Thermodynamics Revision

 

Over 110 Short Q&A for Thermodynamics (Class 11 NCERT)

Part 1: Basic Concepts and First Law of Thermodynamics

Q. No.	Question	Answer
1	What is the System in thermodynamics?	The part of the universe chosen for thermodynamic consideration.
2	What is the Surrounding?	Everything in the universe other than the system.
3	What is an Isolated System?	A system that can exchange neither energy nor matter with the surroundings.
4	What is a Closed System?	A system that can exchange energy but not matter with the surroundings.
5	What is an Open System?	A system that can exchange both energy and matter with the surroundings.
6	Define an Isothermal Process.	A process carried out at constant temperature (ΔT=0).
7	Define an Adiabatic Process.	A process where no heat is exchanged between the system and surroundings (q=0).
8	Define an Isobaric Process.	A process carried out at constant pressure (ΔP=0).
9	Define an Isochoric Process.	A process carried out at constant volume (ΔV=0).
10	What is a Reversible Process?	A process that can be reversed by an infinitesimal change and takes place in infinite steps.
11	What are State Functions?	Properties of the system that depend only on the initial and final states, not on the path taken.
12	Give three examples of State Functions.	Internal Energy (U), Enthalpy (H), Entropy (S), Gibbs Energy (G).
13	Give two examples of Path Functions.	Heat (q) and Work (w).
14	State the First Law of Thermodynamics.	Energy can neither be created nor destroyed; the total energy of the universe is constant.
15	Write the mathematical form of the First Law of Thermodynamics.	ΔU=q+w (Change in internal energy equals heat plus work).
16	What is the sign convention for heat absorbed by the system?	Positive (+q).
17	What is the sign convention for work done on the system (compression)?	Positive (+w).
18	What is the sign convention for work done by the system (expansion)?	Negative (−w).
19	What is the term for the heat content of a system at constant pressure?	Enthalpy (H).
20	Write the relationship between ΔH and ΔU for chemical reactions.	ΔH=ΔU+Δng​RT (for reactions involving gases).

Part 2: Enthalpy Changes ($\Delta H$) and Hess's Law

Q. No.	Question	Answer
21	Define Enthalpy of Reaction (Δr​H).	The enthalpy change accompanying a reaction represented by a balanced equation.
22	What is an Exothermic Reaction?	A reaction where heat is released (ΔH is negative).
23	What is an Endothermic Reaction?	A reaction where heat is absorbed (ΔH is positive).
24	Define Standard Enthalpy of Formation (Δf​H∘).	Enthalpy change when one mole of a compound is formed from its elements in their standard states.
25	What is the standard enthalpy of formation of an element in its reference state?	Zero (By definition, e.g., Δf​H∘ for C(graphite)=0).
26	Define Enthalpy of Combustion (Δc​H∘).	Enthalpy change when one mole of a substance is completely burnt in excess oxygen.
27	Define Enthalpy of Neutralization.	Enthalpy change when one gram equivalent of an acid is neutralized by one gram equivalent of a base in dilute solution.
28	State Hess's Law of Constant Heat Summation.	The total enthalpy change for a reaction is the same regardless of the path taken, provided the initial and final states are the same.
29	What is Bond Enthalpy?	The energy required to break one mole of a particular type of bond in the gaseous state.
30	How is Δr​H calculated using bond enthalpies?	Δr​H=∑Hbonds broken​−∑Hbonds formed​ (Reactants - Products).
31	How is Δr​H∘ calculated using standard enthalpies of formation?	Δr​H∘=∑Δf​Hproducts∘​−∑Δf​Hreactants∘​.
32	Define Enthalpy of Phase Transition.	The enthalpy change when a substance changes from one physical state to another at constant T and P.
33	What is the relationship between ΔHsublimation​ and ΔHfusion​/ΔHvaporization​?	ΔHsub​=ΔHfus​+ΔHvap​.
34	Define Standard State of a substance.	Its most stable state at 1 bar pressure and a specified temperature (usually 298 K).
35	What does the term Δng​ in ΔH=ΔU+Δng​RT represent?	(Moles of gaseous products) - (Moles of gaseous reactants).

Part 3: Second and Third Laws, Entropy, and Spontaneity

Q. No.	Question	Answer
36	State the Second Law of Thermodynamics (Entropy).	The entropy of the universe always increases in the course of every spontaneous process.
37	Define Entropy (S).	A measure of the degree of randomness or disorder in the system.
38	What is the unit of Entropy?	J K−1 mol−1 (or J K−1).
39	How does entropy change during the melting of ice?	Increases (ΔS>0), as liquid is more disordered than solid.
40	How is the change in entropy for a reversible process calculated?	ΔS=qrev​/T.
41	When is a process Spontaneous (based on ΔStotal​)?	When ΔStotal​(system+surr)>0.
42	What is the spontaneity condition based on Gibbs Free Energy (ΔG)?	Process is spontaneous if ΔG<0.
43	Write the equation that relates ΔG,ΔH, and ΔS (Gibbs equation).	ΔG=ΔH−TΔS.
44	What does Gibbs Free Energy (G) represent?	The energy available to do useful work.
45	What does ΔG=0 signify for a reaction?	The system is at Equilibrium.
46	State the Third Law of Thermodynamics.	The entropy of a perfectly crystalline substance at absolute zero (0 K) is zero.
47	How do the signs of ΔH and ΔS lead to spontaneity at all temperatures?	ΔH<0 (Exothermic) and ΔS>0 (Increased disorder).
48	How do the signs of ΔH and ΔS lead to non-spontaneity at all temperatures?	ΔH>0 (Endothermic) and ΔS<0 (Decreased disorder).
49	What condition is required for a ΔH>0,ΔS>0 process to be spontaneous?	High temperature, such that TΔS>ΔH.
50	What condition is required for a ΔH<0,ΔS<0 process to be spontaneous?	Low temperature, such that **$T \Delta S <
51	How does ΔG∘ relate to the Equilibrium Constant (K)?	ΔG∘=−RTlnK (or −2.303RTlogK).
52	If ΔG∘ is positive, is K>1 or K<1?	K<1 (Reaction favors reactants at standard state).
53	If ΔG∘ is negative, is K>1 or K<1?	K>1 (Reaction favors products at standard state).

Part 4: Work and Specific Concepts

Q. No.	Question	Answer
54	What type of work is typically considered in chemical thermodynamics?	Pressure-Volume (PV) Work.
55	What is the formula for PV work done in a reversible expansion?	wrev​=−∫Pext​dV (where Pext​≈Pint​).
56	What is the formula for PV work done in an irreversible expansion (against constant external pressure)?	wirrev​=−Pext​ΔV.
57	Which type of process (reversible or irreversible) does maximum work?	Reversible expansion work is maximum.
58	What is the value of work done in a vacuum (free expansion)?	Zero (w=−Pext​ΔV, and Pext​=0).
59	What is the relationship between the specific heat capacity at constant volume (Cv​) and internal energy?	Cv​=(∂U/∂T)v​.
60	What is the relationship between the specific heat capacity at constant pressure (Cp​) and enthalpy?	Cp​=(∂H/∂T)p​.
61	What is Joule-Thomson effect?	The cooling or heating of a real gas when it is made to expand adiabatically from a region of high pressure to a region of low pressure.
62	What is the relationship between Cp​ and Cv​ for an ideal gas (Mayer's formula)?	Cp​−Cv​=R (where R is the Universal Gas Constant).
63	Define Standard Enthalpy of Atomization (Δa​H∘).	Enthalpy change when one mole of a substance is completely dissociated into atoms in the gaseous state.
64	Define Standard Enthalpy of Solution (Δsol​H∘).	Enthalpy change when one mole of a substance dissolves in a specified amount of solvent.
65	What is the standard pressure taken for gas measurements in NCERT?	1 bar.

Part 5: Standard Gibbs Energy and Equilibrium

Q. No.	Question	Answer
66	Define Standard Gibbs Energy of Formation (Δf​G∘).	Change in Gibbs energy when one mole of a compound is formed from its elements in their standard states.
67	What is the value of Δf​G∘ for O2​ gas?	Zero (element in standard state).
68	What is the relationship between ΔG and ΔG∘?	ΔG=ΔG∘+RTlnQ (where Q is the reaction quotient).
69	At equilibrium, what is the value of the reaction quotient Q?	Q=K (Equilibrium Constant).
70	If ΔH and ΔS are both positive, which factor determines spontaneity?	Temperature (T) (Spontaneous at high T).
71	If K=1, what is the value of ΔG∘?	ΔG∘=0.
72	If ΔH=−10 kJ and ΔS=−100 J/K, calculate Teq​.	T=ΔH/ΔS=100 K.
73	What is the term for a reaction that goes almost to completion?	High K or Large negative ΔG∘.
74	What is the term for a reaction that hardly proceeds?	Low K or Large positive ΔG∘.
75	How does increasing the temperature affect the ΔG of an exothermic reaction?	ΔG=ΔH−TΔS; for ΔH<0, increasing T makes TΔS (positive) more significant, making ΔG less negative (less spontaneous).
76	How does increasing the temperature affect the ΔG of an endothermic reaction?	ΔG=ΔH−TΔS; for ΔH>0, increasing T makes TΔS more significant, potentially making ΔG negative (more spontaneous).

Part 6: Miscellaneous Concepts and $\Delta U$ vs $\Delta H$

$Q.\; No.QuestionAnswer77What\; is\; the\; value\; of\; \Delta V\; for\; a\; reaction\; in\; a\; bomb\; calorimeter?Zero\; (constant\; volume).78What\; quantity\; is\; measured\; directly\; by\; a\; bomb\; calorimeter?\Delta U\; (Change\; in\; Internal\; Energy).79What\; is\; the\; relationship\; between\; \Delta H\; and\; \Delta U\; when\; there\; are\; no\; gaseous\; reactants\; or\; products?\Delta H=\Delta U\; (\Delta ng=0).80Is\; \Delta H\; or\; \Delta U\; usually\; larger\; for\; a\; gaseous\; reaction\; where\; the\; number\; of\; moles\; of\; product\; gas\; is\; greater\; than\; reactant\; gas?\Delta H>\Delta U\; (since\; \Delta ng>0,\; and\; \Delta H=\Delta U+positive\; term).81What\; is\; the\; maximum\; work\; a\; system\; can\; do\; under\; isothermal\; and\; reversible\; conditions?wmax=-2.303nRTlog(V2/V1).82What\; is\; the\; internal\; energy\; (U)\; of\; an\; ideal\; gas\; dependent\; upon?Temperature\; only.83What\; is\; a\; Cyclic\; Process?A\; process\; where\; the\; system\text{'}s\; initial\; and\; final\; states\; are\; the\; same\; (\Delta U=0,\Delta H=0,\dots ).84What\; is\; the\; \Delta U\; of\; a\; cyclic\; process?Zero\; (\Delta U=0).85Is\; the\; cooling\; of\; a\; gas\; during\; adiabatic\; expansion\; endothermic\; or\; exothermic\; from\; the\; system\text{'}s\; perspective?Neither.\; It\; is\; an\; adiabatic\; process\; (q=0).\; The\; work\; done\; by\; the\; system\; decreases\; U,\; causing\; cooling.86Define\; Heat\; Capacity\; (C).The\; heat\; required\; to\; raise\; the\; temperature\; of\; a\; given\; amount\; of\; substance\; by\; 1\circ C\; or\; 1\; K.87Define\; Molar\; Heat\; Capacity\; (Cm).Heat\; capacity\; for\; one\; mole\; of\; a\; substance.88What\; is\; the\; main\; difference\; between\; Heat\; (q)\; and\; Internal\; Energy\; (U)?q\; is\; a\; path\; function\; (energy\; in\; transit);\; U\; is\; a\; state\; function\; (stored\; energy).89What\; is\; the\; sign\; of\; \Delta H\; for\; the\; process\; A(s)\to A(g)\; (Sublimation)?Positive\; (\Delta H>0,\; always\; endothermic).90What\; is\; the\; sign\; of\; \Delta S\; for\; the\; reaction\; 2SO2(g)+O2(g)\to 2SO3(g)?Negative\; (\Delta S<0,\; disorder\; decreases\; as\; \Delta ng=-1).91What\; is\; the\; standard\; enthalpy\; of\; combustion\; of\; a\; fuel\; a\; measure\; of?Its\; calorific\; value\; (how\; much\; heat\; it\; releases).92What\; is\; the\; standard\; temperature\; used\; in\; most\; thermodynamic\; calculations?298\; K\; (or\; 25\circ C).93Name\; the\; device\; used\; to\; measure\; the\; heat\; changes\; at\; constant\; volume.Bomb\; Calorimeter.94Name\; the\; device\; used\; to\; measure\; the\; heat\; changes\; at\; constant\; pressure.Coffee\; Cup\; Calorimeter\; (or\; any\; simple\; calorimeter).95What\; is\; the\; relationship\; between\; the\; spontaneity\; of\; a\; forward\; and\; reverse\; reaction?If\; the\; forward\; reaction\; is\; spontaneous,\; the\; reverse\; reaction\; is\; non-spontaneous\; (\Delta Grev=-\Delta Gfwd).96What\; is\; the\; \Delta H\; value\; for\; an\; ideal\; reversible\; process\; at\; constant\; T\; and\; P?\Delta H=0\; (Isothermal\; process\; for\; ideal\; gas).97What\; is\; the\; significance\; of\; the\; fact\; that\; diamond\; is\; not\; the\; standard\; state\; of\; carbon?Its\; \Delta fH\circ \; is\; not\; zero\; (graphite\; is\; the\; standard\; state).98Is\; \Delta Ssystem\; positive\; or\; negative\; for\; the\; process\; CaCO3(s)\to CaO(s)+CO2(g)?Positive\; (\Delta S>0),\; due\; to\; the\; formation\; of\; a\; gas.99How\; does\; the\; kinetic\; energy\; of\; ideal\; gas\; molecules\; change\; during\; isothermal\; expansion?Remains\; unchanged\; (KE\; depends\; only\; on\; T).100What\; is\; the\; value\; of\; \Delta G\; when\; a\; reaction\; mixture\; is\; composed\; only\; of\; elements\; in\; their\; standard\; states?\Delta G\; is\; the\; \Delta rG\; for\; the\; formation\; of\; the\; product.101What\; term\; refers\; to\; a\; state\; that\; is\; stable\; under\; a\; given\; set\; of\; conditions,\; but\; not\; the\; most\; stable?Metastable\; state\; (e.g.,\; diamond\; at\; 298\; K,1\; bar).102What\; is\; the\; sign\; of\; w\; for\; a\; reaction\; in\; a\; closed\; container\; where\; P\; increases?Zero\; (w=0\; as\; \Delta V=0).103What\; is\; the\; T\Delta S\; term\; in\; the\; Gibbs\; equation\; known\; as?Unavailable\; energy\; (energy\; lost\; to\; disorder/randomness).104What\; is\; the\; \Delta U\; for\; the\; isothermal\; expansion\; of\; an\; ideal\; gas?Zero\; (\Delta U=0).105If\; a\; system\; does\; work\; on\; the\; surrounding\; and\; releases\; heat,\; what\; is\; the\; sign\; of\; \Delta U?Negative\; (\Delta U=q+w,\; where\; q<0\; and\; w<0).106Why\; are\; the\; Cp\; and\; Cv\; values\; for\; liquids\; and\; solids\; nearly\; the\; same?They\; are\; almost\; incompressible,\; so\; \Delta V\approx 0\; even\; at\; constant\; P,\; making\; P\Delta V\approx 0.107What\; is\; the\; criterion\; for\; a\; process\; to\; be\; non-spontaneous?\Delta G>0\; or\; \Delta Stotal<0.108What\; is\; the\; \Delta H\; for\; the\; formation\; of\; one\; mole\; of\; Cl(g)\; from\; Cl2(g)?1/2\times Bond\; Enthalpy\; of\; Cl-Cl.109Is\; work\; always\; zero\; in\; an\; isochoric\; process?Yes,\; because\; \Delta V=0,\; so\; w=-Pext\Delta V=0.110If\; a\; process\; is\; spontaneous,\; is\; it\; necessarily\; fast?No,\; spontaneity\; is\; a\; thermodynamic\; concept;\; speed\; is\; a\; kinetic\; concept.$

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