An apparatus is described, as including a reaction region for contacting a reactant gas with a reactive solid under conditions effective to form an intermediate product, and an opening for allowing an unreacted portion of the gaseous reagent and the intermediate product to exit the reaction region. The apparatus can be beneficially employed to form a final product as a reaction product of the intermediate product and the reactant gas. The reaction of the reactant gas and reactive solid can be conducted in a first reaction zone, with the reaction of the reactant gas and intermediate product conducted in a second reaction zone. In a specific implementation, the reaction of the reactant gas and intermediate product is reversible, and the reactant gas and intermediate product are flowed to the second reaction zone at a controlled rate or in a controlled manner, to suppress back reaction forming the reactive solid.