Energetic of Chemical Relations and Chemical Thermodynamics | Chemistry Class 12

Chemistry | Notes
Energetic of Chemical Relations and Chemical Thermodynamics
Class : 12

Some Thermodynamic Terms 

In order to study the laws of thermodynamics, it is essential to understand certain terms quite oftenly used in thermodynamics. These terms have definite meaning. Some of the most commonly used terms are given below.

System, Boundary and Surrounding 

The part of the universe under consideration is called a system, on which the effect of certain variables such as temperature and pressure, is to be studied. The system is separated from rest of the universe by a definite boundary. The real o r imaginary surface separating the system from the surroundings is called the boundary.

Region outside the boundaries of any system is termed surroundings. A system is the separate from the rest of the universe, i.e., surroundings by a real or imaginary boundary. Everything outside the system is considered as the surroundings. 

Homogeneous System and heterogeneous System 

When a system is uniform throughout, it is called a homogeneous system. In other words, a system is said to be homogeneous when it has the same chemical composition throughout. For Example, a mixture of gases, a pure single liquid or solid or a true solution forms a homogeneous system. 

A heterogeneous system is one which consists of two or more phases. In other words, it is not uniform throughout. Examples of heterogeneous systems are: ice in contact with water, ice in contact with vapours, etc. here, ice, water and vapours constitute separate phases.  Types of System: Depending on the nature of boundary, thermodynamic systems are classified into following three classes. 

1. Open system

A system which can exchange matter as well as energy with surroundings is called an open system. Some examples are given below. 

• Evaporation of water from a beaker represents an open system. Here, water vapours (matter) mover into atmosphere and heat (energy) requi9red is absorbed by water from the surroundings. 
• In the photosynthesis, plants take up carbon dioxide, water (matter) sunlight (energy) in the presence of chlorophyll and produce carbohydrates. In this process, oxygen (matter) is transferred to the surroundings. Hence, plants constitute an open system. 

2. Closed system

A system which may exchange energy but not matter with surroundings is called a closed system. For example, consider boiling water in a closed metallic vessel. Here heat is transferred from the burner (surroundings) to the system; steam remains inside the vessel. Thus, matter is not exchanged. 

3. Isolated system

A system which can neither exchange matter nor energy with the surroundings is called an isolated system. For example, some water is taken in an insulated vessel and put a small piece of sodium metal in it. An exothermic reaction takes place. Neither the hydrogen gas (matter) nor heat (energy) is transferred to the surroundings. 

4. State of the System and State Functions 

A thermodynamic system is said to be in a certain state when all its properties are fixed. The state of a system can be defined completely by the following four properties: 

(i) Mass and composition 

(ii) Temperature 

(iii) Pressure 

(iv) Volume 

Any change in the magnitude of these properties alters the state of system; therefore, these properties are referred to as state variable of state functions or thermodynamic parameters. A change in any function of state is measured as the difference between final state and the initial state, because, the value of these functions depends solely on the thermodynamic properties of the system in a given state and not on its past history. 

In actual practice, it is not necessary to specify all the state variables because some of them are interdependent. For example, in the case of single gas, composition is fixed automatically as it remains always 100 percent. Thus, the state of such a system can be defined by only three variables, pressure (P), volume (V) and temperature (T). 

Further, I the gas is ideal and one mole of the gas is under examination, it obeys the equation the equation PV=RT, where R is the gas constant. Therefore, if only two of three variables are known the third can be easily calculated. The two variables generally specified are temperature and pressure. These are called independent state variables. The third state variable, generally volume, is said to be a dependent state variable because its value depends upon the values of P and T. 

Thus, state of a system containing definite amount of a single gas can be completely defined by specifying only two of the three variables, i.e., pressure, temperature and volume. When we are considering a closed system consisting of one or more components, mass is not a state variable. In order to define the state of a heterogeneous system having more than one substance we must consider and describe each of the phases of the system. For each phase, we must specify the content, i.e., the amount of each substance present and two other independent state variables.

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