Overview
This lecture explains how to use Hess’s Law to calculate the enthalpy change (ΔH) of a chemical reaction by combining given reactions with known enthalpy changes.
Hess’s Law Fundamentals
- Hess’s Law states that the total enthalpy change for a reaction is the same, no matter how the reaction is carried out.
- You can manipulate and combine chemical equations to obtain the desired reaction, adjusting the enthalpy changes accordingly.
Steps to Apply Hess’s Law
- Identify reactants and products in the target reaction that appear only once in the given equations.
- Avoid focusing on species present in more than one equation to simplify calculations.
- Reverse reactions if needed (reverse the sign of ΔH) and multiply/divide by coefficients (adjust ΔH proportionally).
- Line up the equations so canceling species leave only those in the target equation.
- Add adjusted enthalpy values to find the total ΔH for the desired reaction.
Worked Examples
- Example 1: Calculated ΔH for ammonia reacting with oxygen by reversing and multiplying equations, then adding ΔH values.
- Example 2: Found ΔH for hydrogen plus atomic oxygen forming water by scaling and reversing reactions, leading to the sum of adjusted ΔH values.
- Example 3: Determined ΔH for nitrogen monoxide plus atomic oxygen forming nitrogen dioxide by carefully manipulating and summing up three given equations.
Key Terms & Definitions
- Enthalpy Change (ΔH) — The heat absorbed or released during a chemical reaction at constant pressure.
- Hess’s Law — The principle stating the total enthalpy change is independent of the reaction path.
- Reactant — A starting substance in a chemical reaction.
- Product — A substance produced by a chemical reaction.
- Reverse Reaction — Flipping the direction of a chemical equation; changes the sign of ΔH.
Action Items / Next Steps
- Practice applying Hess’s Law with sample problems by manipulating equations and calculating total ΔH.
- Review definitions of enthalpy and related thermochemical terms.