Question

1. For some dental procedures dentists use "laughing gas" (introduced by Sir Humphry Davy in 1799 - he became rather addicted!) as a sedative in lieu of local anesthesics such as lidocaine. Dental laughing gas is a mixture of 30-40% nitrous oxide (N20) and oxygen. Assume a mixture containing 40 mol% N2O which behaves ideally and determine a) AGmixing, AHmixing b) Explain the meaning of the sign for AGmixing irreversibility of the mixing of these gases. and AV mixing AS mixing and how they relate to the reversibility or and AS, тіxing

Answer 1

Since each gas is at the same T and p the chemical potential is given by, »(3) 4= 9 + RT]n Gni niu= n11+n22 i=1 partial pressure of N20 =p1 = x1p(mole fraction of N20 partial pressure of O2 =p2 = x2p(mole fraction of O2) ui=RTln P2 x2p (5) +RTlnx Similarly 2 Gfina Σnμι niuiRTn,lnx n,lnx GninRT -1 nRTxlnx AGmix 1 nRT(xlnx1+ x2lnx2]

aG =-S aGmix + x2lnP] P1 -nR[xlnx1 + xInx2] = nR[xyln -Smix Әт P P2 AGmix AixTASmi nRT[xInx1xlnx2 AHmixnRT[xlnx +xlnx2 ΔΗΚ = 0 aG = V dẹ т = Vmix=0 aGmix aT because AGm is independent of pressure 40 =0.4 100 60 =0.6 100 AGmix = 100RT(0.41n0.4+0.6ln0.6] AGix-121.6RT = 121.6R AS. mix AG is zero for a reversible process when carried out constant temperature,sin ce here AG is a negative quantity the proce ss here is irreversible.AS change is positive which is sum of the entropy changes of eachindividu al gases undergoing expansion from pressure p to its partial pressure pi,where disorder in the system increases due to expansion leading to irreversibilty.