19: The First Law of Thermodynamics
- Page ID
- 11815
- 19.1: Overview of Classical Thermodynamics
- Joule was able to show that work and heat can have the same effect on matter – a change in temperature! It would then be reasonable to conclude that heating, as well as doing work on a system will increase its energy content, and thus it’s ability to perform work in the surroundings. This leads to an important construct of the First Law of Thermodynamics: The capacity of a system to do work is increased by heating the system or doing work on it.
- 19.2: Pressure-Volume Work
- Work in general is defined as a product of a force F and a path element ds. In the case of a cylinder we can introduce the area of the piston A and forget about the vectorial nature The movement of the piston is constrained to one direction, the one in which we apply pressure (P being force F per area A). This particular form of work is called pressure-volume work and will play an important role in the development of our theory. Notice, however that it is only one form of work.
- 19.5: An Adiabatic Process is a Process in which No Energy as Heat is Transferred
- WORK IS A PATH FUNCTION even if reversible
- 19.8: Pressure-Volume Work
- An important point is that pressure-volume work −PdV is only one kind of work. It is the important one for gases but for most other systems we are interested in other kinds of work (e.g. electrical work in a battery).
- 19.11: Enthalpy Changes for Chemical Equations are Additive
- As enthalpy and energy are state functions we should expect additivity of U and H when we study chemical reactions. This additivity is expressed in Hess's Law. The additivity has important consequences and the law finds wide spread application in the prediction of heats of reaction. The reverse reaction has the negative enthalpy of the forward one. If we can do a reaction in two steps we can calculate the enthalpy of the combined reaction by adding up:
- 19.12: Heats of Reactions Can Be Calculated from Tabulated Heats of Formation
- Reaction enthalpies are important, but difficult to tabulate. However, because enthalpy is a state function, it is possible to use Hess’ Law to simplify the tabulation of reaction enthalpies. Hess’ Law is based on the addition of reactions. By knowing the reaction enthalpy for constituent reactions, the enthalpy of a reaction that can be expressed as the sum of the constituent reactions can be calculated.
- 19.13: The Temperature Dependence of ΔH
- It is often required to know thermodynamic functions (such as enthalpy) at temperatures other than those available from tabulated data. Fortunately, the conversion to other temperatures isn’t difficult.
Thumbnail: A thermite reaction using iron(III) oxide. The sparks flying outwards are globules of molten iron trailing smoke in their wake. (CC SA-BY 3.0; Nikthestunned).