Chemical Thermodynamics, Chemistry tutorial

Introduction:

There is no chemical reaction in which the energy is not involved. In the procedure of reacting substances, you either supply energy to the system or the reaction takes place on its own. However then, why and how do chemical reactions take place? What actually is thermodynamics? Why do certain chemical reactions take place devoid of the supply of external energy? All such questions will be answered in this chapter.

Thermodynamics terminology:

In this part, a number of generally used terms in thermodynamics are defined and illustrated. These terms must be comprehended clearly before you carry on further.

Thermodynamics:

In respect of chemical   reactions, we are not just concerned regarding how and why reactions occur however as well why certain substances are even more   reactive than the others. There is no doubt that before a chemical reaction can take place, energy is needed. The energy needed might be associated to the substances undergoing the reaction. Though, where the quantities of energy possessed via the reacting species are not enough to initiate the reaction or take the reaction to completion, energy might be supplied. In short, the chemical reaction in a system needs certain level of energy transformation. We are familiar that the heat is a form of energy.

Thermodynamics is a compound term. Thermo means transfer of heat or mainly concerned with heat and dynamics implies of physical power and forces producing the motion. Thermodynamics can therefore be stated as the science of heat motion. However, you must consider it as the science of heat flow or transfer or disappearance of work attending the physical and chemical processes.

Let us consider some illustrations of thermodynamics, as shown:

1) Natural process: The water flowing down from a hill-top. An illustration is Erin -Ijesha water fall in OsunState.

2) Controlled chemical reactions: We can find out the dissociation constant (pKa), example acetic acid.

3) Performance of engines: Any engine can be considered here, for example, whenever you are evaluating the efficiency of the blender engine. We must draw our attention to one fact that thermodynamics can't answer all the questions due to its ample applicability. For example, it can't:

  • Answer how fast a reaction takes place.
  • Provide the character of the method of change.

Though, it can state us whether a reaction will take place or not.

In short, the word thermodynamics is stated as the study of energy transformation in a system. However, thermodynamics mainly deals with the systems. The system under consideration in this note is a chemical one, therefore, our main consideration of the concept of the chemical thermodynamics. What then is a system?

System:

Any part of the universe which is under study is termed as a system. The systems can be in different states. A system can be as simple as a gas contained in the closed vessel, or as complicated as the rocket shooting towards the moon.

A system might be homogenous or heterogeneous, based on its contents and conditions. A system is homogenous if physical properties and chemical composition are similar all through the system. Such a system is as well termed as a single phase system. A heterogeneous system comprises of two or more phases separated via mechanical boundaries. Now, let us consider the surroundings of the reaction.

Surroundings:

The rest of the universe around the system is taken as its surroundings. A system and its surroundings are for all time separated via boundaries across which matter and energy might be exchanged. The boundaries can be real (that is, fixed or movable) or imaginary.

Based on the exchange of matter and energy between the system and these founding, a system can be categorized into the given three types:

a) Isolated

b) Closed

c) Open

An Isolated system   is one which exchanges neither energy nor matter by its surroundings. There is no perfectly isolated system, however a system which is thermally well-insulated (that is, doesn't let heat flow) and is sealed to inflow or outflow of matter can be taken as an isolated system. A sealed thermos flask containing some matter might be regarded as the isolated system.

A closed system allows for exchange of energy (that is, heat or work) by the surroundings; however matter is not allowed to enter or leave it. A correctly sealed system (to prevent the passage of matter across its boundary) can be taken as the closed system.

An open system permits exchange of both matter and energy with its surroundings. This is the most general system encountered in our everyday life. All living things are illustrations of an open system, as they are capable of exchanging matter and energy freely with their surroundings.

As well, reaction vessels having permeable membranes are the open systems.

State variables:

Any thermodynamic system should be macroscopic, that is, must have adequately big size. This facilitates the measurement of its properties like volume, pressure, temperature, composition and density. These properties are, thus, termed as macroscopic or bulk properties. These are as well termed as state or thermodynamic variables. These don't base on the past history of the system.

A state variable that fully depends on other variables is termed as a dependant variable. Others, on which it is based, are termed as independent variables. For instance, if you write down the ideal gas equation as:

V/P = nRT

Then, 'V' is the dependent variable, while n, T and P are independent variables. We are familiar that 'R' is the gas constant. On the other hand, if you write this equation as,

P = nRT/V

Then, 'P' is the dependent variable, while n, T and V are independent variables. The choice of dependent and independent variables is a matter of ease.

State of a system:

The state of a system can be stated in thermodynamics once you set up a small set of measurable parameters. For illustration, whenever you have a gas confined in the container, the measurable parameters there will comprise volume, pressure, temperature and composition.

In essence, the state of a system is stated if the state variables encompass definite values. It is not essential to specify all the state variables as these are interdependent. For illustration: if the system is an ideal gas, then its volume, pressure, temperature and the amount of gas (that is, number of moles) are associated by the gas equation. Therefore, if we specify three of these, the fourth variable is automatically fixed. Likewise, most of its other properties, like heat capacity, density and so on are as well fixed, however via more complex relations.

We can change the state of a system by modifying either the pressure or the volume.

The Zeroth law of thermodynamics:

The Zeroth law of thermodynamics is mainly based on the concept of thermal equilibrium. This helps us in defining the temperature. If two closed systems are brought altogether in such a way that they are in thermal contact, changes occur in the properties of both systems. However, ultimately a state is reached whenever there is no further change in either of the systems. This is the state of thermal equilibrium. Both the systems are at similar temperature. In order to find out if two systems are at similar temperature, the two can be brought into the thermal contact, and then the changes in their properties noticed. If no changes take place, they are at the similar temperature.

The Zeroth law of thermodynamics defines that if a system 'A' is in thermal equilibrium with system 'C', and  system  'B' is as well  in thermal  equilibrium with  'C', then A and B are as well in the thermal equilibrium with one other. This is an experimental fact, which might be described by supposing that systems 'A' and 'B' are the two vessels having different liquids, and 'C' is an ordinary mercury thermometer. If 'A' is in thermal equilibrium with 'C', then the mercury level in the thermometer will illustrate a constant reading.

This points out the temperature of system 'A' and also that of 'C'. Now, if 'A' is as well in thermal equilibrium with  the 'B', then  the height  of mercury level  in the  thermometer (that is, in contact with the B) is  the similar  as before; 'B' as well consists of  the  similar temperature as 'A'. There is thermal equilibrium in both 'A' and 'B' or these are at similar temperature.

Extensive and intensive variables:

We are familiar with the homogenous and heterogeneous systems. Let us now illustrate the difference between the two, with respect to the value of certain variables. The parameters illustrated earlier are as well termed as variables. There are two kinds of variables, namely Extensive and Intensive variables.

Extensive variables:

An Extensive property of the homogenous system is one which is dependent on the amount or quantity of a phase in the system   or the mass of the system. For a heterogeneous system made up of some phases, the net value of an extensive property is equivalent to the sum of the contributions from its different phases. Mass, volume, and energy are the illustrations of extensive properties. Therefore, if a system at equilibrium comprises of 0.100 kg of ice and 0.100 kg of liquid water at 273.15K, the total volume of the system is the sum of the two volumes, each of which is directly proportional to the mass.

Volume of 0.100 kg of ice = Mass of ice/Density of ice

                                        = 0.100 kg/917kgm- 3

                                        = 1.09 x 10-4 m3

Likewise, the volume of 0.100 kg of water = Mass of water/Density of water

                                                                = 0.100 kg/1.00 x 103 kg m-3

                                                               = 1.00 x 10-4 m3

Total volume = (1.09 + 1.00) x 10-4 m3

                     = 2.09 x 10-4 m3

Intensive variables:

The property that based only on the nature of the substance and not on the amount(s) of the substance(s) present in the system is termed as intensity or intensive property. The general examples of such properties are pressure, temperature, concentration, viscosity, refractive index, density, specific heat, surface tension and so on. The quotient obtained via dividing any extensive variable by the other extensive variable provides an intensive variable. For illustration: density (mass/volume), concentration (moles/liter), mole fraction (n/N) and specific heat (heat capacity per unit mass).

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