Intermediate Topic 336 of 1102 Thermodynamics & Energetics
Internal Energy
The total kinetic and potential energy of all particles in a system.
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In-depth explanation of Internal Energy
Internal energy, symbolized U, is the total energy stored within a system, comprising the kinetic energy of all particles from their translation, rotation and vibration, together with the potential energy of their mutual interactions. It is a state function, and its absolute value cannot be measured or calculated in practice, so only changes in U are meaningful. For an ideal gas, internal energy depends solely on temperature because there are no intermolecular forces to store potential energy. In a chemical reaction, the change in U reflects the difference between the energy stored in bonds and intermolecular arrangements of products and reactants. Changes in U are measured most directly in constant-volume calorimetry.
Going deeper: Internal Energy
Key ideas: Internal energy changes only through heat and work, so delta U equals q plus w. At constant volume, delta U equals q_v. For an ideal gas it depends only on temperature, with delta U equal to n Cv delta T for any process, not only constant volume. In reactions, delta U and delta H differ by delta n R T when gases are involved.
Common mistakes and connections: Students confuse internal energy with total energy and include the motion of the entire container, which is excluded. Others assume delta U is zero in all isothermal processes, which is true only for ideal gases and not for real fluids or reacting systems. It connects to enthalpy through H equals U plus PV.
Visualization of Internal Energy
Internal energy is the sum of molecular kinetic and potential energies; raising T pushes more molecules into higher energy states, increasing U. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Heating a sample of argon gas raises its internal energy because the atoms move faster on average.
Frequently asked questions about Internal Energy
What is Internal Energy?
The total kinetic and potential energy of all particles in a system.
Is Internal Energy a beginner, intermediate or advanced topic?
Internal Energy is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
What should I learn before Internal Energy?
Study these earlier topics in Thermodynamics & Energetics first: Energy, Heat, Work in Chemistry.
What comes after Internal Energy?
The next topics in Thermodynamics & Energetics are First Law of Thermodynamics and Enthalpy.
Can I practise Internal Energy with quizzes?
Yes. The interactive lesson includes a self-check question on Internal Energy, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Where Internal Energy fits in Thermodynamics & Energetics
Internal Energy is topic 8 of 91 in Thermodynamics & Energetics.