Chemistry · Class 12 Science

Ch 4Chemical Thermodynamics — Class 12 Chemistry, concept-first.

You know that the transformation of liquid water into vapour, of solid ice into liquid water, or the burning of carbon to form carbon dioxide, , are accompanied by a change in energy.

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Pressure-Volume Work

When a gas confined in a cylinder by a movable piston expands or is compressed against an opposing external pressure Pext, the system and surroundings exchange pressure-volume (PV) work.

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Chapter contents

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4.1

Introduction

You know that the transformation of liquid water into vapour, of solid ice into liquid water, or the burning of carbon to form carbon dioxide, , are accompanied by a change in energy.

+Can you recall?i2 questions

4.2

Thermodynamic reasoning rests on a small set of precisely defined terms. The subsections that follow introduce them one by one: the system and its surroundings, the kinds of systems, the properties of…

4.2.1

System and surrounding

Consider a gas enclosed in a cylinder equipped with a movable piston, as shown in Fig. 4.1. Suppose we undertake a study of the change in volume of the gas, and of the amount of energy released or gai…

4.2.2

Types of system

Three types of systems are shown in Fig. 4.2.

+Observe and discuss 4.2.21 question

Q1Observe Fig. 4.2 and discuss with reference to transaction of energy and matter. ![Refers to the textbook's Fig. 4.2 (three coffee-cup sketc…Preview

4.2.3

Properties of system

i. Extensive property : A property which depends on the amount of matter present in a system is called an extensive property.

4.2.4

State functions

As shown in Fig. 4.1, a certain amount of a gas is enclosed in a cylinder fitted with a movable piston.

4.2.5

Path Functions

The properties which depend on the path are called path functions. For example, work and heat .

4.2.6

Thermodynamic equilibrium

Consider a gas enclosed in a cylinder fitted with a movable piston, shown in Fig. 4.1. The gas has temperature , pressure and volume .

4.2.7

Process and its types

A transition from one equilibrium state to another is called a process. Processes are of different types.

4.3

Nature of heat and work

Heat and work are the two ways in which a system exchanges energy with its surroundings. The two subsections below examine the nature of each in turn — first work, with two chemical reactions that per…

4.3.1

Nature of work (W)

In mechanics, work is defined as the energy by which a body is displaced through a distance by an application of force . Thus,

4.3.2

Nature of heat (Q)

Like work, heat is a form of energy by which the system exchanges energy with its surroundings. When the system and its surroundings are at different temperatures, heat either flows into the system or…

4.3.3

Sign conventions of W and Q

The energy changes for the system are considered hereafter. The energy entering the system from the surroundings has a positive value, while the energy leaving the system and flowing into the surround…

4.4

Expression for pressure-volume (PV) work

Consider a certain amount of gas at constant pressure enclosed in a cylinder fitted with a frictionless, rigid movable piston of area . This is shown in Fig. 4.7.

4.4.1

Free expansion

A free expansion means expansion against zero opposing force. Such expansion occurs in vacuum. The work done by a system during such expansion is given by Eq. (4.5), .

4.4.2

Units of energy and work

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4.5

Concept of maximum work

Eq. (4.5) shows that the amount of work performed by a system is governed by the opposing force . The larger the opposing force, the more work is done by the system to overcome it.

4.5.1

Expression for the maximum work

Consider moles of an ideal gas enclosed in a cylinder fitted with a frictionless movable rigid piston.

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4.6

Internal energy (U)

Every substance is associated with a definite amount of energy. This energy stored in a substance is its internal energy, denoted by .

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Q125 kJ of work is done on the system and it releases 10 kJ of heat. What is $\Delta U$?Preview

4.7

First law of thermodynamics

The first law of thermodynamics is simply the conservation of energy. According to this law, the total energy of a system and surroundings remains constant when the system changes from an initial stat…

4.7.1

Formulation of first law of thermodynamics

A system exchanges energy with its surroundings either by transfer of heat or by doing work. An energy supplied to the system increases its internal energy; on the other hand, removal of heat or work…

4.7.2

First law of thermodynamics for various processes

i. Isothermal process : Temperature is constant in such a process, so the internal energy is constant. Hence, . For an isothermal process,

4.8

Enthalpy (H)

Enthalpy of a system is the sum of the internal energy of the system and the energy equivalent to work:

4.8.1

Relationship between ΔH and ΔU for chemical reactions

4 Q

At constant pressure, and are related as

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+Intext Questions 4.8.1i1 question

4.1Under what conditions $\Delta H = \Delta U$?Preview

4.8.2

Work done in chemical reaction

The work done by a system at constant temperature and pressure is . Assuming ,

4.9

Enthalpies of physical transformations

Physical transformations — melting, vaporization, sublimation, and changes at the level of atoms and molecules such as ionization, atomization and dissolution — each have a characteristic enthalpy cha…

4.9.1

Enthalpy of phase transition

In a phase transition, one phase of a substance is converted into another at constant temperature and pressure, without a change in chemical composition.

4.9.2

Enthalpy for the atomic / molecular change

i. Enthalpy of ionization () : It is the enthalpy change accompanying the removal of an electron from one mole of gaseous atoms. For example,

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Q1For KCl, $\Delta_L H$ = 699 kJ/mol$^{-1}$ and $\Delta_{hyd} H$ = -681.8 kJ/mol$^{-1}$. What will be its enthalpy of solution? > [!NOTE] > Th…Preview

4.10

Thermochemistry

Thermochemistry deals with the enthalpy changes in chemical reactions.

4.10.1

Enthalpy of chemical reaction (ΔrH)

Consider the reaction

4.10.2

Exothermic and endothermic reactions

The enthalpy of a reaction can be positive or negative, depending on and .

4.10.3

Standard enthalpy of reaction (ΔrH0)

To compare the enthalpy changes of different reactions, they have to be reported under a similar set of conditions.

4.10.4

Thermochemical equation

A thermochemical equation is the balanced chemical equation in which the enthalpy change, the physical states and the number of moles of reactants and products have been specified.

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Q1Given the thermochemical equation, $\mathrm{C_2H_2(g)} + \frac{5}{2}\,\mathrm{O_2(g)} \rightarrow 2\mathrm{CO_2(g)} + \mathrm{H_2O}(l)$, $\D…Preview

4.10.5

Standard enthalpy of formation (ΔfH0)

Consider

4.10.6

Standard enthalpy of reaction from standard enthalpies of formation

The standard enthalpies of formation of compounds are used to determine the standard enthalpies of reactions. Calculations of from the of compounds are based on the following:

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