6.2
First Law of Thermodynamics
Energy Conservation
Energy can be converted from one form into another, but all of the energy present before a change occurs always exists in some for…
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.
Q1: What does the first law of thermodynamics state?
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.
Q2: How does a car engine demonstrate energy conversion?
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.
Q3: What is internal energy and why is it a state function?
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.
Q4: How does the change in a system's internal energy relate to surroundings?
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.
Q5: Why is internal energy different from heat and work?
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.
Q6: What types of energy contribute to a substance's internal energy?
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.
Q7: How do mass-energy conversions differ between chemical and nuclear reactions?
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.