Problem 20 - Chemistry The Central Science In Si Units

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?

Step-by-Step Solution

Answer

(a) No, detonation is not reversible. (b) The sign of q is negative. (c) Work w is negative.

Step 1: Analyze the Nature of Detonation

A detonation is a rapid reaction involving a high explosion, producing gases and heat at a high rate. It is highly unlikely that the original conditions can spontaneously reoccur due to the entropy increase and energy dispersion. Therefore, a detonation is not a reversible process.

Step 2: Determine the Change in Heat (q)

In a detonation, heat is released to the surroundings as the chemical energy stored in the TNT is converted to heat and kinetic energy. Since heat is lost from the system, the sign of the heat change, q, is negative.

Step 3: Analyze the Work done (w)

The rapid expansion of gases in a detonation performs work on the surroundings by pushing them back. Since the system does work on the surroundings, the work done (w) is negative, following the convention that work is negative when done by the system.

Key Concepts

Irreversible Processes

When we delve into the concept of irreversible processes, it is essential to understand how these differ from reversible processes. Irreversible processes are changes where the system cannot easily return to its initial state. Think of it as a one-way street.
One common characteristic of these processes is the loss of energy to the surroundings that cannot be completely recovered.

A classic example is the detonation of TNT. Once the explosive reaction occurs, gases and heat are produced in such a large amount and at such a high rate that returning to the original state is practically impossible.

• Irreversible processes usually entail a significant increase in entropy.
• They often involve spontaneous reactions, like explosions or chemical decompositions.

Understanding this concept is crucial because many real-world processes are, in fact, irreversible, giving insight into energy efficiency and system dynamics.

System Energy Changes

In thermochemistry, system energy changes during processes like chemical reactions are critical for analyzing what happens within the system. Energy within a system can transform from one type to another, but the total energy of the system and surroundings remains constant according to the first law of thermodynamics.

When TNT detonates, the chemical energy initially stored within the TNT is drastically converted into heat and kinetic energy. As a result, energy disperses outward, causing a notable change in the system’s internal energy.

• The energy that used to keep the TNT's molecules together transforms into other forms during and after detonation.
• This dispersion ordinarily results in an energy loss to the surroundings, acknowledged by a change in heat, denoted as Δq.

These energy transitions are measured to understand better how efficient a reaction is and its impact on the surroundings.

Work and Heat in Chemical Reactions

Work and heat are two key factors of energy transfer in chemical reactions. They determine how the energy of a system changes during a process.

Work is done when a system applies a force over a distance, such as when gases from a detonation push against the surroundings. According to thermodynamic conventions, when the system does work, such as expanding gases during TNT detonation, the work is considered negative.

• Negative work signifies the energy flowing out from the system to the surroundings.
• Conversely, if the surroundings perform work on the system, the work is positive.

In terms of heat, it is energy transferred solely due to temperature differences. If heat escapes from the system, like in the TNT explosion where heat radiates into the environment, the sign of heat ( q ) is negative. Ultimately, being mindful of these transformations helps in predicting reaction behaviors and managing energy conservation.