Hess' Law and Enthalpy Cycles | A-level Chemistry | OCR, AQA, Edexcel

Introduction

This tutorial explains Hess' Law and enthalpy cycles, which are essential concepts in A-level Chemistry. Understanding these topics is crucial for calculating enthalpy changes indirectly when direct measurements are not possible. This guide will help you grasp these concepts and apply them effectively in your studies.

Step 1: Understand Hess' Law

Hess' Law states that the total enthalpy change of a reaction is independent of the pathway taken between reactants and products. This means that you can calculate the enthalpy change regardless of whether the reaction occurs in one step or multiple steps.

Key Points

• The enthalpy change depends only on the initial and final states, not on the reaction route.
• This concept allows you to use known enthalpy changes to find unknown values.

Step 2: Learn About Enthalpy Cycles

Enthalpy cycles are visual representations that illustrate the different routes a reaction can take. They help in calculating enthalpy changes indirectly.

Characteristics of Enthalpy Cycles

• Triangle Shape: The cycle is typically represented as a triangle.
• Chemical Species at Corners: Each corner of the triangle represents a different chemical species involved in the reaction.
• Arrows as Reaction Paths: The sides of the triangle are arrows that indicate the direction of the chemical reaction.

Practical Advice

• Ensure that the number of each type of atom is the same in all corners of the triangle, reflecting the conservation of matter.
• Use the known enthalpy changes of combustion or formation to complete your cycle.

Step 3: Example of Enthalpy Change of Combustion

To calculate the enthalpy change for combustion reactions, you can use the enthalpy changes of combustion for the involved substances.

Example Process

1. Identify the Combustion Reaction: For example, the combustion of carbon and hydrogen to form carbon dioxide (CO2) and water (H2O).
2. Draw the Enthalpy Cycle: Place reactants and products at the corners of the triangle.
3. Use Known Values: Incorporate known combustion enthalpy values to find the unknown enthalpy change.

Step 4: Example of Enthalpy Change of Formation

For formation reactions, you can similarly use enthalpy changes to find the enthalpy change of the reaction.

Example Process

1. Identify the Formation Reaction: For example, the formation of nitrogen monoxide (NO) from nitrogen (N2) and oxygen (O2).
2. Draw the Enthalpy Cycle: Position the reactants and products in the triangle.
3. Utilize Known Formation Values: Plug in known enthalpy changes of formation to calculate the unknown enthalpy change.

Conclusion

Hess' Law and enthalpy cycles are powerful tools for calculating enthalpy changes in chemical reactions. By understanding how to construct and utilize enthalpy cycles, you can solve complex problems that involve enthalpy changes without direct measurement. To further your understanding, practice creating enthalpy cycles for various reactions, using known enthalpy values, and solidifying your grasp of these essential chemistry concepts.