work equations


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Alyssa Bryan 3F
Posts: 61
Joined: Fri Sep 28, 2018 12:26 am
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work equations

Postby Alyssa Bryan 3F » Sun Feb 10, 2019 5:03 pm

Can someone explain when we use this equation to solve for work w = - P ∆V versus when we use this equation w = -nRT ln V2/V1 ?

aisteles1G
Posts: 117
Joined: Fri Sep 28, 2018 12:15 am

Re: work equations

Postby aisteles1G » Sun Feb 10, 2019 5:12 pm

w=-nRTln(v2/v1) is for a Reversible rxn, w=-p(deltaV) is for Irreversible reaction, be careful though because whenever you use w=-PV your answer will not be in joules and you have to multiply by 101.325 to get to joules whereas the w=-nRTln(v2/v1) will already be in joules. The question should state whether the reaction is reversible or irreversible so just memorize which one goes with which

Megan_Ervin_1F
Posts: 78
Joined: Fri Sep 28, 2018 12:18 am

Re: work equations

Postby Megan_Ervin_1F » Sun Feb 10, 2019 7:55 pm

irreversible can also be called "constant pressure"

Timothy_Yueh_4L
Posts: 57
Joined: Fri Sep 28, 2018 12:28 am

Re: work equations

Postby Timothy_Yueh_4L » Sun Feb 10, 2019 8:34 pm

To add on, you use the equation, w = -nRT ln V2/V1, to calculate the total amount of work as the pressure equalizes at infinitesimally small intervals (it sums up the maximum amount of work that is able to be done). On the other hand, we use w = - P ∆V when dealing with irreversible/sudden expansion because it accounts only for the amount of work performed between 2 volumes (initial and final).

katie_sutton1B
Posts: 61
Joined: Fri Sep 28, 2018 12:17 am

Re: work equations

Postby katie_sutton1B » Sun Feb 10, 2019 8:50 pm

Basically - Isothermic, Reversible --> w=-nRTln(v2/v1)
Irreversible --> w = -P ΔV


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