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Some situations you have to use both and then add them together to find the total entropy change of the system- it all just depends on what is given. If the problem gives you 2 temperatures and 2 volumes and then asks for entropy change, chances are you'll be using both. However if there is just a temperature change, then you would just use the temperature change equation only.
Danny Elias Dis 1E wrote:Can someone please explain the derivation of deltaS = nRlnV2/V1 ? I do not understand why this can be used.
This equation is used in an isothermal reaction so temperature is constant which means that deltaU is 0, this allows q=-w, because w=-nRTln(v2/v1) then q=nRTln(v2/v1), then, since deltaS=q/T, you can substitute the q that you just found into this equation making deltaS=nRTln(v2/v1)/T, then you can cancel out the T’s making the equation: deltaS=nRln(v2/v1)
If you have an initial temp and final temp (that is temp is changing and volume is not), then you would use the equation with ln(T2/T1). Versus if its the opposite (temp is constant and volume is changing), the you would use the equation with ln(V2/V1).
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