What went wrong when the king rode under the arch?

It was too low and knocked off his crown

How does the chain of blame unfold in the poem?

Each accused person blames another, ending with the king's own changes

State the first law of thermodynamics.

The energy of an isolated system is constant: \(\Delta U=q+w\); energy can be neither created nor destroyed.

Distinguish open, closed and isolated systems.

An open system exchanges both matter and energy, a closed system exchanges only energy, and an isolated system exchanges neither.

What is the sign convention for \(q\) and \(w\)?

Both are positive when heat is absorbed by the system and when work is done on the system.

How are \(\Delta H\) and \(\Delta U\) related for a gaseous reaction?

\(\Delta H=\Delta U+\Delta n_g RT\), where \(\Delta n_g\) is moles of gaseous products minus gaseous reactants.

State Hess's law of constant heat summation.

If a reaction occurs in steps, its standard reaction enthalpy is the sum of the standard enthalpies of the individual steps.

What is the standard enthalpy of formation of an element in its reference state?

It is taken to be zero by convention.

Define a reversible process.

A change carried out so slowly through a series of near-equilibrium states that it could be reversed by an infinitesimal change.

Why can an endothermic reaction be spontaneous?

Because the total entropy change of system plus surroundings can still be positive, making \(\Delta G=\Delta H-T\Delta S\) negative.

How is Gibbs energy related to the equilibrium constant?

By \(\Delta G^{\circ}=-RT\ln K=-2.303RT\log K\).

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Chapter 5: Thermodynamics — Class 11 Chemistry

Chemistry · 9 chapters

Summary, key terms, important questions and a practice quiz with AI diagnosis for each.

Chapter 5: Thermodynamics

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The first law of thermodynamics is expressed as:

Summary

Thermodynamics studies the energy changes accompanying physical and chemical processes in macroscopic systems, considering only initial and final equilibrium states, not the rate or mechanism of change. The system, surroundings and boundary are defined, and systems are classified as open, closed or isolated by their exchange of matter and energy. The internal energy \(U\) is a state function whose change in an adiabatic process equals the work done; combined with heat it gives the first law of thermodynamics, \(\Delta U=q+w\), an expression of energy conservation. Pressure-volume work is examined for reversible, irreversible and free expansions. Enthalpy is defined as \(H=U+pV\), with the heat absorbed at constant pressure equal to \(\Delta H\), related to \(\Delta U\) by \(\Delta H=\Delta U+\Delta n_g RT\), and with \(C_p-C_v=R\) for an ideal gas. Calorimetry (bomb and constant-pressure) measures these quantities. Standard reaction, formation, combustion, atomisation, bond, lattice (via the Born-Haber cycle) and solution enthalpies are introduced, together with Hess's law of constant heat summation. Spontaneity requires more than a fall in enthalpy: the entropy \(S\), a measure of disorder, and the second and third laws are introduced, and Gibbs energy \(G=H-TS\) provides the criterion \(\Delta G<0\) for a spontaneous change. Finally, \(\Delta G^{\circ}=-RT\ln K\) links Gibbs energy to chemical equilibrium.

System, surroundings and types of systemsInternal energy and the first lawPressure-volume workEnthalpy and heat capacityCalorimetryReaction, formation and bond enthalpiesHess's law and Born-Haber cycleEntropy, spontaneity and Gibbs energyGibbs energy and equilibrium

Key terms

Internal energy

The total energy of a system, a state function whose change equals the adiabatic work done on the system.

Enthalpy

A state function defined as \(H=U+pV\); its change equals the heat absorbed at constant pressure.

State function

A property such as \(U\), \(H\), \(p\), \(V\) or \(T\) whose value depends only on the state of the system, not on the path taken.

Entropy

A state function measuring the degree of randomness or disorder; for a reversible process \(\Delta S=q_{rev}/T\).

Gibbs energy

The function \(G=H-TS\); a negative change \(\Delta G\) indicates a spontaneous process.

Hess's law

The total enthalpy change of a reaction is the same whether it occurs in one step or several steps.

Important questions

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Internal energy

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The total energy of a system, a state function whose change equals the adiabatic work done on the system.

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Practice quiz · Thermodynamics

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Thermodynamics

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