Ch 5Thermodynamics — Class 11 Chemistry, concept-first.

"It is the only physical theory of universal content concerning which I am convinced that, within the framework of the applicability of its basic concepts, it will never be overthrown." — Albert Einst…

Every chemical reaction involves some change in energy — heat may be given out or absorbed, and work may be done on or by the system.

Thermodynamics is the study of energy changes that accompany physical and chemical processes. To analyse these changes in a disciplined way, we must first define the region of the universe we are focu…

A system can exchange matter and/or energy with its surroundings, or neither — three possibilities, giving three types of system:

An open system can exchange both matter and energy with its surroundings.

A closed system can exchange energy with its surroundings, but not matter.

An isolated system can exchange neither matter nor energy with its surroundings.

To do any useful calculation in thermodynamics, you must first describe the system. This means specifying its measurable properties quantitatively — things like pressure (), volume (), temperature (),…

When a chemical system gains or loses energy, we need a quantity that represents the total energy stored inside it — chemical, electrical, mechanical, and every other form combined.

Let us first examine how the internal energy changes when only work is done. Imagine a system containing some water inside a thermos flask or an insulated beaker.

We can also change the internal energy of a system by transferring heat from the surroundings to the system (or vice versa) without doing any work.

Now consider the general case in which a change of state is brought about both by doing work and by transferring heat. The change in internal energy for this case is:

We now turn from defining the state functions of a system to seeing how they behave in real processes.

In thermodynamics, work is not just any kind of effort — it is a precisely defined mode of energy transfer between a system and its surroundings.

Applying the first law to a closed system requires two more state functions: enthalpy (), and the heat capacity that relates a temperature change to the heat absorbed.

When a chemical reaction is carried out in a beaker or flask open to the atmosphere, the pressure remains constant (atmospheric pressure) but the volume may change.

In thermodynamics, properties of a system are classified into two fundamental categories.

When heat is transferred to a system, the temperature of the system rises. The increase in temperature is proportional to the amount of heat transferred:

The heat capacity of a system depends on the conditions under which the heating occurs. Two special cases are particularly important:

Both and are measured experimentally using calorimetry — tracking the temperature change of a known quantity of surrounding fluid (usually water) to work out how much heat a process released or absorb…

For chemical reactions, the heat absorbed or evolved at constant volume, , is measured using a bomb calorimeter.

In a chemical reaction, reactants are converted into products: reactants products. The enthalpy change accompanying a reaction is called the reaction enthalpy, :

Enthalpy of a reaction depends on the conditions under which the reaction is carried out. Temperature, pressure, and physical states all affect the value.

Phase transformations involve energy changes. When ice melts, heat is absorbed. This melting occurs at constant pressure (atmospheric pressure) and during the phase change, temperature remains constan…

The standard enthalpy of formation is the standard enthalpy change for the formation of one mole of a compound from its elements in their most stable states of aggregation (also called reference state…

A balanced chemical equation together with the value of its is called a thermochemical equation. The physical state (including allotropic state) of each substance must be specified.

Enthalpy is a state function. Therefore, the change in enthalpy is independent of the path between the initial state (reactants) and the final state (products).

Not every enthalpy change is a full reaction enthalpy — chemists also tabulate enthalpies for specific, standardised physical/chemical events so they can be combined via Hess's law:

Combustion reactions are exothermic — they release heat. They are important in industry, rocketry, and everyday life (for example, the burning of cooking gas or fuel in a vehicle).

Definition: The enthalpy of atomization is the enthalpy change when one mole of bonds is completely broken to obtain atoms in the gas phase.

Chemical reactions involve the breaking and making of chemical bonds. Energy is required to break a bond, and energy is released when a bond is formed.

Definition: The lattice enthalpy of an ionic compound is the enthalpy change which occurs when one mole of an ionic compound dissociates into its ions in the gaseous state.

Definition: The enthalpy of solution of a substance is the enthalpy change when one mole of it dissolves in a specified amount of solvent.

The enthalpy of solution is the enthalpy change associated with the addition of a specified amount of solute to a specified amount of solvent at constant temperature and pressure.

Knowing is not enough to predict whether a reaction happens on its own. This part builds the real criterion for spontaneity — entropy, and then Gibbs energy, which combines enthalpy and entropy into a…

When we observe phenomena like water flowing downhill or a stone falling to the ground, we see a net decrease in potential energy in the direction of change.

What drives a spontaneous process when there is no change in enthalpy? Consider the diffusion of two gases into each other in a closed container that is isolated from the surroundings.

We have seen that it is the total entropy change, , that decides the spontaneity of a process. But most chemical reactions occur in closed or open systems, not isolated ones.

For an isolated system, the change in energy remains constant. Therefore, an increase in entropy in such systems is the natural direction of a spontaneous change.

Molecules of a substance can move in a straight line in any direction (translational motion), spin like a top (rotational motion), and have bonds that stretch and compress (vibrational motion).

We have already seen that the sign and magnitude of the standard Gibbs energy change, , tells us two things: whether a reaction is spontaneous, and how much useful work it can provide.

- System, surroundings, and boundary define the thermodynamic system. The universe = system + surroundings.

The chapter closes with 22 numbered Exercises that test the ideas developed above — thermodynamic terms and the first law, work and heat for ideal-gas processes, enthalpy and calorimetry, Hess's law a…

Higher-order thinking / exemplar-style practice problems.