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Hess’s Law

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Hess’s Law


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You will learn about Hess’s law of heat summation.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • Use thermochemical equations to calculate energy changes that occur in chemical reactions.
  • Relate the enthalpy change of a reaction to its intermediate steps.
  • Solve problems concerning enthalpy in multi-step reactions.

Everything You'll Have Covered

Enthalpy is the sum of the internal energy of the system plus the product of the pressure of the gas in the system and its volume. Enthalpy is the amount of energy in a system capable of doing mechanical work. While every chemical reaction involves a change in enthalpy it is not possible to empirically determine the enthalpy change of every reaction. For example, the oxidation of carbon graphite to pure carbon monoxide is very difficult to carry out in a lab. However, most chemical reactions are actually a net result of a series of intermediate steps. Hess's Law states that the change in enthalpy for a reaction can be found by adding together the enthalpy changes for each intermediate step. It also states that the enthalpy value of a reaction is the same no matter how many intermediate reactions occur. Importantly, Hess's Law applies whether the reaction actually occurs in a series of steps or can simply be broken down conceptually. Therefore, Hess's Law enables us to estimate an enthalpy of a reaction that is otherwise not possible to determine experimentally.

To apply Hess's Law, a chemical equation is broken down into a series of thermochemical equations, each with an associated, known ?H value. Each of the reaction equations must be multiplied by a factor to ensure that the number of each atom is consistent with the net reaction. An intermediate reaction may also need to be reversed. In applying Hess's Law:

1) If an intermediate equation is multiplied by a factor, its associated enthalpy change (?H) value must also be multiplied by the same factor.

2) If an intermediate reaction must be reversed, the sign of its ?H value must also be reversed.

An example of the application of Hess's Law:

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts Learners should know how to balance chemical reactions, distinguish between exothermic and endothermic reactions, and understand the concept of enthalpy.
Course Chemistry
Type of Tutorial Concept Development
Key Vocabulary enthalpy, Hess’s law, intermediate step