Thermodynamics: Calculating work done in irreversible expansion

In summary, the problem asks to calculate the heat and work done by a system undergoing an isothermal expansion at 350C, with a change in volume from 50ml to 2L. The first part of the problem, done via a reversible path, results in a work of -5738.79 J and an entropy change of 9.209 J.K. However, there is not enough information provided to solve the second part, as it depends on the pressure imposed by the surroundings during the irreversible change.
  • #1
Rocalvic
1
0

Homework Statement



Suppose 100g of ethane (C2H6) expands isothermally at 350C from 50ml to 2L. Calculate the heat and work done by the system and the change in entropy if:

i)The process is via a reversible path.
ii)The process is non-reversible.

I don't know how to answer ii)

Homework Equations


w=-nRTln(v2/v1)

The Attempt at a Solution



I have managed to answer i)

Work = -NRTln (v2/v1) = (0.30026)(8.315)(623.15)ln(2/0.050)
Work = -5738.79 J.

Entropy change = nRln (v2/v1) = (0.30026)(8.3145)ln(2/0.050)
Entropy Change = 9.209 J.K

Heat: Q=-w, so Heat = 5738.79.

ii) I am unsure how to answer this question, do I apply the same principle? Or is this a result of free expansion and therefore the work done is 0?
Thanks
 
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  • #2
Hi Rocalvic,

Welcome to Physics Forums!

You analyzed part i correctly, and you are also correct to note that there is not enough information provided to solve part ii (except for the change in entropy, of course). To do part ii, you need to know the pressure imposed by the surroundings during the irreversible change.
 

Related to Thermodynamics: Calculating work done in irreversible expansion

1. What is thermodynamics and why is it important?

Thermodynamics is the branch of physics that deals with the study of energy and its transformations, particularly heat and work. It is an important field of study as it helps us understand the behavior of matter and energy in various systems, including chemical reactions, engines, and power plants.

2. What is irreversible expansion in thermodynamics?

In thermodynamics, irreversible expansion refers to a process in which a gas expands against a constant external pressure, but the expansion is not reversible and results in a loss of usable energy. This can happen due to friction, turbulence, or other non-ideal conditions.

3. How do you calculate the work done in irreversible expansion?

The work done in irreversible expansion can be calculated using the formula W = -PΔV, where W is the work done, P is the external pressure, and ΔV is the change in volume. This formula is based on the first law of thermodynamics, which states that the change in internal energy of a system is equal to the heat transferred to the system minus the work done by the system.

4. What are some real-life examples of irreversible expansion?

One common example of irreversible expansion is the compression and expansion of gases in a car engine. The explosion of fuel in the engine creates pressure, which pushes the piston and expands the gases, resulting in work being done. However, this process is not completely reversible, as some energy is lost due to friction and heat transfer.

5. How does irreversible expansion affect the efficiency of a system?

Irreversible expansion can decrease the efficiency of a system since it results in a loss of usable energy. This energy loss can be seen in various systems, such as power plants, where some energy is lost as heat due to non-ideal conditions. In order to improve efficiency, efforts are made to minimize irreversible processes and increase the reversible ones.

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