Question

Consider what happens when a sample of the explosive TNT is detonated under atmospheric pressure. (a) Is the detonation a reversible process? (b) What is the sign of \(q\) for this process? (c) Is \(w\) positive, negative, or zero for the process?

Short answer

(a) The detonation of TNT is NOT a reversible process, since it's a chemical reaction that cannot be changed back to its original state. (b) The sign of q for this process is positive, as it releases a large amount of heat. (c) The sign of w for the process is positive, since the system does work on the surroundings through rapid expansion.

Step-by-step solution

Understanding Reversible Process

A reversible process is a process that can be reversed by an infinitesimally small change in a variable, such that both the system and surroundings return to their initial states. In simple terms, it's a process which can be changed back to the original state by making a small change in any variable.

Evaluating whether the detonation is reversible or not

Detonation involves a chemical reaction releasing a large amount of energy in a very short period of time. This chemical reaction is, by nature, irreversible. Once the explosive material is detonated, it cannot be changed back to its original state. So, the detonation of TNT is an irreversible process. (a) Answer: The detonation is NOT a reversible process.

Evaluating the sign of q (heat)

In a detonation, the chemical reaction releases a large amount of heat. Therefore, the heat (q) generated by the system is positive since the energy is being released into the surroundings. (b) Answer: The sign of q for this process is positive.

Understanding Work (w)

Work (w) is the amount of energy transferred by the system to the surroundings through mechanical or other forms of energy transfer. In the case of detonation of an explosive, the system does work on the surroundings by producing an expansion.

Evaluating the sign of w (work)

For the TNT detonation, the expansion occurs rapidly and causes a release of energy (work) on the surroundings. Therefore, work (w) done by the system is positive. (c) Answer: The sign of w for the process is positive.

Key concepts

Reversible Process

In chemical thermodynamics, a reversible process is akin to a perfectly choreographed dance where every step can be traced back with precision. Imagine pushing a swing: if you push it gently, it moves forward and naturally swings back. With just the right touch, it would return to your hand, and the motion could continue indefinitely without losing energy. This is how we think of a reversible process: it's an idealistic scenario where the system undergoing change can switch direction with the slightest nudge, leaving no trace of its journey in the surroundings.

In reality, no process is perfectly reversible; real-life processes always involve some form of friction or resistance, which prevents complete recoverability. For students grappling with this concept, it's important to recognize the theoretical value of reversible processes: they serve as a standard against which real processes are compared. In the world of thermodynamics, perfection is a guide, not an expectation.

Detonation of TNT

When dealing with the detonation of TNT (trinitrotoluene), we step into the realm of rapid, violent reactions that are anything but subtle. The detonation of TNT is a striking example of an irreversible process where molecules are rearranged in an instant, releasing a massive burst of energy and gases. The explosion of TNT is fast, uncontrollable, and final—you can't unexplode an explosive.

This process serves as a stark counterpoint to the reversible processes mentioned earlier. Understanding the irreversibility of TNT detonation is important because it highlights the one-way nature of certain chemical reactions, which can help students differentiate between processes that can reach equilibrium and those that are unidirectional.

Sign of q in Thermodynamics

The term 'q' in thermodynamics represents the heat exchanged between a system and its surroundings. Imagine heating a pot of water on the stove—the heat flows from the burner to the pot, ultimately causing the water to boil. This direction of heat flow (outward from the burner) is described as being positive.

From a teaching perspective, it's crucial to emphasize that the sign of q can help us understand which direction heat is flowing. If q is positive, the system has lost heat to the surroundings, which happens during the detonation of TNT. Conversely, if q is negative, the system is gaining heat. Encouraging students to think about heat flow in terms of energy transfer can simplify the concept and make it more approachable.

Work (w) in Thermodynamics

Work, represented by 'w' in thermodynamics, is about energy movement just like heat, but it’s the type of energy that's transferred when an object is moved by a force. A simple way to think about work is pushing a ball up a hill. If you push the ball, you're doing work on it. Similarly, when TNT explodes, it does work on the air around it as it expands.

Here's the twist: in thermodynamics, when a system does work on its surroundings, like pushing them expansively outward during an explosion, we consider it as positive work. This concept can occasionally confuse learners because it feels backwards—doing work usually means expending energy, yet we call it positive. It's crucial to teach this with clear examples and emphasize that the sign of work is about perspective—positive when the system does work on its surroundings, and negative when the surroundings do work on the system.