Chapter 5 - Thermodynamics

Thermodynamics studies heat, work and energy changes during physical and chemical processes. It explains how energy is conserved, how reaction enthalpy is measured and what determines whether a process is spontaneous.

System, surroundings and state

The system is the part being studied; everything else is the surroundings. Systems may be open, closed or isolated.

The state of a system is described by variables such as pressure, volume, temperature and composition.

• Intensive properties do not depend on amount.

• Extensive properties depend on amount.

• State functions depend only on initial and final states.

• Path functions depend on the route followed.

Students also distinguish isothermal, adiabatic, isobaric, isochoric, reversible and irreversible processes.

First law of thermodynamics

The first law expresses conservation of energy:

[
\Delta U = q + w
]

Using the chemistry convention, heat absorbed by the system is positive and work done on the system is positive.

For pressure-volume work against constant external pressure:

[
w=-P_{ext}\Delta V
]

Enthalpy and calorimetry

Enthalpy is defined by (H=U+PV). At constant pressure, heat exchanged equals enthalpy change.

Calorimetry measures heat changes using heat capacity, specific heat or a calorimeter constant. Students learn constant-pressure and constant-volume situations.

Types of enthalpy change

The chapter includes:

• Enthalpy of reaction

• Standard enthalpy of formation

• Enthalpy of combustion

• Enthalpy of atomisation

• Bond enthalpy

• Enthalpy of phase transition

• Enthalpy of solution

• Enthalpy of dilution

Hess’s law

Because enthalpy is a state function, the enthalpy change depends only on initial and final states. Thermochemical equations can be reversed, multiplied and combined to calculate unknown values.

Spontaneity, entropy and Gibbs energy

A spontaneous process can proceed under the stated conditions without continuous external intervention. Spontaneous does not mean rapid.

Entropy describes energy dispersal or disorder. Gibbs energy combines enthalpy and entropy:

[
\Delta G=\Delta H-T\Delta S
]

• Negative (\Delta G): spontaneous forward process

• Positive (\Delta G): non-spontaneous forward process

• Zero (\Delta G): equilibrium

What can students study on Teachoo?

• Definitions and comparison tables

• Sign-convention questions

• Internal-energy numericals

• Pressure-volume work

• Calorimetry

• Reaction enthalpy

• Hess’s law

• Bond-enthalpy calculations

• Entropy comparisons

• Gibbs-energy numericals

• NCERT solutions, MCQs and case studies

Common mistakes

• Using the wrong sign for work

• Confusing heat with temperature

• Mixing (\Delta U) and (\Delta H)

• Forgetting to reverse the sign when reversing a thermochemical equation

• Assuming every exothermic process is spontaneous

• Assuming spontaneous means fast

• Using Celsius instead of kelvin in Gibbs-energy calculations

Best way to study this chapter

Before solving, identify the system, process condition and sign convention. Write the governing equation before inserting numbers. Keep all energy units consistent.

Frequently asked questions

What does thermodynamics study?

It studies heat, work and energy changes in physical and chemical processes.

What is the difference between a state function and path function?

A state function depends only on initial and final states. A path function depends on how the change occurs.

What is the difference between internal energy and enthalpy?

Internal energy is the total microscopic energy of a system. Enthalpy includes internal energy and the pressure-volume term.

Is every exothermic reaction spontaneous?

No. Spontaneity depends on both enthalpy and entropy and may vary with temperature.

Does spontaneous mean fast?

No. Thermodynamics predicts feasibility, while kinetics determines rate.

Why is Hess’s law valid?

Enthalpy is a state function, so its change is independent of the path.

Learn Thermodynamics with Teachoo for clear concepts, correct sign conventions, numerical practice and complete NCERT solutions.