Question

Use the second law of thermodynamics to explain why we can use oil only once as a fuel, or in other words, why we cannot recycle its high-quality energy.

Short answer

We cannot recycle oil's energy because the conversion increases entropy, leading to energy degradation and irreversibility.

Step-by-step solution

Understanding the Second Law of Thermodynamics

The second law of thermodynamics states that the entropy of an isolated system always increases over time. Entropy can be understood as a measure of disorder or randomness in a system. According to this law, energy transformations are not 100% efficient, and some energy is always lost as waste heat, leading to an increase in entropy.

Energy Degradation During Combustion

When oil is burned as a fuel, chemical energy stored in the hydrocarbons is transformed into thermal energy (heat), which can be used to do work. However, because of the increase in entropy, not all of the energy is converted into useful work. Some energy is lost to the environment as waste heat, causing a permanent loss of usable energy.

Irreversibility of Energy Transformation

Due to the entropy increase associated with the conversion of energy from one form to another (i.e., chemical energy to thermal energy), it is a thermodynamic principle that this process is irreversible. Once the high-quality chemical energy is transformed into heat and dispersed, it cannot be fully converted back into its original, more orderly form.

Conclusion: Impossibility to Recycle

As a result of the reasons described above, after combustion, the energy from oil is left in a more random, less useful state and cannot be recomposed back into its high-quality, ordered state. This is why oil can be used only once as a fuel; its energy degrades and becomes less useful in the process.

Key concepts

Entropy

Entropy is an important concept that relates to disorder or randomness within a system. In thermodynamics, when a system undergoes a transformation, its entropy tends to increase. This increase in entropy means that energy becomes more spread out and less organized. Entropy can be imagined as a gradual shift from order to chaos. - Every energy change results in some increase in entropy. - As reactions occur, particularly when burning fuel, systems move from organized to disorganized states. This natural drive towards increased entropy is why it's impossible to perfectly convert energy between forms without some waste.

Energy Transformation

Energy transformation is the process of changing energy from one form to another. In the context of fuel use, chemical energy stored in oil is transformed into thermal energy through combustion.

• This transformation is not perfectly efficient, as heat energy often disperses into the environment.
• The second law of thermodynamics reveals that energy transformations lead to an increase in entropy.
• Energy transformations result in the conversion of high-quality energy into less useful forms.

Understanding energy transformation helps to explain why efficient energy use is so challenging.

Energy Degradation

Energy degradation refers to the process by which usable energy becomes less useful after a transformation. - In a combustion reaction, while some chemical energy is converted to useful work, a significant portion becomes waste heat. - This heat cannot be wholly recovered or used again, representing degraded energy. As energy degrades, its capacity to perform useful work diminishes. This is a key reason why high-quality energy resources, like oil, cannot be recycled for additional use.

Irreversibility

Irreversibility is a core concept of thermodynamics. When high-quality energy undergoes a transformation, like oil combusting into heat, the process cannot be reversed.

• Once high-energy molecules in oil are burnt, the arrangement of atoms and energy is permanently altered.
• The dispersed heat can't easily reform into structured, high-energy chemical bonds.

This irreversible transformation underscores why we cannot use oil repeatedly as a fuel source and why recycling such energy without loss is theoretically and practically impossible.