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The first law of thermodynamics states that energy is always conserved and cannot be created or destroyed. During chemical or physical changes, energy can be converted from one form into another, but the total amount of energy remains constant. In chemical systems, this principle ensures that when reactants transform into products, the change in internal energy reflects the difference between the energy of reactants and products.

In a car engine, chemical energy stored in gasoline is converted into heat through combustion. The heat from the reaction causes gases to expand, pushing the piston and turning the crankshaft, converting thermal energy into mechanical work. This process exemplifies how potential energy transforms into kinetic energy, with all conversions following the first law of thermodynamics.

Internal energy (U) is the total of all possible kinds of energy present in a substance, including kinetic energy from atomic and molecular motion. It is a state function because its value depends only on the current state of a system, not on how that state was reached. Whether a car's fuel is used to drive 300 or 50 miles, the internal energy when the tank is empty depends only on the final state, not the path taken.

Because energy is conserved, any change in a system's internal energy (ΔU) must be accompanied by an equal and opposite change in the energy of the surroundings. If a system loses internal energy, that energy transfers to the surroundings, and vice versa. This reciprocal relationship ensures that total energy remains constant throughout any process.

Internal energy is a state function, meaning its value depends only on the system's current state. Heat and work, however, are not state functions because their values depend on how a process occurs. For example, reaching a mountain summit has the same elevation regardless of the route taken, but the distance traveled differs. Similarly, internal energy change is path-independent, while heat and work are path-dependent.

Internal energy includes all forms of kinetic energy stored in a substance, such as translational motion (straight-line movement), vibrations, and rotations of atoms or molecules. When thermal energy is added, these motions intensify and kinetic energy increases. When thermal energy is lost, the intensities of these motions decrease, lowering the substance's total internal energy.

In chemical reactions, energy changes are modest and mass changes are too small to measure, so the laws of conservation of matter and energy hold well. In nuclear reactions, energy changes are much larger by factors of a million, making mass changes measurable and matter-energy conversions significant. This distinction reflects the much greater energy release in nuclear processes compared to chemical reactions.