First law of thermodynamics
Energy cannot be created or destroyed, only moved. The change in a system's internal energy equals the heat added to it plus the work done on it: ΔU = q + w. Heat in and work done on the system are positive.
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Analogy
A bank account: deposits (heat in, work done on) raise the balance, withdrawals lower it.
Example
Heat added: q = +50 JGas expands, does work: w = −20 JΔU = q + wΔU = +30 J
In practice
A car engine turns some fuel energy into work and the rest leaves as heat. Together they equal the energy released by burning.
Practice
Flashcards
- State the first law as an equation.ΔU = q + w.
Questions people ask
What is a system and what are the surroundings?
The system is the part of the universe you are studying, such as the reaction in a flask. The surroundings are everything else, like the water bath and the air. Energy flows between them, and the total energy of both together stays constant.
What is a state function?
A state function depends only on the present condition of the system, not on how it got there. Internal energy, enthalpy, entropy and Gibbs energy are state functions. Heat and work are not, because they depend on the path taken. This is why Hess's law works.