# Chapter 5

Thermodynamics is the study of energy and how it is converted from one form to another. Thermodynamic principles are found in physics, chemistry, biology, engineering, and material science courses, as well as many other scientific disciplines. In any situation where heat or energy are considered, thermodynamics plays a vital role. With the exception of the sun, most of the energy we use in our everyday lives comes from chemical reactions. By studying the energy changes that accompany chemical reactions, chemists can use thermodynamic principles to improve a variety of applications, from heating your apartment, to designing materials to line your jacket and keep you warm on a cold winter day, to finding the proper mixture of fuel to power your automobile.

In order to have an educated discussion about energy we must first have an understanding of the units chemists use and we need to be able to define a system and its surroundings.

5.1 The Nature of Energy

I’m sure at some point you have heard the phrase “Energy cannot be created nor destroyed, but it can change from one form to another.” This Law of conservation of energy is know as the 1st Law of Thermodynamics. In order to understand just how this observation can be applied, we must first have a firm understanding of heat.

5.2 Heat and the First Law of Thermodynamics

Once we have an understanding of heat and the First Law of Thermodynamics, we can use these concepts to solve example problems involving heat lost and heat gained.

5.2 Relationships Involving Heat Example Problem

Now that we have an understanding of some Thermochemical properties, we can use principles from the 1st law of Thermodynamics to come up with a sign convention indicating whether or not a reaction gives off heat or absorbs heat. We refer to such reactions as exothermic and endothermic respectively.

5.2 Thermochemical Reactions

Most chemical reactions we observe in our everyday lives occur under constant pressure. Enthalpy (H) is a state function used to describe heat flow under constant pressure. Once enthalpy is defined, we can use it to determine the change in enthalpy for chemical reactions.

5.3 & 5.4 Enthalpy and Enthalpies of Reaction

Since Enthalpy is a state function (independent of path) we are only concerned with the initial state of the reactants and the final state of the products. It does not matter if this happens in one step, or in multiple steps, the overall change in enthalpy will be the same. This means the enthalpy change for an overall chemical reaction can be calculated by manipulating a series of chemical reactions. Two examples of such a process are shown below.

5.6 Using Hess’s Law to Determine the Enthalpy of a Reaction Example Problem

5.6 Using Hess’s Law to Determine the Enthalpy of a Reaction Example Problem #2

The standard enthalpy of formation is the change in enthalpy for a reaction producing one mole of a product in its standard state from its elements in their standard states. A table of standard enthalpies of formation can be found in your textbook and using these values in combination with Hess’s Law, we can calculate the standard enthalpy change for chemical reactions provided we know the standard enthalpy values for all of the products and reactants.

5.7 Standard Enthalpies of Formation

5.7 Standard Enthalpies of Formation Example Problem

We can now calculate the amount of heat released or absorbed in a given chemical reaction by using the concepts from this chapter along with the stoichiometry of chemical reactions concepts from Chapter 3.

5.7 Enthalpy and Reaction Stoichiometry