System, Surroundings and First Law
State functions, internal energy, work and heat. (Chemistry › Thermodynamics, NEET UG syllabus.)
What is System, Surroundings and First Law?
The part of the universe chosen for thermodynamic study, separated from the rest by a boundary.
Key formula / rule: First Law of Thermodynamics
Key points
- Define system, surroundings, and boundary, and classify different types of systems.
- Distinguish between state functions and path functions with examples.
- Explain the concept of internal energy and its significance.
- Define heat and work as modes of energy transfer.
Common exam trap
Confusing state functions with path functions.
Definitions
- Term
System
- Meaning
The part of the universe chosen for thermodynamic study, separated from the rest by a boundary.
- Term
Surroundings
- Meaning
Everything in the universe outside the system that can interact with it.
- Term
Boundary
- Meaning
The real or imaginary surface that separates the system from its surroundings.
- Term
Open System
- Meaning
A system that can exchange both matter and energy with its surroundings.
- Term
Closed System
- Meaning
A system that can exchange energy but not matter with its surroundings.
- Term
Isolated System
- Meaning
A system that can exchange neither matter nor energy with its surroundings.
- Term
State Function
- Meaning
A thermodynamic property whose value depends only on the current state of the system, independent of the path taken to reach that state (e.g., U, H, S, G).
- Term
Path Function
- Meaning
A thermodynamic property whose value depends on the path or manner in which the change from initial to final state occurs (e.g., heat (q), work (w)).
- Term
Internal Energy (U)
- Meaning
The total energy contained within a thermodynamic system, including kinetic and potential energies of its constituent particles. It is a state function.
- Term
Heat (q)
- Meaning
Energy transferred between a system and its surroundings due to a temperature difference.
- Term
Work (w)
- Meaning
Energy transferred between a system and its surroundings by means other than temperature difference, typically involving a force acting over a distance (e.g., expansion/compression work).
- Term
First Law of Thermodynamics
- Meaning
States that energy cannot be created or destroyed, only converted from one form to another. Mathematically expressed as ΔU = q + w.
Learning objectives
Define system, surroundings, and boundary, and classify different types of systems.
Distinguish between state functions and path functions with examples.
Explain the concept of internal energy and its significance.
Define heat and work as modes of energy transfer.
State and apply the First Law of Thermodynamics (ΔU = q + w) to various processes.
Correctly apply sign conventions for heat and work in thermodynamic calculations.
Calculate work done in expansion/compression processes under different conditions.
Formulae
- Name
First Law of Thermodynamics
- Note
Change in internal energy (ΔU) equals heat (q) added to the system plus work (w) done on the system.
- Expression
ΔU = q + w
- Name
Work done by expansion/compression against constant external pressure
- Note
Pext is external pressure, ΔV is change in volume (Vfinal - Vinitial). Negative sign indicates work done by the system (expansion) decreases internal energy.
- Expression
w = -PextΔV
- Name
Work done in reversible isothermal expansion/compression of an ideal gas
- Note
n is moles, R is gas constant, T is temperature, V1/V2 are initial/final volumes, P1/P2 are initial/final pressures. Applicable for reversible processes at constant temperature.
- Expression
w = -nRT ln(V2/V1) = -nRT ln(P1/P2)
Prerequisites
Basic understanding of energy and its forms (kinetic, potential).
Knowledge of states of matter (solid, liquid, gas).
Familiarity with ideal gas laws (PV=nRT).
Basic algebra and logarithmic functions.
Common mistakes
Confusing state functions with path functions.
Incorrectly applying sign conventions for heat (q) and work (w). Remember: 'on the system' is positive, 'by the system' is negative for work; 'absorbed by system' is positive, 'released by system' is negative for heat.
Forgetting that ΔU = 0 for a cyclic process, as it returns to its initial state.
Assuming work is always -PΔV; this is only for constant external pressure. For reversible processes, Pext is replaced by Pinternal.
Not understanding that the First Law applies to the system, not the surroundings, though energy is conserved for the universe.
Keywords
Thermodynamics
System
Surroundings
Boundary
Open System
Closed System
Isolated System
State Function
Path Function
Internal Energy
Heat
Work
First Law
Conservation of Energy
Expansion Work
Sign Convention
Practice preview
In thermodynamics, what constitutes the 'universe'?…
easy
Which of the following equations correctly represents the First Law of Thermodynamics, using the IUPAC convention where work done *on* the system is positive?…
easy
Which of the following thermodynamic properties is classified as a state function?…
medium