Lesson 1 of 12 · 7 min

System, surroundings and state functions

NCERT § "Thermodynamic Terms"

Riya's cooker, the flame under it, the kitchen air and Riya herself are all exchanging energy. Before any accounting, we must decide what we are keeping accounts for.

The story this chapter follows: Riya's kitchen and its cooker

Riya cooks dal in a pressure cooker on a piped-gas stove. The gas is methane, the cooker holds 1.00 kg of water, and to see the gas's work clearly we trap 0.40 mol of air at 300 K under a sliding piston: 5.0 L at 2 bar, so nRT = 10 L bar. Every idea in this chapter, from heat and work to Gibbs energy, is a question asked in that kitchen.

The lesson in notes

In short

The system is the part of the universe under study; everything else is the surroundings, and the two together make up the universe.

An open system exchanges both energy and matter with its surroundings (reactants in an open beaker); a closed system exchanges energy but not matter (a sealed flask); an isolated system exchanges neither (an ideal thermos flask).

The state of a system is fixed by measurable state variables such as pressure, volume, temperature and amount.

A state function depends only on the present state of the system, not on how that state was reached: p, V, T, U, H, S and G are state functions.

Heat (q) and work (w) are path functions; their values depend on how the change is carried out, though their sum ΔU does not.

Internal energy U is the total energy of the system (chemical, electrical, mechanical and other forms); only its change ΔU can be measured.

In adiabatic changes no heat passes between system and surroundings (q = 0), so the work done on the system equals ΔU.

An isothermal process takes place at constant temperature; for an ideal gas this means ΔU = 0.

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System, surroundings and state functions

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