Choosing work equation

$w=-P\Delta V$
and
$w=-\int_{V_{1}}^{V_{2}}PdV=-nRTln\frac{V_{2}}{V_{1}}$

Jessica Castellanos
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Choosing work equation

How do you determine which work equation to use based on reversible and irreversible processes?

Andrew Pfeiffer 2E
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Re: Choosing work equation

For an irreversible expansion, use w=-P(deltaX).

For a reversible expansion (isothermal), use w=-nRT(ln deltaV2/deltaV1).

PranaviKolla2B
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Re: Choosing work equation

Does anyone have any helpful links explaining reversible and irreversible processes in relation to work equations?

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Re: Choosing work equation

Andrew Pfeiffer 2E wrote:For an irreversible expansion, use w=-P(deltaX).

For a reversible expansion (isothermal), use w=-nRT(ln deltaV2/deltaV1).

Would the reversibility/irreversibility of a system be given explicitly in a problem, or would that be something that we have to figure out for ourselves? If so, how would we determine if a system was reversible or not?

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Re: Choosing work equation

As on the midterm, I expect that we will be told on the exams when a reaction is reversible or irreversible. However, a useful thing to keep in mind is that if the arrow on a reaction is not pointing at the reverse as well as the forward, this probably means that the products cannot become reactants, a sign that the reaction is irreversible. This is due to the fact that an infinitesimal change is not enough to shift the reaction to either side.

Hussain Chharawalla 1G
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Re: Choosing work equation

For the most part, you will be explicitly told when the process is reversible. If you are not told, you can most likely tell from the context of the problem (pressure is constant, etc), that the process is irreversible.

AniP_2D
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Re: Choosing work equation

Andrew Pfeiffer 2E wrote:For an irreversible expansion, use w=-P(deltaX).

For a reversible expansion (isothermal), use w=-nRT(ln deltaV2/deltaV1).

Would the reversibility/irreversibility of a system be given explicitly in a problem, or would that be something that we have to figure out for ourselves? If so, how would we determine if a system was reversible or not?

This information will most likely be given. It may be possible that two work values are given and then you must say which is the value corresponding to an irreversible or reversible process. In this case, the reversible process always does more work than the irreversible process.

Charlene Datu 2E
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Joined: Wed Sep 11, 2019 12:16 am

Re: Choosing work equation

AniP_2D wrote:
Andrew Pfeiffer 2E wrote:For an irreversible expansion, use w=-P(deltaX).

For a reversible expansion (isothermal), use w=-nRT(ln deltaV2/deltaV1).

Would the reversibility/irreversibility of a system be given explicitly in a problem, or would that be something that we have to figure out for ourselves? If so, how would we determine if a system was reversible or not?

This information will most likely be given. It may be possible that two work values are given and then you must say which is the value corresponding to an irreversible or reversible process. In this case, the reversible process always does more work than the irreversible process.

Could you explain why the reversible process always does more work than irreversible process? Are you talking about the work done on or done by the system?

Posts: 81
Joined: Fri Sep 28, 2018 12:28 am

Re: Choosing work equation

AniP_2D wrote:
Andrew Pfeiffer 2E wrote:For an irreversible expansion, use w=-P(deltaX).

For a reversible expansion (isothermal), use w=-nRT(ln deltaV2/deltaV1).

Would the reversibility/irreversibility of a system be given explicitly in a problem, or would that be something that we have to figure out for ourselves? If so, how would we determine if a system was reversible or not?

This information will most likely be given. It may be possible that two work values are given and then you must say which is the value corresponding to an irreversible or reversible process. In this case, the reversible process always does more work than the irreversible process.

Why would the reversible process always do more work? Also will there every be an instance where the irreversible process does more work?

AniP_2D
Posts: 95
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Re: Choosing work equation

AniP_2D wrote:
Would the reversibility/irreversibility of a system be given explicitly in a problem, or would that be something that we have to figure out for ourselves? If so, how would we determine if a system was reversible or not?

This information will most likely be given. It may be possible that two work values are given and then you must say which is the value corresponding to an irreversible or reversible process. In this case, the reversible process always does more work than the irreversible process.

Why would the reversible process always do more work? Also will there every be an instance where the irreversible process does more work?

I believe the reversible process always does more work just because it has to push against more external pressure than the irreversible one. In the book it states that the reversible one always does more work so I'm pretty sure there isn't going to ever be a case in which the irreversible one does more work. Hope this helped!

AniP_2D
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Re: Choosing work equation

Charlene Datu 2E wrote:
AniP_2D wrote:
Would the reversibility/irreversibility of a system be given explicitly in a problem, or would that be something that we have to figure out for ourselves? If so, how would we determine if a system was reversible or not?

This information will most likely be given. It may be possible that two work values are given and then you must say which is the value corresponding to an irreversible or reversible process. In this case, the reversible process always does more work than the irreversible process.

Could you explain why the reversible process always does more work than irreversible process? Are you talking about the work done on or done by the system?

The reversible process has to push against more external pressure than the irreversible one, leading it to do more work. I found this link in another post regarding this topic. I hope it helps!
http://i.imgur.com/pvRr9Tk.png?1