NCERT Solutions Class 11 Chemistry Chapter 5 Thermodynamics PDF

The class 11 chemistry NCERT solutions chapter 5 Thermodynamics cover every intext and back-exercise question, according to the latest 2026-27 CBSE syllabus, and help students prepare for the CBSE Boards, JEE Main, JEE Advanced, NEET and CUET. Each answer is worked step by step, from the first law and enthalpy to Hess's law, entropy and the Gibbs energy test for spontaneity.

This chapter builds the energy language that runs through equilibrium, electrochemistry and every reaction you study later, so the sign rules here are worth getting right early.

• CBSE Weightage: 6 to 8 marks, part of the Physical Chemistry unit that carries most of the numerical questions.
• Topics covered: system and surroundings, internal energy, first law, enthalpy, heat capacity, Hess's law, bond enthalpy, entropy and Gibbs energy.
• Exercise count: 22 back-exercise questions plus intext questions, a mix of sign-convention theory and short numericals.

These class 11 chemistry NCERT solutions chapter 5 Thermodynamics are curated by subject experts, based on the 2026-27 NCERT textbook, and checked against the last five years of CBSE Board, JEE Main and NEET papers.

Why Thermodynamics Matters and What the Chapter Covers

Thermodynamics studies the energy changes that go with chemical and physical processes. It answers two big questions: how much heat a reaction gives out or takes in, and whether the reaction will happen on its own. These ideas decide which reactions run in a battery, a furnace or a living cell.

• Everyday role: fuels, cold packs, and metabolism all depend on the heat that reactions release or absorb.
• Core skill: tracking energy as heat and work, and keeping the signs correct at every step.
• Why it is central: the Gibbs energy test defined here decides spontaneity for every later chapter.

Thermodynamics tells you if a reaction is possible, not how fast it goes. Speed is the job of kinetics, a separate chapter. Students who fix the sign conventions in this chapter early find the numericals much easier. The class 11 chemistry NCERT solutions chapter 5 Thermodynamics below follow the same order as the NCERT textbook.

System, Surroundings and State Functions

Every problem starts by choosing what you are studying. The system is the part of the universe under observation, and the surroundings are everything else. The boundary separates the two. Getting this split clear makes the sign of heat and work obvious later.

A state function depends only on the present state of the system, not on how it got there. Internal energy, enthalpy, entropy and Gibbs energy are all state functions. Heat and work are path functions, so their values depend on the route taken. The change in a state function is the same for every path between the same two states. That single idea is what makes Hess's law work.

Internal Energy, Work and Heat in a Chemical System

The total energy stored inside a system is its internal energy, written as U. You cannot measure U directly, but you can measure its change, ΔU, when a system moves from one state to another. This change happens through two routes: heat and work.

• Heat (q): energy that flows because of a temperature difference between system and surroundings.
• Work (w): energy transferred when a force acts, most often pressure-volume work when a gas expands or is compressed.
• Sign rule: heat added to the system is positive; work done on the system is positive.

For a gas expanding against a constant external pressure, the work done by the system is w = −p_extΔV. The NCERT sign convention treats heat gained and work done on the system as positive. This is the most common source of lost marks in the chapter, so write the sign next to every number as you go. A reversible expansion does the maximum work, which is a favourite short-answer point.

First Law of Thermodynamics and Its Equation

The first law of thermodynamics is the law of conservation of energy applied to heat and work. Energy is neither created nor destroyed; it only changes form. For any process, the change in internal energy equals the heat added plus the work done on the system.

The working equation is ΔU = q + w. Here ΔU is the change in internal energy, q is the heat, and w is the work. From this one line you can find any of the three quantities when the other two are given, which is exactly what several back-exercise numericals ask.

At constant volume the heat supplied equals the change in internal energy. Learn the three special cases above, because the intext questions test them directly. The first law fixes the total energy but says nothing about direction, which is why entropy is needed later.

Enthalpy and Heat Capacity of a Substance

Most reactions in the lab happen in open containers at constant pressure, not constant volume. For these, chemists use enthalpy, written H, defined as H = U + pV. The heat exchanged at constant pressure equals the change in enthalpy, ΔH.

The link between the two heat measures is ΔH = ΔU + Δn_gRT, where Δn_g is the change in the number of moles of gas. When no gas moles change, ΔH and ΔU are equal. This relation appears in at least one numerical every year.

• Heat capacity (C): the heat needed to raise the temperature of a substance by one degree.
• C_p and C_V: heat capacities at constant pressure and constant volume; for an ideal gas C_p − C_V = R.
• Specific and molar: specific heat is per gram; molar heat capacity is per mole.

Enthalpy is a state function, so ΔH depends only on the initial and final states. An exothermic reaction has a negative ΔH because it releases heat, while an endothermic reaction has a positive ΔH. Keep this sign link firmly in mind before the next section.

Enthalpy of Reactions, Hess's Law and Bond Enthalpy

Different reactions have their own named enthalpy changes, and the NCERT exercise expects you to define each one cleanly. These standard enthalpies are measured at 298 K and one bar pressure.

Hess's law states that the total enthalpy change of a reaction is the same whether it happens in one step or several. It follows directly from enthalpy being a state function. You can add reactions like algebra to reach a target equation, then add their ΔH values the same way. Hess's law lets you find an enthalpy that is hard to measure directly, such as the formation of carbon monoxide. The bond enthalpy route gives another estimate: ΔH equals the bonds broken minus the bonds formed. Both methods appear in the back exercise.

Entropy, Gibbs Energy and Spontaneity

The first law cannot tell you which way a reaction goes on its own. That job needs entropy, written S, a measure of the disorder or randomness of a system. Processes tend to move towards greater total disorder. Melting ice and mixing gases both raise entropy.

• Entropy rises when a solid melts, a liquid boils, or the number of gas molecules increases.
• Second law: the total entropy of the universe increases in every spontaneous process.
• Gibbs energy (G): combines enthalpy and entropy into a single test for the system alone.

The Gibbs energy change is ΔG = ΔH − TΔS, where T is the temperature in kelvin. A reaction is spontaneous when ΔG is negative, at equilibrium when ΔG is zero, and non-spontaneous when ΔG is positive. A negative ΔG means the reaction can proceed on its own at that temperature. Because the equation carries T, a reaction that is non-spontaneous when cold can become spontaneous when heated, which explains why temperature is so often the twist in these questions. This is the single most tested idea in the whole chapter.

Thermodynamics Exercise-wise Breakdown

The NCERT back exercise has 22 questions that mix theory with short numericals. The intext questions check one idea each. The table below maps the question blocks to their topics so you can plan your practice.

The intext questions before the exercise are shorter and check one idea, such as the sign of ΔH or a single first-law substitution. Solve the intext set first, then the back exercise. Every question in the class 11 chemistry NCERT solutions chapter 5 Thermodynamics PDF is solved with each step shown, so you can compare your working line by line.

Common Mistakes Students Make in the Thermodynamics Chapter

Most marks are lost on signs and definitions, not on hard ideas. Each slip below costs 1 to 2 marks, so watch for them at the exact step.

Student Feedback on the Thermodynamics Solutions

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