The Second Law of Thermodynamics


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The two laws of thermodynamics.
Source: Urry, Lisa A.. Campbell Biology (p. 145). Pearson Education. Kindle Edition.

The Second Law of Thermodynamics (Campbell Biology)

If energy cannot be destroyed, why can’t organisms simply recycle their energy over and over again? It turns out that during every energy transfer or transformation, some energy becomes unavailable to do work. In most energy transformations, the more usable forms of energy are at least partly converted to thermal energy and released as heat. Only a small fraction of the chemical energy from the food is transformed into the motion of the brown bear shown in the photo above; most is lost as heat, which dissipates rapidly through the surroundings.

In the process of carrying out chemical reactions that perform various kinds of work, living cells unavoidably convert other forms of energy to heat. A system can put this energy to work only when there is a temperature difference that results in thermal energy flowing as heat from a warmer location to a cooler one. If temperature is uniform, as it is in a living cell, then the heat generated during a chemical reaction will simply warm a body of matter, such as the organism. (This can make a room crowded with people uncomfortably warm, as each person is carrying out a multitude of chemical reactions!)

A consequence of the loss of usable energy as heat to the surroundings is that each energy transfer or transformation makes the universe more disordered. We are all familiar with the word “disorder” in the sense of a messy room or a rundown building. The word “disorder” as used by scientists, however, has a specific molecular definition related to how dispersed the energy is in a system, and how many different energy levels are present. For simplicity, we use “disorder” in the following discussion because our common understanding (the messy room) is a good analogy for molecular disorder.

Scientists use a quantity called entropy as a measure of molecular disorder, or randomness. The more randomly arranged a collection of matter is, the greater its entropy. We can now state the second law of thermodynamics: Every energy transfer or transformation increases the entropy of the universe. Although order can increase locally, there is an unstoppable trend toward randomization of the universe as a whole.

The physical disintegration of a system’s organized structure is a good analogy for an increase in entropy. For example, you can observe the gradual decay of an unmaintained building over time. Much of the increasing entropy of the universe is more abstract, however, because it takes the form of increasing amounts of heat and less ordered forms of matter. As the bear in the photo above converts chemical energy to kinetic energy, it is also increasing the disorder of its surroundings by producing heat and small molecules, such as the carbon dioxide it exhales, that are the breakdown products of food.

The concept of entropy helps us understand why certain processes are energetically favorable and occur on their own. It turns out that if a given process, by itself, leads to an increase in entropy, that process can proceed without requiring an input of energy. Such a process is called a spontaneous process. Note that as we’re using it here, the word spontaneous does not imply that the process would occur quickly; rather, the word signifies that it is energetically favorable. (In fact, it may be helpful for you to think of the phrase “energetically favorable” when you read the formal term “spontaneous,” the word favored by chemists.) Some spontaneous processes, such as an explosion, may be virtually instantaneous, while others, such as the rusting of an old car over time, are much slower.

A process that, on its own, leads to a decrease in entropy is said to be non-spontaneous: It will happen only if energy is supplied. We know from experience that certain events occur spontaneously and others do not. For instance, we know that water flows downhill spontaneously but moves uphill only with an input of energy, such as when a machine pumps the water against gravity. Some energy is inevitably lost as heat, increasing entropy in the surroundings, so usage of energy means that a non-spontaneous process also leads to an increase in the entropy of the universe as a whole.


Urry, Lisa A.. Campbell Biology. Pearson Education. Kindle Edition.


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