Reversible vs Irreversible Entropy Calculation

[Anna Lockhart 2B]

Wondering how we know which change in entropy equation to use in a reversible reaction versus an irreversible reaction.

[Simrah_Ahmed1J]

For ΔS...

reversible(isothermal): there is a change in V, no change in T
ΔS= n*R*ln(V₂/V₁)

irreversible: there is a change in T, no change in V
ΔS= n*C_v*ln(T₂/T₁)

[Alexis Sanft 1E]

For ΔS...

reversible(isothermal): there is a change in V, no change in T
ΔS= n*R*ln(V₂/V₁)

irreversible: there is a change in T, no change in V
ΔS= n*C_v*ln(T₂/T₁)

So irreversible is never isothermal?

[SashaAnand2J]

Hi Anna! Just want to add on that for an irreversible reaction, deltaS(univ)=deltaS(sys), and the entropy of the surroundings does not change. However, for a reversible reaction, deltaS(univ)=0, and therefore deltaS(sys)=-deltaS(surr).

[Kaihan_Danesh_2J]

The pathway should not matter for entropy, change in S is always the same for both pathways, it is the change in S of the surroundings that is different and which allows for some of these processes to occur. Entropy is a state function, so whether the pathway is irreversible or reversible, the change in S is always the same for a certain reaction. So, just use whichever change in S equation you need, the one with changing temperature if that is occurring in the reaction given or the changing volume change in S equation if volume is changing.

[Simrah_Ahmed1J]

For ΔS...

reversible(isothermal): there is a change in V, no change in T
ΔS= n*R*ln(V₂/V₁)

irreversible: there is a change in T, no change in V
ΔS= n*C_v*ln(T₂/T₁)

So irreversible is never isothermal?

yeah! so basically irreversible pathways will be isothermal since volume is conserved, the change in entropy (ΔS) is coming from a change in temperature.