The objectives of the present study is to investigate the nature of the velocity field of an unsteady heat and mass transfer flow of a chemically reacting fluid past a semi infinite vertical plate with viscous dissipation. The method of the solution is valid only for small perturbation approximation. Results for the velocity field and the effect of various critical parameters are illustrated graphically. It is noticed that as Schmidt number increases the velocity field is found to be decreasing and after at a certain distance away from the plate , reverse trend is noticed. It is observed that at certain stage backward flow is noticed, and there exist a situation where (irrespective of value of Schmidt number) the velocity is constant. It is seen that profiles for velocity field are found to be more parabolic and subsequently, the parabolic nature is being lost as Schmidt number increases. Also, as time increases the velocity increases when Solutal Grashof number is maintained at 0.80. Initially as chemical reaction parameter increases, the velocity field increases and as we move far away from plate a reverse trend is noticed and also a backward flow is observed. At a very small interval of time (t = 2.50) the backward flow can be attributed due to the increased concentration of the fluid medium. An interesting observation is that, initially as Solutal Grashof number increases the velocity field increases and thereafter as we move far away from the plate a reverse effect is noticed.