The reaction of sodium bicarbonate and sulfuric acid is quite endothermic... it seems. Of course, the chemical change is actually exothermic, but from what I've read, the production of carbon dioxide gas is what makes it appear endothermic. I'm assuming that as the gas is produced, it expands and this absorbs heat from the surrounding water. Does the gas expand because as the gas rises, the pressure drops? And does this expansion mean the gas is doing work against the surroundings, making the enthalpy change negative?