Irreversible and Reversible Expanision

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

Andrea_Torres
Posts: 98
Joined: Sun Sep 15, 2019 12:15 am

Irreversible and Reversible Expanision

What is irreversible and reversible expansion? How do we know which is which when faced with a problem?

Daniel Honeychurch1C
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Joined: Thu Jul 11, 2019 12:15 am

Re: Irreversible and Reversible Expanision

Usually the problem will tell you. If not, in reversible expansion, the pressure of the system equals the external pressure (which decreases during the expansion). In irreversible expansion, the external pressure is constant.

Kylie Lim 4G
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Re: Irreversible and Reversible Expanision

In irreversible expansion, there is a sudden significant change in external pressure that allows the contents of the system to expand. In a reversible expansion, external pressure is reduced gradually and in small increments so that Pexternal = Psystem. Usually in problems it will state whether it is a reversible or irreversible reaction.

Nick Lewis 4F
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Joined: Wed Sep 18, 2019 12:18 am

Re: Irreversible and Reversible Expanision

How can you differentiate when to use -nRlnV2/V1 versus using -PV? I feel like I always get mixed up when I should use which equation. Any tips for knowing which one to use when?

TarynD_1I
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Joined: Sat Aug 24, 2019 12:17 am

Re: Irreversible and Reversible Expanision

Nick Lewis 4F wrote:How can you differentiate when to use -nRlnV2/V1 versus using -PV? I feel like I always get mixed up when I should use which equation. Any tips for knowing which one to use when?

this is the link to a chart that a TA made that really helped me distinguish which equation to use when

lauraxie2e
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Re: Irreversible and Reversible Expanision

it usually will tell you if it is irreversible or reversible

Gurmukhi Bevli 4G
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Joined: Wed Nov 14, 2018 12:20 am

Re: Irreversible and Reversible Expanision

Generally the problem should specify which one it is. If not, for a reversible expansion, Psystem = external pressure, which will decrease as the expansion takes place. For an irreversible expansion, the external pressure will be constant.

Altamash Mahsud 1I
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Joined: Fri Aug 09, 2019 12:16 am

Re: Irreversible and Reversible Expanision

In an irreversible expansion, the external pressure on something has changed a huge (relative) amount in a short span of time. For an irreversible expansion, the external pressure is constant. For a reversible expansion, the internal and external pressure change in small, infinitesimal increments so that when the internal pressure changes, the external pressure reacts to that change in a similar way (and vice versa). This means that the internal pressure (pressure of the system) equals the external pressure the whole way through (Psystem = Pexternal). When given a problem about gaseous expansion, the problem will tell you whether it is an irreversible or a reversible expansion.

Posts: 100
Joined: Sat Jul 20, 2019 12:15 am

Re: Irreversible and Reversible Expanision

So far, we have only been exposed to questions that has this information given.

Robert Tran 1B
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Joined: Thu Jul 11, 2019 12:15 am

Re: Irreversible and Reversible Expanision

In addition to what has already been said, for a reversible expansion, there is heat transfer to maintain the temperature. However, for an irreversible expansion, there is no heat transfer because the change occurs too quickly.