A reaction in which hydrogen is detached from a molecule. The reaction is strongly endothermic; therefore, heat must be supplied to maintain the reaction temperature. Since carbon-carbon bonds are easier to break than carbon-hydrogen bonds, highly selective reaction conditions are required to cause the molecule to dehydrogenate rather than break apart into small molecules (for example, methane) and carbon (coke). Superheated steam (725°C or 1337°F) is often mixed with the dehydrogenation reactor feed to provide the needed heat of reaction, lower the partial pressure and enhance the equilibrium conversion, and decoke the catalyst to extend the reactor onstream time. In addition, excess hydrogen is sometimes added to the feed to depress coke formation and diminish the breakup of large molecules (for example, styrene) into smaller molecules of much less economic value.