Within the framework of a two species reaction-diffusion model coupled
to the hydrodynamic equations we discuss how a chemical reaction driv
es convective motion in a fluid layer. The model has an oscillatory in
stability with vanishing wavenumber at onset. We derive from the model
equations an envelope equation of Ginzburg-Landau type with complex c
oefficients. To cubic order in the envelope the generated convective v
elocity is slaved to the concentration variables and has no independen
t dynamics. Feedback from the flow in the chemical reaction arises at
higher orders in the amplitude. We compare our results with existing e
xperimental data.