An exact solution to the problem of free convection flow of a viscous incompressible chemically reacting fluid past an infinite vertical plate with the flow due to impulsive motion of the plate with Newtonian heating in the presence of variable mass diffusion is presented. The resulting governing equations are non-dimensionalized to a system of coupled linear partial differential equations for the velocity, the temperature and the concentration; and their solutions are obtained with the help of Laplace transform technique. The effects of various flow parameters such as the chemical reaction parameter, thermal Grashof number, mass Grashof number, Schmidt number, Prandtl number and time on the velocity, temperature, concentration, skin friction and Nusslet number for both water and air in the cases of both cooling and heating of the plate are analyzed in detail. © Research India Publications.