, the first step is to draw the energy cycle
• What is a direct pathway in a chemical reaction?

  Straight route to products
• An indirect pathway involves a route through intermediates
• Hess's Law states that ΔH₁ = ΔH₂ + ΔH₃ for a direct and indirect pathway, respectively.
• Steps to calculate enthalpy changes using energy cycles and Hess's Law

  1️⃣ Draw the Energy Cycle

  2️⃣ Determine Known Enthalpy Changes

  3️⃣ Apply Hess's Law

  4️⃣ Calculate ΔH₁
• Hess's Law depends on the specific reaction pathway taken to calculate the enthalpy change.

  False
• Why is Hess's Law important?

  Calculates unmeasurable ΔH
• Hess's Law uses alternative routes through intermediate substances
• According to Hess's Law, the total enthalpy change for a reaction is the same regardless of the pathway taken.
• Match the reaction pathway with its enthalpy change:

  Direct Pathway ↔️ ΔH₁

  Indirect Pathway ↔️ ΔH₂ + ΔH₃
• Components of an energy cycle in Hess's Law

  1️⃣ Reactants

  2️⃣ Products

  3️⃣ Direct Pathway (ΔH₁)

  4️⃣ Indirect Pathway (ΔH₂ + ΔH₃)
• In Hess's Law, ΔH₁ is equal to ΔH₂ + ΔH₃ for a complete reaction.
• What are the enthalpy changes for the indirect pathway in Hess's Law?

  ΔH₂ and ΔH₃
• Hess's Law states that ΔH₁ = ΔH₂ + ΔH₃, which aligns with the principle of energy conservation
• An energy cycle is a visual representation used in Hess's Law to illustrate direct and indirect pathways of a reaction.
• Match the components of an energy cycle with their descriptions:

  Reactants ↔️ Starting materials of the reaction

  Products ↔️ Substances formed after the reaction

  Direct Pathway ↔️ Straight route from reactants to products

  Indirect Pathway ↔️ Route through intermediates
• Steps to calculate enthalpy changes using energy cycles:

  1️⃣ Draw the Energy Cycle

  2️⃣ Determine Known Enthalpy Changes

  3️⃣ Apply Hess's Law

  4️⃣ Calculate ΔH₁
• Hess's Law can be used to determine standard enthalpy changes when direct measurement is impractical.
• Hess's Law states that ΔH° depends only on initial and final states
• What is the key principle of Hess's Law in terms of enthalpy changes?

  ΔH₁ = ΔH₂ + ΔH₃
• According to Hess's Law, the enthalpy change for the direct pathway is equal to the sum of the enthalpy changes for the indirect pathway.