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

The detonation of TNT is an irreversible process. The sign of q (heat) for this process is negative, as it releases heat into the surroundings (exothermic). The sign of w (work) for the process is positive, as the system does work on the surroundings when the explosive expands.

Step-by-step solution

a) Reversible Process

Reversible processes are processes that can be reverted to their original state while also returning the surroundings to their original state. In the case of an explosive such as TNT detonating, it's improbable that the process could revert back to its initial conditions. Thus, the detonation of TNT is an irreversible process.

b) Sign of q (Heat)

When the TNT detonates, it releases a large amount of heat into the surroundings. This heat transfer is essentially from the system (TNT) to the surroundings, therefore making it an exothermic process. In an exothermic process, the heat (q) is considered to be negative. Hence, the sign of q for this process is negative.

c) Sign of w (Work)

During detonation, the explosive TNT expands rapidly and does work on the surroundings. According to thermodynamics sign conventions, when a system does work on the surroundings, the work (w) is considered to be positive. Therefore, in this case, w is positive for the explosion process.

Key concepts

Reversible Process

A reversible process in thermodynamics is a theoretical concept where a system changes its state in such a way that the entire process can be reversed without leaving any change in both the system and the surrounding environment. Imagine a pendulum swinging perfectly symmetrically back and forth without any friction slowing it down. That would be a reversible process. However, in reality, friction and other dissipative forces prevent true reversibility.
For a process to be reversible, it would need to be carried out infinitely slowly so that the system remains in equilibrium at all times. This is practical only in theory because real-world processes always run into some type of friction or inefficiency.
If you consider an explosive like TNT, once it detonates, it’s impossible to go back to the initial state without changing its surroundings. Therefore, the process becomes entirely irreversible due to the rapid chemical reaction and the abrupt energy release.

Exothermic Process

An exothermic process is a chemical reaction or physical change that releases energy in the form of heat or light. The energy is transferred from the system to the surroundings, typically making the surroundings warmer. Ever watched a piece of firewood burning in a bonfire? The heat and light emitted are due to an exothermic reaction.
In thermodynamic terms, when the system loses heat to the surroundings, the heat change (or q) is considered negative. This means that the system’s energy content decreases as it releases energy to the environment. For example, when TNT detonates, it releases a significant amount of energy, and the heat flow is from the TNT to the surroundings, confirming it an exothermic process with negative heat flow.

• Characterized by energy release.
• Surroundings become warmer due to the reaction.
• Sign of heat, q, is negative in one's calculations.

Thermodynamic Work

Thermodynamic work is an energy transfer related to systems that involve some form of expansion or contraction. When a system does work on the surroundings, it exchanges energy through a force applied over a distance. Think of a balloon being inflated: as it expands, it does work on the air surrounding it.
In the context of the TNT detonation, during the explosion, the gasses expand rapidly against the atmospheric pressure. This expansion is a classic example of a system performing work on its surroundings. According to thermodynamic conventions, when work is done by the system on the surroundings, the work, represented by w, is positive.

• Energy is transferred as work when the system expands.
• The environment is acted upon by a force through some distance.
• Work (w) is positive if done by the system on the surroundings.