Thermochemistry+3b+-+Hess+Law

Unit 1: Thermochemistry

Hess's Law Overview

  • Hess's Law: The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same.

  • Law of conservation of energy states that the heat of reaction is constant, whether through one or multiple reactions.

  • Useful in situations where direct measurement of enthalpy change is impractical due to cost, rarity, or reaction time.

Predicting Enthalpy Changes

  • Enthalpy change (DrH) can be predicted through:

    • Algebraic Combination of Reactions: Long and detailed method.

    • Standard Enthalpies of Formation: Short and simplified approach.

Practical Examples Using Hess's Law

Example of Simple Reactions

  1. Combustion reactions might need transformations:

    • C(s) + ½ O2(g) → CO(g) (ΔfH = -110.5 kJ)

    • CO(g) + ½ O2(g) → CO2(g) (ΔfH = -283.0 kJ)

    • Overall: C(s) + O2(g) → CO2(g) (ΔfH = -393.5 kJ)

Algebraic Steps for Combining Reactions

  1. Write the net reaction if it is not provided.

  2. Manipulate equations so they add to the net equation.

  3. If equations are multiplied or divided, adjust ΔH accordingly.

  4. If an equation is reversed, invert ΔH's sign.

  5. Simplify by canceling entities common on both sides.

  6. End result should be the desired net equation.

Sample Calculation of ΔrH

  • Target Reaction: H2O2(l) → H2O(l) + ½ O2(g)

  • Given Reaction Steps:

    • H2O2(l) → H2(g) + O2(g) (ΔdH = +187.8 kJ)

    • H2(g) + ½ O2(g) → H2O(l) (ΔfH = -285.8 kJ)

  • Final ΔrH calculated to be -98.0 kJ.

Model Reactions and Calculations

  • In the steel industry example:

    • Fe2O3(s) + 3 CO(g) → 3 CO2(g) + 2 Fe(s)

    • Adjustments for enthalpy (ΔH) based on standard reaction enthalpies.

Guided Practice Problems

  1. Combustion of Acetylene:

    • Calculate ΔrH for the combustion reaction using standard enthalpy changes.

  2. Formation of Wolfram Bromides:

    • Calculate the standard enthalpy change for the reaction involving WBr4(s) and WBr6(s).

  3. Oxidation of Ethyl Alcohol:

    • Calculate heat of reaction for the conversion of ethyl alcohol to acetic acid using Hess's Law.

Conclusion

  • Hess's Law is an essential tool in thermochemistry that allows for the calculation of enthalpy changes in complex reactions. Using both algebraic methods and standard enthalpy of formation data provides a comprehensive approach to understanding how energy is transferred in chemical processes.