Hess's Law and Its Applications in Chemistry

Course Structure Overview

  • Completion of three topics
  • Upcoming instructions: one additional lab, a second quiz, one review day, and a unit test.
  • Schedule for the unit test: scheduled a day before teacher convention in two weeks.
  • Note: Two short weeks are coming up back-to-back.

Hess's Law

Definition and Explanation

  • Hess's law states that:
    • Chemical reactions can be expressed as the algebraic sum of two or more reactions.
    • The total enthalpy change for the reaction is the algebraic sum of the enthalpy changes for the individual steps.
    • This means that if you have multiple reactions, you can eliminate all reactants and products to derive a target reaction by combining the enthalpic changes of the individual reactions.

Application of Hess's Law

  • First Law: This is referred to as Hess's first law.
    • Used when provided with a list of smaller reactions.
    • Objective: Combine smaller reactions to derive a larger or net reaction.
  • Manipulation Steps for Hess's Law:
    1. Identify all reactions that contribute to the final reaction.
    2. Manipulate the given reactions (e.g., multiply, divide, or reverse) to yield the net reaction.

General Rules for Manipulation

  • When multiplying or dividing the equation by a constant:
    • Perform the same operation to the enthalpy change ($\Delta H$).
  • For reversing a reaction:
    • The enthalpy change ($\Delta H$) must be multiplied by -1.

Important Notes

  • Reaction Manipulation: Perform the same manipulation to the enthalpy change as done to the reactions.
    • If a reaction is reversed, it needs an adjustment to the enthalpy value ($\Delta H \times -1$).
    • For multiplying the entire reaction by a number X, $\Delta H$ becomes $\Delta H \times X$.
  • Reactants and Products:
    • Add the reactants and products from each reaction to yield the net reaction.
  • Cancellation Process:
    • To cancel unwanted chemicals, ensure:
    • The same number of each chemical appears on both sides of the equation.
    • Unwanted chemicals can be eliminated if they are equal in quantity on both sides.

Example Application

  • Begin with straightforward examples involving only two reactions to illustrate the principles effectively.
  • Designate reactions for reference:
    • Label them as reaction number one and reaction number two for clarity.
    • Address any anomalies in chemical representation with a note that such exceptions exist in chemistry.