# Thermo - Prove that a rapid chemical reaction is irreversible.

1. Feb 27, 2012

1. The problem statement, all variables and given/known data

Show that processes involving rapid chemical reactions are irreversible by considering the combustion of a natural gas (e.g. methane) and air mixture in a rigid container.

2. Relevant equations

$$Q_{in}+W_{e,in}-W_{b}=\Delta U+\Delta KE+ \Delta PE$$

3. The attempt at a solution

I know that an irreversible process is one that, when reversed, have some effect on the surroundings. Common irreversibilities are friction, unrestrained expansion, mixing of two fluids, heat transfer across a finite temperature difference, electrical resistance, inelastic deformation of solids, and chemical reactions.

This problem deals with the mixing of two fluids. Does it also deal with unrestrained expansion?

The problem says that the chemical reaction is "rapid" which immediately sets off the idea of irreversibility, and it also says the combustion happens in a "rigid" container which means the total volume inside the container never changes.

I know that combustion causes temperature and pressure to increase.

The heat produced by the gas during combustion equals the amount of work it would require to put the surroundings back to their original state, right?

How do I put all of these thoughts into a statement proving the irreversibility of combustion in a rigid chamber?

2. Mar 1, 2012

There were replies to my original post, but here is some light on the situation for all you struggling engineering students:

Combustion Reaction:

$$CH_4+2O_2 \to CO_2+2H_2O+Energy(Heat)$$

methane + oxygen yields carbon dioxide + water + energy (heat)

The → symbol in this reaction indicates that the combustion reaction is irreversible. In other words, the CO2 + 2H2O created in the reaction do not spontaneously turn back into methane and oxygen.

To reverse the reaction and cause the carbon dioxide and water to turn back into methane and oxygen, energy input would be required. Thus, the chemical reaction is irreversible.

Only reversible processes can return to their original states without energy input from their surroundings.