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CBSE Class 11 · Chemistry

Thermodynamics

Official NCERT chapter from Chemistry Part I–II (book code kech1). ExamMaster notes are original teaching at CBSE Class 11 depth.

This lesson follows the official chapter “Thermodynamics” in Chemistry Part I–II. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 11
  • Medium level
  • 8 concepts

1Thermodynamic Terms

Terms: system (the bit you watch), surroundings, universe = both. Open, closed, isolated as taught. U is internal energy; q heat; w work. A term without “what is in / out” is a mute word. This is chemistry thermo — a reaction-energy story — not a piston-chapter dump.

Calling the whole lab “the system” with no boundary is a miss.

Figure. The boundary decides the system. Open lets matter and energy cross; closed keeps the matter and still lets heat in; isolated exchanges neither.

How it works

  1. Draw a boundary — system versus surroundingsThe terms.
  2. Name open / closed / isolated as taughtThe sort.
  3. Keep U, q, w as the energy-wordsThe kit.

2Applications

Applications (school): a reaction heat, an expansion-work, a first-law write. ΔU = q + w or ΔU = q − w — name the sign convention the lesson used and keep it. One application is enough; do not swap to physics engines.

Mixing two sign conventions in one line is a silent fail.

Figure. Chemistry writes ΔU = q + w from the system. Heat arriving is +q; expansion work leaving is −P_ext ΔV, so w is negative when the piston moves out. Arrows are not to force scale.

How it works

  1. Name the convention (q+w or q−W)The first law.
  2. Apply it to one reaction or expansionThe application.
  3. Keep chemistry-size, not a turbine dumpThis class.

3Measurement of ∆U and ∆H: Calorimetry

Calorimetry measures ΔU or ΔH as the lesson framed: a bomb (constant volume) versus a coffee-cup (constant pressure) if taught. q_cal = −q_rxn as the sign-story. A calorimeter is a heat-bucket, not a mole-counter.

A thermometer-only photo with no energy-write is not calorimetry.

Figure. A sealed bomb cannot expand, so the heat it records is ΔU. An open cup sits at constant pressure, so the heat it records is ΔH. Same reaction, two constraints, two heats.

How it works

  1. Name constant-V or constant-P as taughtWhich Δ.
  2. Write q_cal and flip the sign for the reactionThe measure.
  3. Keep joules, then ÷ moles if Δr was askedHonest.

4Enthalpy Change, ∆rH of a Reaction – Reaction Enthalpy

Reaction enthalpy ΔrH is the enthalpy change for the as-written moles. An exothermic write has ΔrH negative as taught. ΔrH is per the equation as balanced — doubling the equation doubles ΔrH.

Using a per-gram heat as ΔrH without moles is a miss.

Figure. ΔrH is products minus reactants. When the products sit lower, heat left the system and ΔrH is negative. Levels are ordered, not drawn to a kJ scale.

How it works

  1. Copy the balanced equationThe given.
  2. Attach ΔrH with the taught signReaction enthalpy.
  3. Scale if the equation was doubledThe caution.

5Enthalpies for Different Types of Reactions

Different enthalpy types: formation, combustion, atomisation, bond — as named. ΔfH of an element in the standard state is zero as taught. A type-name tells you the reaction-kind, not a second first-law.

Calling every negative ΔH “combustion” is a steal.

Figure. Hess: H is a state function, so C + O2 → CO2 is −394 kJ either straight or via CO. −111 + −283 = −394; the two routes close.

How it works

  1. Name the type (formation, combustion, …)The heading.
  2. Write the matching reaction-kindThe use.
  3. Keep ΔfH(element, standard) = 0 if taughtThe zero.

6Spontaneity

Spontaneity (school): a change that can run without a continuous push, as framed. ΔS universe > 0 is the taught entropy-test; a negative ΔH is a hint, not the whole test. Ice melting at room temperature can be spontaneous even if it “feels cold”.

Calling every exothermic change automatic-forever skips entropy.

Figure. Spontaneity is the sign of ΔS_univ = ΔS_sys + ΔS_surr, not the sign of the system alone. A plus universe change runs; a minus does not.

How it works

  1. Ask whether it can run without a continuous pushSpontaneous as taught.
  2. Bring ΔS or ΔG if the lesson used themThe test.
  3. Refuse “hot = spontaneous” as the only ruleHonest.

7Gibbs Energy Change and Equilibrium

Gibbs: ΔG = ΔH − TΔS as taught. ΔG < 0 is the school spontaneous-at-this-T write; ΔG = 0 is equilibrium. Gibbs energy change links a reaction-heat-and-disorder to a yes/no at a named T. Units kJ, T in kelvin.

Using t in °C inside TΔS is a unit-miss.

Figure. For ΔH = +30 kJ and ΔS = +0.10 kJ/K the line is ΔG = 30 − 0.10 T. It crosses the dashed ΔG = 0 rule at 300 K: below that T the change is not spontaneous; above it, it is. ΔG° = −RT ln K sits at equilibrium on that same zero.

How it works

  1. Copy ΔH, ΔS, T (kelvin)The given.
  2. Form ΔH − TΔSΔG.
  3. Read <0, 0, or >0 as taughtSpontaneous / eq / not.

ΔG sign

ΔH=−40 kJ, ΔS=−80 J/K = −0.080 kJ/K, T=300 K. Find ΔG.

  • TΔS300×(−0.080)=−24 kJ
  • ΔG=ΔH−TΔS−40−(−24)=−16 kJ
  • Sign<0 so spontaneous at 300 K as taught

Pro tip. Match ΔH and TΔS units; T in kelvin.

8A definition is a test you can run

Thermo-word is a test: system, ΔU/ΔH, a calorimeter, ΔrH, or ΔG. If you only say “energy”, you have a heading.

A flame-sticker is not the test.

Figure. A state function is a test: if A and B are fixed, ΔU is the same on every route. q and w fail that test — they change when the path changes.

How it works

  1. Name a term, a Δ, or a calorimeter-lookThe object.
  2. Give the school sentenceThe test.
  3. Then the word has contentThe definition ran.
ΔfH of an element in the standard state is
  1. Zero as taught
  2. Always −100 kJ
  3. A mole-count

The formation-zero.

Notes

  • Mapped to the official NCERT chapter “Thermodynamics”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Formulas

  • ΔU = q + w (name the lesson’s sign)
  • ΔG = ΔH − TΔS
  • ΔfH(element, standard) = 0

Recap

Hold these pegs from the official chapter “Thermodynamics”. The wording is ExamMaster’s teaching, not a textbook recap.

Thermodynamic Terms
Terms: system (the bit you watch), surroundings, universe = both.
Applications
Applications (school): a reaction heat, an expansion-work, a first-law write.
Measurement of ∆U and ∆H: Calorimetry
Calorimetry measures ΔU or ΔH as the lesson framed: a bomb (constant volume) versus a coffee-cup (constant pressure) if taught.
Enthalpy Change, ∆rH of a Reaction – Reaction Enthalpy
Reaction enthalpy ΔrH is the enthalpy change for the as-written moles.
Enthalpies for Different Types of Reactions
Different enthalpy types: formation, combustion, atomisation, bond — as named.
Spontaneity
Spontaneity (school): a change that can run without a continuous push, as framed.

Practise Thermodynamics

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