Some of us will ask these questions when we buy that next car or truck. And we like to brag about the speed, power or efficiency of the engines that drive us. When they design the powerful and efficient engines, are the engineers at Ford and Daimler Chrysler and Porsche and General Motors dealing with a long history and lots of data, or do they apply chemical principles. Well, fundamental chemical principles guide the designers - principles that are both limiting and enabling. They are limiting in that thermochemistry teaches that there is only so much energy is available for moving the car down the road and they are enabling in that thermodynamics shows us how to maximize the energy available for doing the work of propulsion.
Thermochemistry deals with energy, heat and work.
Much of what we study deals with static systems in which no change is taking place. But we are not content with an automobile or truck that just sits in the driveway, so this discussion deals with situations undergoing change. The change in energy of a system that is going through any process is the sum of the heat added TO the system and the work done ON the system. If heat flows from the system and work is done by the system as in the case of fuel combustion, then both delta q and delta e are negative and delta E is likewise negative. This situation expresses the conversion of the potential or stored energy of the fuel converted to heat and kinetic energy and transferred from the system to its surroundings.