Chemical Energetics - Basic Terms - July Dec Revision

System

a specified part of universe which is under observation is called the system.

A system is homogeneous system if physical properties nad chemical composition are identical throughout the system. It is heterogeneous if it consists of parts each of which has different physical and chemical properties.

surroundings

The remaining portion of the universe which is not part of the system is termed the surroundings.

Open system: A system which can exchange matter as well as energy with the surroundings is called an open system.

Closed system: A system which can exchange energy but not matter with the surroundings is called a closed system.

Isolated system: A system which can neither exchange matter nor energy with the surroundings is called an isolated system.

Macroscopic properties

Pressure, Volume etc. are related to the behavior of the bulk of the material. These properties are called macroscopic properties.

the macroscopic properties are divided into types.

1. Intensive properties 2. Extensive properties

1. Intensive properties: These properties have no relation to the amount of substance present in a system. Examples: temperature, pressure, viscosity, surface tension, refractive index etc.

2. Extensive properties: The value of these properties depends upon the amount of substance present in the system.

Examples: Mass, volume, surface area, energy, enthalpy, entropy, free energy, heat capacity

State Variables and State Functions

The state of a system is described by macroscopic properties when they are stable and have definite values. If any of the macroscopic properties of the system changes, the state of the system changes.

We describe a system by its state variables. A system having ideal gas can be described by three state variables. These three variables are : temperature (T), pressure (p) and volume (V). Once these three variables are specified all the other variables will be definite and can be easily calculated.

State function is a property of the system whose value depends only upon the state of the system and is independent of the path or manner by which the state is reached.

A system is said to be in thermodynamic equilibrium when the macroscopic properties do not change with time.




Processes

Isothermal: Temperature of the system is constant.

Adiabatic: No heat flows into or out of the system.

Isochoric: volume of the system remains the same.

Isobaric: Pressure of the sytem remains the same.

Reversible: The system changes in infinitesimal steps and they can be reversed.

Irrevesible: Real life systems do not satisfy the reverbility criterion and hence irreversible.

Cyclic: a process in which the system undergoes a series of changes and ultimately returns to its original state is called a cyclic process.

Modes of transfer of energy between system and surroundings

1. Heat (Q): Energy is exchanged between the system and the surroundings as heat if they are at different temperatures.

2. Another modes of transfer of energy is work. Work is said to be performed if th point of application of a force is displaced in the direction of the force.

Pressure volume work

Pressure volume work is mechanical work. It is the work done when the gas expands or contracts against external pressure.

It is equal to force multiplied by distance moved or pressured mulitiplied by change in volume.

Units of Heat and Work

S.I. unit of heat is joule or kilojoule
S.I. unit of work is also joule or kilojoule

Joule and calories are related by the relation

1 cal = 4.184 J
1 kcal = 4.184 kJ