Laws of Thermodynamics

of energy (potential or heat energy) gained by another aspect of the reaction (because of the law of conservation of energy).

As mentioned earlier in this course, chemical reactions can be exothermic and will give off heat, or endothermic and will store or take on energy. Some chemical reactions can be written in such a way as to account for the heat given off by stating that there is heat as part of the equation. A thermochemical equation is one that has heat put into it or given off as a byproduct of the reaction. An example of an endothermic reaction is that of turning solid H2O to liquid H2O. This reaction requires the input of heat and is said to be endothermic.

LAWS OF THERMODYNAMICS

There are three laws of thermodynamics that apply to all physical and chemical systems. The first law has been described, which is that energy cannot be created or destroyed. In chemistry, it also involves the concept that in all systems, the tendency of all chemical (and nonchemical) systems is to move toward the state of the lowest possible energy (which in chemistry, is the most stable system). This means that potential energy is more likely to become kinetic energy.

In biochemical systems, catabolic reactions are those that take a bigger molecule (which stores energy in its chemical bonding) to be metabolized into smaller molecules plus CO2 and water. Because these are biological systems, however, the energy is given off as heat to a mild degree but goes into creating high-energy molecules like adenosine triphosphate or ATP, so that the energy is not destroyed or created but is transferred into another form of potential energy, although these molecules, too, will go on to become lower energy molecules.

Even the building of molecules like glucose from CO2 and water, which are endothermic, requires the input of energy from the universe in the form of photons of light energy from the sun to make photosynthetic organisms build glucose and other molecules. In such cases, this is an open system because it requires the input of energy (photons) from the universe. In such cases, energy is not created because energy from the universe is necessary.

In an example from the previous chapter, there is pressure-volume work done by an expanding gas. In the equation for this work equals negative P multiplied by the change in volume. What this means is that, an expanding gas at the same pressure uses up energy as the volume increases. A contracting gas holds potential energy, as it will spontaneously expand when the container is breached. Think of the potential energy in a balloon under pressure.

According to the first law of thermodynamics, there are two kinds of energy processes, heat and work, that can lead to a change in the internal energy of a system. If energy flows into a system from the outside, both the change in heat and the change in work are positive. If there is work done by the system or heat given off, the change in heat and work are negative.

The second law of thermodynamics says that the entropy of a system always increases. Entropy is the measure of disorder of a system. The best way to think of the second law of thermodynamics is to think of small children in a neat and tidy playroom. The natural course of things is that the room will become increasingly messy until energy is put back into the system from an external (or internal source) who can return the room to its former state. Even in this system, the entropy of the universe is always increasing. Figure 18 shows the idea of entropy in a system:

Figure 18.