8.2.1 Application of Hess's Law

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

  It is the same
• Hess's Law states that the total enthalpy change depends only on the initial and final states
• The overall enthalpy change for a reaction is the same regardless of the path taken.
• What does enthalpy change measure during a chemical reaction?

  Heat released or absorbed
• Exothermic reactions have a negative value of \Delta H
• Exothermic reactions release heat to the surroundings.
• What type of reaction absorbs heat from the surroundings?

  Endothermic
• In Hess's Law, a direct route involves the reactants transforming into products in a single step
• An indirect route in Hess's Law involves multiple intermediate steps.
• Arrange the steps of the indirect route for the formation of methane in the correct order.

  1️⃣ C(s) + ½O₂(g) → CO(g) $\Delta H_{1} =$$- 110.5 \text{ kJ / mol}$

  2️⃣ CO(g) + 2H₂(g) + ½O₂(g) → CH₄(g) + H₂O(l) $\Delta H_{2} =$$- 242 \text{ kJ / mol}$

  3️⃣ H₂O(l) → 2H₂(g) + O₂(g) $\Delta H_{3} =$$285.8 \text{ kJ / mol}$
• Match the reaction type with its corresponding enthalpy change (\Delta H</latex>).

  Exothermic ↔️ $\Delta H < 0$

  Endothermic ↔️ $\Delta H > 0$
• Exothermic reactions release heat, resulting in \latex{\Delta H < 0}
• Endothermic reactions absorb heat, leading to \latex{\Delta H > 0}.
• Match the reaction type with its enthalpy change and heat flow:

  Exothermic ↔️ \latex{\Delta H < 0}, Released

  Endothermic ↔️ \latex{\Delta H > 0}, Absorbed
• Enthalpy change is a key factor in Hess's Law, which calculates the enthalpy change regardless of the pathway
• Enthalpy change measures the heat released or absorbed during a chemical reaction at constant pressure.
• Exothermic reactions release heat, resulting in \latex{\Delta H < 0}
• Endothermic reactions absorb heat, leading to \latex{\Delta H > 0}.
• Match the reaction type with its enthalpy change and heat flow:

  Exothermic ↔️ \latex{\Delta H < 0}, Released

  Endothermic ↔️ \latex{\Delta H > 0}, Absorbed
• Enthalpy change is a key factor in Hess's Law, which calculates the enthalpy change regardless of the pathway
• Steps in applying Hess's Law to identify direct and indirect routes:

  1️⃣ Distinguish between direct and indirect routes

  2️⃣ Direct route involves a single step

  3️⃣ Indirect route involves multiple steps

  4️⃣ Calculate the enthalpy change for each step
• Hess's Law ensures that the overall enthalpy change is the same regardless of the path taken.
• Hess cycles illustrate how Hess's Law applies by showing the direct and indirect routes
• Steps in drawing a Hess cycle:

  1️⃣ Draw a single arrow for the direct route with \latex{\Delta H}

  2️⃣ Draw arrows for each step of the indirect route with respective \latex{\Delta H} values

  3️⃣ Ensure the total enthalpy change is the same for both routes
• What is a Hess cycle used for?

  Calculating enthalpy change
• A Hess cycle is a diagrammatic representation used to apply Hess's Law
• Steps to draw a Hess cycle

  1️⃣ Identify the reactants and products

  2️⃣ Determine the direct route

  3️⃣ Identify an indirect route
• The direct route in a Hess cycle represents the reaction from reactants to products in a single step.
• What is the overall goal of a Hess cycle?

  Calculate enthalpy change
• The first step in drawing a Hess cycle is to identify the reactants and products
• Hess's Law states that the overall enthalpy change of a reaction is independent of the path taken.
• Steps to draw a Hess cycle

  1️⃣ Identify the reactants and products

  2️⃣ Determine the direct route

  3️⃣ Identify an indirect route
• For the reaction $C(s) +$$2H_{2}(g) \rightarrow CH_{4}(g)$, the direct route is $C(s) +$$2H_{2}(g) \rightarrow CH_{4}(g)$itself
• What law is applied when using a Hess cycle?

  Hess's Law
• The indirect route in a Hess cycle involves intermediate steps to convert reactants to products.
• Steps to draw a Hess cycle

  1️⃣ Identify the reactants and products

  2️⃣ Determine the direct route

  3️⃣ Identify an indirect route
• What is one possible intermediate compound in the indirect route of the reaction $C(s) +$$2H_{2}(g) \rightarrow CH_{4}(g)$?


  $CO_{2}(g)$
• Hess cycles always involve at least two routes: a direct route and an indirect route.
• What is the term for the diagrammatic representation used to apply Hess's Law?

  Hess cycle
• In the indirect route of the reaction C(s) + 2H_{2}(g) \rightarrow CH_{4}(g)</latex>, one intermediate step is $C(s) +$$O_{2}(g) \rightarrow CO_{2}(g)$alone