Hi thanks for reading. I have 2 questions regarding the topics mentioned that I'm having trouble with. Hope you guys can help me out here :)
1) The first law is ΔU=Q+W so we can rearrange this to get ΔH=ΔU+PΔV so would this indicate that ΔH only measures the heat given out at constant pressure? Because ΔU=ΔH-PΔV so they would cancel out right? But I was thinking even if pressure isn't constant, won't the heat evolved be the same since they would still cancel out?
2) Then when combining ΔH and ΔS, my teacher briefly mentioned that ΔH is doing non expansive work which makes sense if what I said in the first paragraph above was correct that enthalpy change only tells us how much heat is evolved or absorbed. But in the example where enthalpy change was negative and entropy change was negative also but the overall Gibbs free energy was still negative. So, he said some energy from the enthalpy change is used for the non spontaneous entropy change for the expansive work. What does that mean?
I understand mathematically that ΔG=ΔH-TΔS so one factor might outweigh the other in terms of magnitude. However, I don't quite understand the theory on why the terms expansive work were used in explaining these concept and how I should explain it if it was the other way round where ΔS is positive while ΔH is positive also. I can explain mathematically that if they are subtracted to give a negative answer, then there is free energy. But I don't understand the reasons on why expansive work is done and the general theory behind them.
Sorry for the long post. Hope you guy can help me out here :) thanks
1) The first law is ΔU=Q+W so we can rearrange this to get ΔH=ΔU+PΔV so would this indicate that ΔH only measures the heat given out at constant pressure? Because ΔU=ΔH-PΔV so they would cancel out right? But I was thinking even if pressure isn't constant, won't the heat evolved be the same since they would still cancel out?
2) Then when combining ΔH and ΔS, my teacher briefly mentioned that ΔH is doing non expansive work which makes sense if what I said in the first paragraph above was correct that enthalpy change only tells us how much heat is evolved or absorbed. But in the example where enthalpy change was negative and entropy change was negative also but the overall Gibbs free energy was still negative. So, he said some energy from the enthalpy change is used for the non spontaneous entropy change for the expansive work. What does that mean?
I understand mathematically that ΔG=ΔH-TΔS so one factor might outweigh the other in terms of magnitude. However, I don't quite understand the theory on why the terms expansive work were used in explaining these concept and how I should explain it if it was the other way round where ΔS is positive while ΔH is positive also. I can explain mathematically that if they are subtracted to give a negative answer, then there is free energy. But I don't understand the reasons on why expansive work is done and the general theory behind them.
Sorry for the long post. Hope you guy can help me out here :) thanks