17.4 Hess’s Law
17.4 Hess’s Law
Lesson Objectives
- Use Hess’s law of heat summation to add chemical reactions together in such a way as to produce a desired
final equation. Calculate the enthalpy change for that final reaction. - Define the standard heat of formation of a compound.
- Use known values for standard heats of formation to calculate a previously unknown standard heat of reaction.
Lesson Vocabulary
- heat of combustion
- Hess’s law of heat summation
- standard heat of formation
Check Your Understanding
Recalling Prior Knowledge
- What is a combustion reaction?
- How does a compound compare to an element?
Calorimetry is an experimental technique used to directly measure a heat of reaction in a laboratory setting. However,
many reactions for which it may be desirable to know the heat of reaction are too difficult to perform in a controlled
manner. In this lesson, you will learn about two indirect methods that will allow you to find the enthalpy change for
almost any chemical reaction.
Adding Heats of Reaction
It is sometimes very difficult or even impossible to measure the enthalpy change for a reaction directly in the
laboratory. Some reactions take place extremely slowly, making a direct measurement unfeasible. In other cases, a
given reaction may be an intermediate step in a series of reactions. Some reactions may be difficult to isolate because
multiple side reactions may occur at the same time. Fortunately, it is possible to measure the enthalpy change for a
reaction by an indirect method.Hess’s law of heat summationstates that if two or more thermochemical equations
are added together to give a final equation, then the heats of reaction for those equations can also be added together
to give a heat of reaction for the final equation.
An example will illustrate how Hess’s law can be used. Acetylene (C 2 H 2 ) is a gas that burns at an extremely high
temperature (3300°C) and is used in welding (Figure17.10). On paper, acetylene gas can be produced by the