The apparatus described herein comprises a first reaction chamber having an inlet for a hydrocarbon medium, particularly a gas having the composition CnHm, and an outlet. Means for decomposing the hydrocarbons into carbon particles and hydrogen by introducing heat are provided between the inlet and the outlet in the first reaction chamber. The apparatus also comprises a second reaction chamber having an elongated configuration and having a first inlet at one end and an outlet at the opposite end, wherein the first inlet of the second reaction chamber is connected with the outlet of the first reaction chamber, and wherein the second reaction chamber comprises a widening flow cross-section (measured perpendicular to the longitudinal dimension of the second reaction chamber) between the inlet and the outlet. In addition, at least one second inlet into the second reaction chamber is provided, wherein the second inlet can be connected to a source for CO2 and/or H2O. Preferably, the second inlet is connected with a source of CO2, and therefore CO2 is injected therewith during operation. A method for operating this apparatus is also described. The energy balance of a synthesis gas production can be improved with the apparatus and the method for operating compared with known methods.