Intermediate Topic 331 of 1102 Thermodynamics & Energetics
State Function
A property depending only on the current state, such as enthalpy, entropy and internal energy.
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In-depth explanation of State Function
A state function is a property whose value depends only on the current condition of the system, not on how that condition was reached. Pressure, volume, temperature, internal energy, enthalpy, entropy and Gibbs energy are all state functions. Because of this, the change in a state function between two states is the same for every path connecting them, which is the foundation of Hess's law and of tabulated thermodynamic data. Heat and work are not state functions, so their values depend on the route. Thermodynamic tables can therefore give a single value for a substance, allowing reaction energies to be computed without knowing the mechanism.
Going deeper: State Function
Key ideas: The test for a state function is path independence: delta X depends only on the initial and final states. Tabulated quantities such as enthalpy of formation, entropy and Gibbs energy are all state functions, which is why they can be added and subtracted around a cycle. Mathematically the change around any closed loop is zero, so state functions have exact differentials.
Common mistakes and connections: Do not assume that because delta U equals q plus w, both q and w are state functions. Only their sum is. Another trap is treating a quantity as a state function in a reaction where the path changes the final state, such as different products. This idea is the logical basis of Hess's law and Born-Haber cycles.
Visualization of State Function
Two different routes join the same start and end states. State functions such as U, H, S and G change by the same amount along both, regardless of the path. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Climbing from sea level to a mountain summit changes altitude by the same amount whichever trail is taken.
Frequently asked questions about State Function
What is State Function?
A property depending only on the current state, such as enthalpy, entropy and internal energy.
Is State Function a beginner, intermediate or advanced topic?
State Function is a intermediate topic in Thermodynamics & Energetics. It builds on the vocabulary of beginner topics in this area.
What should I learn before State Function?
Study these earlier topics in Thermodynamics & Energetics first: System and Surroundings, Open Closed Isolated Systems.
What comes after State Function?
The next topics in Thermodynamics & Energetics are Path Function and Energy.
Can I practise State Function with quizzes?
Yes. The interactive lesson includes a self-check question on State Function, and the Thermodynamics & Energetics quiz draws on every topic in this area.
Where State Function fits in Thermodynamics & Energetics
State Function is topic 3 of 91 in Thermodynamics & Energetics.