Hess's law

Hess's Law

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A Hess cycle illustrating Hess's Law

Hess's Law is a fundamental principle in thermochemistry that states the total enthalpy change for a chemical reaction is the same, regardless of the pathway by which the chemical reaction occurs, provided the initial and final conditions are the same. This law is a manifestation of the law of conservation of energy, which states that energy cannot be created or destroyed.

History

Hess's Law is named after Germain Hess, a Swiss-born Russian chemist who formulated the law in 1840. Hess's work laid the foundation for the development of the concept of enthalpy and the broader field of thermochemistry.

Explanation

Hess's Law can be understood by considering a chemical reaction as a series of steps. According to Hess's Law, the total enthalpy change for the reaction is the sum of the enthalpy changes for each step, regardless of the number of steps or the specific nature of those steps. This allows chemists to calculate the enthalpy change of a reaction that is difficult to measure directly by using known enthalpy changes of other reactions.

Applications

Hess's Law is widely used in chemistry to calculate the enthalpy changes of reactions. It is particularly useful in determining the enthalpy changes of reactions that are difficult to measure directly, such as those involving intermediate steps or reactions that occur at high temperatures or pressures.

Example

Consider the formation of carbon dioxide from carbon and oxygen:

C(s) + O_(g) _ CO_(g)

This reaction can be broken down into two steps:

1. C(s) + 1/2 O_(g) _ CO(g) 2. CO(g) + 1/2 O_(g) _ CO_(g)

According to Hess's Law, the enthalpy change for the overall reaction is the sum of the enthalpy changes for these two steps.

Related Concepts

• Enthalpy
• Thermochemistry
• Law of conservation of energy
• Standard enthalpy of formation

Related pages

• Germain Hess
• Thermodynamics
• Chemical reaction