Thermodynamics — Class 11 MCQs with Answers

Thermodynamics — important questions & MCQs with answers (Class 11 Chemistry)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Enthalpy H =

   AU + PVBU − PVCPVDQ
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   Answer: U + PV

   Explanation: Enthalpy is defined as H = U + PV, internal energy plus the pressure-volume product, so it tracks heat exchanged at constant P. Writing U − PV instead flips the sign of the PV term and gives a different, undefined quantity.

2. Q2 of 6Easy

   SI unit of molar entropy:

   AJ K⁻¹ mol⁻¹BJ mol⁻¹CJDK mol⁻¹
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   Answer: J K⁻¹ mol⁻¹

   Explanation: Entropy is heat divided by temperature (J K⁻¹); per mole of substance, molar entropy's SI unit is J K⁻¹ mol⁻¹, e.g. S°(H₂O,l) ≈ 70 J K⁻¹ mol⁻¹. Dropping the K⁻¹ (just J mol⁻¹) confuses entropy's unit with enthalpy's.

3. Q3 of 6Medium

   Relation ΔH = ΔU + Δn_g RT applies to:

   ASolids onlyBGas-phase reactionsCLiquids onlyDSolutions
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   Answer: Gas-phase reactions

   Explanation: The added term Δn_g·RT in ΔH = ΔU + Δn_g·RT accounts for PV work from a change in moles of GAS, so it matters only when gases are reactants or products. For solids or liquids, Δn_g ≈ 0 and ΔH ≈ ΔU already.

4. Q4 of 6Medium

   Entropy is a measure of:

   AMassBEnergy releasedCDisorder/randomnessDVolume only
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   Answer: Disorder/randomness

   Explanation: Entropy quantifies the number of accessible microstates, i.e. the disorder or randomness of a system, e.g. gas molecules spreading through a larger volume. Picking mass or volume only confuses entropy with a measure of how much matter there is.

5. Q5 of 6Hard

   Hess's law: ΔH of overall reaction depends only on:

   APressureBPath takenCTemperature aloneDInitial and final states
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   Answer: Initial and final states

   Explanation: Hess's law holds because enthalpy is a state function: the overall ΔH depends only on the initial and final states of the reaction, never on the path or number of steps taken. Picking 'path taken' directly contradicts this state-function property.

6. Q6 of 6Hard

   Second law: in an isolated system, entropy:

   ATends to increaseBDecreasesCStays zeroDBecomes negative
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   Answer: Tends to increase

   Explanation: The second law says the entropy of an isolated system spontaneously tends to increase over time, ΔS_universe ≥ 0, driving processes toward greater disorder. Picking 'decreases' would violate the second law entirely.

Thermodynamics notes: key concepts (Class 11 Chemistry)

Thermodynamics accounts for the energy that flows when matter changes, without asking how fast. This chapter defines systems and surroundings, state functions, the first law linking heat and work, enthalpy and its measurement, Hess's law, entropy as a measure of disorder, and Gibbs energy as the true criterion of spontaneity.

System and surroundings

The system is the part of the universe under study; everything else is the surroundings. Systems are open, closed or isolated by what they exchange.

State function

A property fixed by the present state, not the path taken to reach it — like U, H, S and G. Heat q and work w are path functions, not state functions.

Internal energy (U)

The total kinetic and potential energy stored in a system; a state function whose change ΔU equals the heat and work exchanged with the surroundings.

First law of thermodynamics

Energy is conserved: ΔU = q + w. Heat absorbed BY the system and work done ON the system are both taken as positive in this convention.

Work of expansion

For pressure–volume work against a constant external pressure, w = −pₑₓ ΔV; a gas expanding (ΔV > 0) does work and so loses internal energy.

Enthalpy (H)

Defined as H = U + pV; at constant pressure the heat exchanged equals ΔH, which makes enthalpy the practical measure of a reaction's heat.

ΔH and ΔU relation

For reactions producing or consuming gases, ΔH = ΔU + ΔnₐRT, where Δnₐ is the change in the number of moles of gaseous species only.

Hess's law of heat summation

The overall ΔH of a reaction is the sum of the enthalpy changes of its steps, whatever the route, because enthalpy is a state function.

Standard enthalpy changes

Measured at 1 bar and a stated temperature: enthalpies of formation, combustion and neutralisation are tabulated so reaction heats can be combined.

Exothermic and endothermic

Exothermic reactions release heat to the surroundings (ΔH < 0); endothermic ones absorb heat (ΔH > 0). The sign of ΔH gives the direction of flow.

Entropy (S)

A state function measuring molecular disorder or randomness; for a reversible change ΔS = q_rev ÷ T. Entropy rises on melting, boiling and mixing.

Second law and spontaneity

A change is spontaneous when the total entropy of system plus surroundings increases; the universe tends of its own accord toward greater disorder.

Heat engine and efficiency

A heat engine turns heat drawn from a hot reservoir into work while rejecting some to a cold one; the second law caps its efficiency below 100%.

Gibbs energy (G)

Defined as G = H − TS; at constant T and p, ΔG = ΔH − TΔS folds enthalpy and entropy into a single, usable criterion for spontaneity.

Gibbs energy and equilibrium

ΔG < 0 is spontaneous, ΔG > 0 non-spontaneous and ΔG = 0 at equilibrium; the standard value links to K by ΔG° = −RT ln K.