Important conditions


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Important conditions

Postby WYacob_2C » Fri Feb 07, 2020 10:45 pm

I was wondering which conditions should we know for our midterm? So far I know we should understand an isothermal system and an adiabatic system; are there any other important conditions we should know and could you give a small description of it as well. Thank you in advance.

Osvaldo SanchezF -1H
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Re: Important conditions

Postby Osvaldo SanchezF -1H » Fri Feb 07, 2020 10:55 pm

We should also know when it is isochloric, and isobaric. A question might state this and we would have to know what it means. Isochloric means that it has constant volume and isobaric is when the pressure is constant. Also a key not to remember for isothermal is that because there is no change in temperature the internal energy would have to equal 0 and thus the same ammount of work done on or from the system would have to be equal and this q=-w. One thing to also also to keep in mind is that if a problem says free expansion it means that w=0.

Hui Qiao Wu 1I
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Re: Important conditions

Postby Hui Qiao Wu 1I » Sun Feb 09, 2020 11:32 am

I think it's also good to know about reversible and irreversible reactions, and how to differentiate between the two.

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Re: Important conditions

Postby Mulin_Li_2J » Sun Feb 09, 2020 6:56 pm

Isothermal, adiabatic, isobaric, isochoric, reversible, and irreversible are all important terms you need to know.

Isothermal means ΔT = 0. This also implies that ΔU = 0, and because ΔU = q + w, q = -w. You can also use the formula for reversible expansion w = -nRTln(V2/V1), and the second law of thermodynamics ΔS = q/T as they all assume constant temperature.

adiabatic means q = 0. This also implies that ΔH = 0, and because ΔU = q + w, ΔU = w only. This does not mean ΔS = 0 because you still have positional/residual entropy.

isobaric means Δp = 0. This implies that ΔU = ΔG. Also, under constant pressure, you can use Cp(molar heat capacity under constant pressure). Note that this p refers to the pressure within a system, not the external pressure. When the external pressure is 0, no work can be done so ΔU = q only.

isochoric means ΔV = 0. You can use Cv(molar heat capacity under constant volume).

reversible means that we use very small increment of pressure or temperature when changing the system so the system is constantly at equilibrium. Isothermal reversible expansion of gas is the maximum amount of work a system can do. Note that reversible typically infers isothermal.

irreversible means that the we use noticeable increment of pressure or temperature when changing the system. Typically, for an irreversible reaction, it's also not isothermal.

Hope this can help! -_+

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