reaction energy
 

excuse me to disturb

When calculating reaction energy using CHEMKIN I cann't understand where following formula come from. If you know please discribe it detail

DDX1(J,1)=(SX1(J,1)+(DX1(J,1)-SX1(J,1)*PD(J,1))*DT)/(1.0-PD(J,1)*DT)-SX1(J,1)

SX concentration DX reactin rate comcentration/time PD ?