6.7 Hess's Law

• The overall enthalpy change for a reaction is the sum of the enthalpy changes for each individual step.


  True
• What is enthalpy change (ΔH) in a chemical reaction?

  Heat energy released or absorbed
• The overall enthalpy change for a reaction is the same whether it occurs in one step or multiple steps.


  True
• Hess's Law states that the overall enthalpy change for a chemical reaction is the same whether it occurs in one step or multiple steps
• What is the fundamental principle of Hess's Law regarding the overall enthalpy change of a reaction?

  Independent of reaction pathway
• What is the key concept of Hess's Law regarding the enthalpy change of a reaction?

  Overall ΔH is pathway-independent
• Exothermic reactions have a negative ΔH and release heat to the surroundings.


  True
• Steps to calculate enthalpy change using Hess's Law:

  1️⃣ Identify the target reaction

  2️⃣ Arrange known reactions

  3️⃣ Adjust reactions if needed

  4️⃣ Sum adjusted enthalpy changes
• Steps to apply Hess's Law to calculate the enthalpy change of a target reaction.

  1️⃣ Identify the target reaction

  2️⃣ Arrange known reactions

  3️⃣ Adjust reactions if needed

  4️⃣ Sum adjusted enthalpy changes
• Match the action with its explanation in Hess's Law.

  Identify the target reaction ↔️ Clearly state the reaction whose ΔH needs to be calculated

  Adjust reactions if needed ↔️ Reverse or multiply reactions to match the target, adjusting ΔH

  Sum adjusted enthalpy changes ↔️ Add the ΔH values of each step to find the overall ΔH
• Multiplying equation 1 (C(s) + ½ O₂(g) → CO(g)) by 2 results in a new ΔH'₁ value of -221.0 kJ/mol.
• Hess's Law can be used to calculate the enthalpy of formation of a compound.


  True
• Match the reaction step with its enthalpy change:

  Step 1 ↔️ ΔH₁

  Step 2 ↔️ ΔH₂

  Step 3 ↔️ ΔH₃

  Overall ↔️ ΔH₁ + ΔH₂ + ΔH₃
• Order the steps to calculate the overall enthalpy change using Hess's Law:

  1️⃣ Identify the target reaction

  2️⃣ Break the target reaction into steps

  3️⃣ Determine the enthalpy change for each step

  4️⃣ Sum the enthalpy changes
• What is the fundamental principle of Hess's Law?

  Overall ΔH is constant
• What does the enthalpy change (ΔH) measure in a chemical reaction?

  Heat energy released or absorbed
• The total ΔH for a reaction is the sum of the enthalpy changes for each individual step, regardless of the reaction pathway.


  True
• Hess's Law states that the enthalpy change for a chemical reaction is the same whether the reaction occurs in one step or in multiple steps
• For the formation of CO₂(g) from C(s) and O₂(g), the overall enthalpy change is -393.5 kJ/mol
• What is the enthalpy change for the decomposition of calcium carbonate (CaCO₃)?

  +178 kJ/mol
• To calculate the ΔH for CO₂(g) → CO(g) + ½ O₂(g), you must reverse reaction 2, resulting in a ΔH'₂ value of +283.0 kJ/mol.
• What is the new enthalpy change (ΔH'₂) for the reversed reaction CO₂(g) → CO(g) + ½ O₂(g)?

  +283.0 kJ/mol
• Reversing a reaction in Hess's Law requires changing the sign of its enthalpy change.


  True
• Steps to calculate the enthalpy change for the reaction 2CO(g) + O₂(g) → 2CO₂(g) using Hess's Law.

  1️⃣ Multiply equation 1 by 2

  2️⃣ Multiply equation 2 by 2

  3️⃣ Reverse equation 1

  4️⃣ Combine equations
• What is one common application of Hess's Law in AP Chemistry?

  Combustion enthalpy calculations
• Hess's Law states that the enthalpy change for a chemical reaction is the same whether the reaction occurs in one step or in multiple steps
• The overall enthalpy change in a reaction is independent of the reaction pathway.


  True
• Hess's Law states that the sum of enthalpy changes for each step equals the total enthalpy change of the reaction
• What does it mean for enthalpy change to be independent of the pathway in Hess's Law?

  Total ΔH remains constant
• Hess's Law states that the overall enthalpy change (ΔH) for a reaction is the same, whether the reaction occurs in one step or in multiple steps
• Match the step with its corresponding enthalpy change:

  Step 1 ↔️ ΔH₁

  Step 2 ↔️ ΔH₂

  Step 3 ↔️ ΔH₃

  Overall ΔH ↔️ ΔH₁ + ΔH₂ + ΔH₃
• If a reaction can be broken into steps with known enthalpy changes, their sum equals the overall enthalpy change.

  True
• Match the reaction type with its enthalpy change and heat release/absorption:

  Exothermic ↔️ Negative ΔH, heat released

  Endothermic ↔️ Positive ΔH, heat absorbed
• To calculate the enthalpy change of a target reaction using Hess's Law, the first step is to identify the target reaction
• Hess's Law states that the enthalpy change of a reaction is independent of the path taken to reach the final state.


  True
• When reaction 1 (C(s) + ½ O₂(g) → CO(g)) and the reversed reaction 2 are combined, the final ΔH for the reaction C(s) + O₂(g) → CO₂(g) is +172.5 kJ/mol.
• What is the overall ΔH for the reaction CO₂(g) → C(s) + O₂(g) using the given enthalpy changes?

  +172.5 kJ/mol
• What is the final ΔH for the reaction 2CO(g) + O₂(g) → 2CO₂(g) using Hess's Law?

  -566.0 kJ/mol