Thermodynamic Systems & First Law Why thermodynamics matters for NEET Thermodynamics tells us how energy moves and changes form — in a boiling beaker, a pressure cooker, your body, even car engines. For NEET, the First Law and process calculations (q, w, U) are frequent. We will learn to classify systems, identify state/path functions, use the correct sign convention, and compute work and heat for common processes. System, surroundings, and boundary A thermodynamic system is the part we choose to study; everything else is surroundings. The boundary separates them and can be real (walls of a flask) or imaginary (a surface around a flame). Choosing the system correctly makes problems simple. Specified part of the universe we study (e.g., the gas in a cylinder). System Everything outside the system that can exchange energy or matter with it. Surroundings Real or imaginary surface separating system and surroundings; may be fixed/movable, diathermal/adiabatic, permeable/impermeable. Boundary Open vs closed vs isolated systems: arrows show matter and/or energy crossing the boundary (beaker, sealed flask, thermos). System type Matter exchange? Energy exchange? Everyday example Type Types of systems — exchange and examples Open Yes Yes (heat/work) Boiling water in an open beaker (vapour escapes; heat enters) Closed No (mass fixed) Yes Sealed flask with gas (can be heated/cooled; no mass leaves) Isolated No Ideally no Thermos flask approximates isolation for short times Macroscopic vs microscopic; intensive vs extensive properties We usually describe systems by macroscopic (bulk) properties like pressure, volume, temperature, density. Some properties do not depend on the amount taken (intensive), while others scale with size (extensive). A quick test: if you divide the system into two equal parts, intensive properties remain the same; extensive properties halve. Independent of the amount of substance (e.g., temperature T, pressure P, density, molar mass). Intensive property Extensive property Depends on the amount of substance (e.g., volume V, internal energy U, enthalpy H, entropy S, mass, number of moles n). Intensive Temperature (T), Pressure (P), Density ( ), Refractive index, Molar heat capacity (per mole) Extensive Volume (V), Internal energy (U), Enthalpy (H), Entropy (S), Gibbs energy (G), Mass, Number of moles (n) Intensive vs Extensive — common examples Property type Category Examples State functions vs path functions A state function depends only on the current state (P, V, T, composition) — not on how we reached there. Internal energy U, enthalpy H, entropy S, Gibbs energy G, volume V, pressure P, temperature T are state functions. Heat (q) and work (w) are not stored in a system; they are modes of transfer during a process, so they depend on the path taken between states — hence path functions. State functions U, H, S, G, V, P, T Depend only on initial and final states Path functions q, w Depend on how the change is carried out (path) State vs Path functions — at a glance Category Members Key idea Function type Heat (q) and work (w) are path functions — they depend on the path of the process. Internal energy (U) is a state function: U depends only on initial and final states. Heat (Q) and work (W) are state functions just like internal energy. Modern IUPAC sign conventions for q and w We will use the modern IUPAC sign convention throughout: heat absorbed by the system is positive (q > 0). Work done on the system is positive (w > 0); work done by the system is negative (w < 0). Keep this straight for accurate First-Law calculations. Sign conventions (IUPAC) with examples Transfer q sign w sign Example Case System absorbs heat Heating a gas at constant volume: q > 0, w = 0 System releases heat Cooling water in a bath: q < 0 Work done on system Compressing a gas by pushing the piston in: w > 0 Work done by system Gas expansion lifting a piston: w < 0 The signs of heat (Q) and work (W) can be chosen arbitrarily or swapped across problems. Stick to one convention: IUPAC uses q > 0 for heat absorbed by the system, and w > 0 for work done on the system. Expansion work done by the system is negative. First Law of Thermodynamics Piston–cylinder showing heat in/out and work on/by the system, with U tracking the energy change of the system. Here Q and W represent heat and work with the chosen sign convention. We will use lowercase q, w in worked problems. First Law (v1 preserved) The First Law states energy is conserved. The internal energy change of the system ( U) equals heat added (q) plus work done on the system (w). Because U is a state function, U depends only on initial and final states, not on the path. Common thermodynamic processes Isothermal T constant U = 0 (ideal gas) w rev = -nRT ( V 2 / V 1 ); q = -w Adiabatic q = 0 U = w For reversible adiabatic: pV = const (preview; not needed here) Isobaric P constant q p = H Work w = -P V Isochoric (isometric) V constant w = 0 q v = U Cyclic State repeats Sum over cycle: U cycle = 0 q cycle = - w cycle Process types and key relations Process What is constant? Key relation Sample result (ideal gas) Type At constant volume, no expansion work is possible (w = 0). Constant volume heat (v1 preserved) At constant pressure, heat equals enthalpy change. Full enthalpy discussion comes next chapter. Constant pressure heat (preview; v1 preserved) Isothermal ideal-gas internal energy For an ideal gas, internal energy depends only on T, so isothermal implies no change in U. Adiabatic energy balance With q = 0 in adiabatic process, any change in U equals work. Work of expansion and compression Mechanical work in gas expansion/compression shows up as area under the P–V curve. For a constant external pressure (irreversible) step, work depends only on V. For a reversible isothermal expansion, the system is always in near-equilibrium with surroundings — the process is quasi-static and maximises (in magnitude) the work for the same end states. Irreversible expansion work (v1 preserved) For a single-step expansion/compression against constant external pressure. Sign is negative for expansion (work done by the system). Reversible isothermal work (ideal gas) At constant T for an ideal gas, use either volume or pressure ratio. Area under P–V curve equals work: larger magnitude for reversible isothermal than for a one-step irreversible expansion to the same final state. gpt-image-2 P–V diagram comparing isothermal reversible vs irreversible expansion: smooth hyperbola curve for reversible from (P1,V1) to (P2,V2), and a horizontal line at P ext for a single-step irreversible jump. Shade area under each curve; label w rev and w irrev with magnitudes. Clean vector style, red arrows for processes, axes labeled P (y), V (x). No text inside figure. 2026-05-26T17:04:21.306Z neet-alert For the same initial and final states, | w rev | > | w irrev | in expansion of an ideal gas. Reversible (quasi-static) gives maximum work output in magnitude. Data: 1.0 mol ideal gas at T = 300 K expands isothermally from V1 = 10.0 L to V2 = 20.0 L. Irreversible case against P ext = 1.0 atm: V = 10.0 L = 1.0 10 -2 m 3 . w = - P ext V = -(1.01325 10 5 Pa)(1.0 10 -2 m 3 ) -1.01 kJ. Reversible case: w rev = -nRT (V2/V1) = -(1)(8.314 J mol -1 K -1 )(300 K) 2 -(2494)(0.693) -1.73 kJ. Isothermal ideal gas: U = 0, so q = -w. Therefore q irrev +1.01 kJ; q rev +1.73 kJ. Worked example: isothermal expansion (ideal gas) Cyclic processes and real engines (preview) In a complete cycle, a system returns to its initial state, so the net internal energy change over one cycle is zero: U cycle = 0. Hence, q cycle = - w cycle . Real devices like internal combustion engines (Otto/Diesel) and refrigerators run on cycles. Their full analysis (efficiency, entropy) comes later; for now, remember the U cycle = 0 rule. Carnot cycle on a P–V diagram (preview): two isothermal and two adiabatic legs forming a closed loop; net work equals loop area. gpt-image-2 2026-05-26T17:04:21.489Z Clean P–V diagram of an idealised Carnot cycle: isothermal expansion ( T h ) hyperbola, adiabatic expansion, isothermal compression ( T c ), adiabatic compression. Arrows showing cycle direction, loop area shaded as net work. Vector textbook style, labels Th, Tc, P, V. No embedded text beyond axis labels. Your body is an open system: matter and energy flow in and out while total energy is conserved globally. Open-system life: You take in food (chemical energy) and O2, do work (movement), and release heat and waste. The First Law explains how input energy redistributes — none is lost, only transformed. remember Reversible vs irreversible: what 'reversible' really means A reversible process is an ideal limit carried out in infinitely small steps (quasi-static), always near equilibrium, and can be exactly reversed by infinitesimal changes. It is not 'fast' — actually the opposite. Real processes are irreversible to some degree; we use the reversible ideal as a benchmark for maximum work (in expansion) or minimum work (in compression). Reversible means the process happens very quickly or with no friction only. Reversible means quasi-static and exactly reversible by infinitesimal changes. It is an ideal, infinitely slow limit that maximises (in magnitude) the work in expansions. High-yield NEET traps to avoid State vs path: U, H, S, G, V, P, T are state; q and w are path. Isothermal (ideal gas) does not mean q = 0; it means U = 0, so q = -w. Adiabatic means q = 0, not T = 0. Temperature usually changes in adiabatic steps. At constant volume: w = 0, hence q v = U (v1). At constant pressure: q p = H (v1, preview). For same end states, | w rev | > | w irrev | in expansion. Predicting reaction feasibility without calculation. Spontaneity Criteria (Gibbs Energy) TREND Thermodynamics Gibbs Free Energy Reaction Feasibility NEET Chemistry Exo-Positive is always Go, Endo-Negative is always No; for matching signs, High T favors disorder and Low T favors heat. H Sign S Sign G at Low T G at High T Spontaneity Here are a few precise prompt variations based on your request, optimized for AI image generators (like Midjourney, DALL-E 3, or Stable Diffusion). They range from a strict decision tree to a matrix-style chart common in NEET prep materials. Option 1: The Logical Decision Tree (Best for algorithmic visualization) > Prompt: A professional scientific flowchart diagram illustrating the decision process for Spontaneity Criteria (Gibbs Free Energy). The chart begins with the equation G = H - T S at the top. It branches into a decision tree based on the signs of Enthalpy ( H ) and Entropy ( S ). Four distinct terminal boxes display the outcomes: "Always Spontaneous," "Always Non-Spontaneous," "Spontaneous at High T," and "Spontaneous at Low T." Use sharp black vector lines, crisp sans-serif typography, and color-coding (red for positive, blue for negative). Style: Labeled textbook vector, high contrast, scientific accuracy, white background, educational infographic. Option 2: The "Trend" Matrix Table (Best for the specific "NEET Table" context) > Prompt: A clean, high-contrast scientific table visualization showing the trends of Thermodynamic Spontaneity. A 2x2 layout or structured list. Columns labeled: Enthalpy ( H ), Entropy ( S ), and Resulting Spontaneity. Visual icons represented by arrows pointing up (positive) and down (negative). The layout highlights the four scenarios: Exothermic/Disordered (Always Spontaneous), Endothermic/Ordered (Never Spontaneous), and the two Temperature-dependent cases. Green checks and red crosses for visual cues. Style: Labeled textbook vector, high contrast, scientific accuracy, white background, flat design. Option 3: The Temperature Scale Diagram (Best for visualizing the "Trend" of T) > Prompt: A vector chemistry diagram focusing on the effect of Temperature on Gibbs Energy ( G ). A central flowchart design where the inputs are H and S . The diagram visually splits into a temperature scale showing how specific reactions become spontaneous only above or below certain temperatures (equilibrium points). Mathematical symbols clearly visible. Minimalist color palette of primary colors against white. Style: Labeled textbook vector, high contrast, scientific accuracy, white background, chemistry study guide aesthetic. Tips for Best Results: Aspect Ratio: If using Midjourney, add --ar 3:2 or --ar 4:3 for a wider layout that fits flowchart text better. Text Handling: Most AI struggles with specific text. You may need to specify "Blank labels" if you intend to add the specific text ( G , H , etc.) yourself in Photoshop/Canva later, or use DALL-E 3 which handles text generation better than others. - H (Exothermic) + S (Disorder) - G - G Spontaneous at all temperatures + H (Endothermic) - S (Order) + G + G Non-spontaneous at all temperatures - H (Exothermic) - S (Order) - G (Enthalpy-driven) + G (Entropy-driven) Spontaneous at low T only + H (Endothermic) + S (Disorder) + G (Enthalpy-driven) - G (Entropy-driven) Spontaneous at high T only H = T S G = 0 Equilibrium State Equilibrium State System is at equilibrium Isothermal A process occurring at a constant temperature throughout the change. T (Temperature) U = 0 , PV = constant , Q = -W Adiabatic A process in which no heat enters or leaves the system. Q = 0 (Heat transfer) PV = constant , U = W Isobaric A process occurring at a constant pressure. P (Pressure) V/T = constant , W = -P(V 2 - V 1) , Q = nC p T Isochoric A process occurring at a constant volume. V (Volume) W = 0 , P/T = constant , Q = U = nC v T Cyclic A series of processes where the system returns to its initial state. Internal Energy (Initial = Final) U total = 0 , Q net = -W net Thermodynamic Processes Glossary Clear distinction between process types to apply First Law correctly. GLOSSARY Iso-Thermal (T stays), Adia-No-Heat, Iso-Baric (Bar for Pressure), Iso-Choric (Chorus-Volume stays). Definition Constant Variable Key Equation Condition Process Thermodynamics Physics First Law NEET High Yield