First Law of Thermodynamics

When gasoline fuels a car engine, the energy produced is released as heat and work.

In a car’s combustion engine, the fuel and air mixture contains chemical energy, which is a type of potential energy. This potential energy is then transformed into kinetic energy through a combustion reaction that generates heat. 

The heat coming from the reaction is then converted to work by the expanding hot gases pushing on the piston, which then turns the crankshaft, ultimately bringing the car into motion.

All these energy interconversions must follow the first law of thermodynamics — energy is always conserved and cannot be created or destroyed.

To study energy changes in a system all sources of energy must be considered, including potential and kinetic energy. The total energy within a system at any given time is called the internal energy, which has the symbol capital-U, or sometimes symbolized as capital-E.

A system’s internal energy can change. The internal energy of a car with a full tank — its initial state — is different from the total energy while the car is running and is again different when the tank is empty — its final state. 

Whether all of the fuel is used to drive 300 or 50 miles is irrelevant, when the tank is empty, the car reaches a new internal state. Thus, internal energy is described as a state function, which is not affected by how the system came to be in its current state. 

Therefore, the change in a system’s internal energy, ΔU, is measured as the difference between the values of its final and initial states. 

Because energy is conserved, the change in a system’s internal energy must be accompanied by an equal and opposite change in the energy of the surroundings. 

In chemical systems, the change in internal energy is described by the difference of the reactant’s initial state and product’s final state. It is used to understand the energy flow of a system.