Thermodynamics

Energetics of Chemical Reactions

Energy is a very important commodity in the modern world. No life is possible without energy. The concept of energy is quite familiar but energy itself is rather difficult to define. Energy is defined as the capacity to perform work. Energy is taken in various forms such as heat energy, electrical energy, light energy, nuclear energy and so on. Energy can be converted from one form to another but can neither be created nor destroyed. This is the law of conservation of energy. The law of conservation of energy is often called the first law of thermodynamics.

Thermodynamics deals with the interconversion of one form of energy to another. The chemical reaction is also accompanied by a change in energy. When kerosene is burned, thermal energy is generated. When food is digested, the energy stored in the food material is converted in our body which makes us able to talk, walk and perform work. Energy can be used to carry out a chemical reaction. The interrelation between chemical reaction and energy change in the field of thermodynamics. (thermo=heat, dynamics=motion).

Let us consider some changes:

1. Water stored in an open vessel gets cooled after some time.
2. Solid melts and liquid boils when they are heated.
3. Plants produce carbohydrates from atmospheric carbon dioxide and water in the presence of Sunlight.
4. Fuel liberates energy in the form of heat when it is burnt.

Among these few examples given below the common property in all the changes is the energy change during the process- either some energy has to be supplied to bring the changes or some energy is liberated when the process is accomplished.

It is therefore either of these processes completed by the change in energy. The branch of science which deals with energy changes during chemical and physical is called energetics and such change in energy during a chemical reaction is called energetics of chemical reaction.

In a physical change, the molecule remains unaffected and undergoes molecular rearrangement. In processes like the melting of solids and evaporation of liquid, molecules move against the force of attraction. Such a process occurs with the absorption of energy. In processes like condensation and solidification, molecules come closer to each other and some energy is released.

In chemical changes, the molecule breaks and atoms rearrange to form another molecule. This is to say one of the reactants breaks and new products are formed. In both cases, energy is involved. Energy is required when a bond is broken and energy is released when a bond is formed. The net energy change depends upon whether the energy required is more than the energy released and vice versa. Chemical bonds are much stronger than molecular interaction( physical process), therefore energy changes in chemical processes are larger than in physical processes.

The energy possessed by an object can be classified as kinetic energy. It is the energy possessed by a body by virtue of its motion. Potential energy is the energy possessed by a body by virtue of its position. The total energy content of the body is the sum of kinetic and potential energy.

Total\ energy= K.\ E.+\ P.\ E.

Total energy remains constant when a body changes its position from one form of energy to another. S.I. unit of energy is Joule and various other units of energy are related to one another as follows:

\begin{align*} 1\ Joule &= 10^{7}\ ergs\\ 1\ Calorie &= 4.184\ Joules\\ 1\ Calorie &= 4.184\times 10^{7}\ ergs\\ 0.082\ lit.atm. &= 1.987\ Calorie \end{align*}

Some thermodynamic terms

System

The part of the universe which is under thermodynamic study containing a specific amount of matter is called a system. A certain amount of a solid, liquid, gas or a mixture of substances contained in a container is a system.

Surrounding

The rest of the universe or environment around the system is called surrounding. It represents the portion of the universe with which a system interacts.

The system is a definite part of the universe while the surrounding is unlimited. The system consists of those molecules which are reacting and surrounding everything else. Examples:
i. Chemicals in a test tube are the system, while the rest are the surroundings.
ii. LPG gas in a cylinder is a system and the remaining part is surrounding.

Boundary

A real or imaginary interface that separates the system from the surroundings is called a boundary. The Wall of the test tube is the boundary of a chemical system in a test tube.

The whole part of a system, surrounding and the boundary is called the universe.

Types of thermodynamic systems

1. Open system

If a boundary allows to exchange of both matter as well as energy between the system and surroundings, this system is called an open system. In this case, the boundary is imaginary and open. eg: boiling water placed in a glass.

2. Closed system

If a boundary allows to exchange of energy but not matter between the system and surroundings, this system is called a closed system. In this case, the boundary is real, closed but not insulated. eg. boiling water kept in a kettle.

3. Isolated system

If the boundary neither allows to exchange of energy nor matter between the system and surroundings, this system is called an isolated system. In this case, the boundary is real closed and insulated. eg. hot water kept in a thermos flask.

Thermodynamic properties

There are two types of observable thermodynamic properties:

1. Extensive properties

The thermodynamic property whose magnitude depends upon the amount of substance present in that system is called extensive property. This property can be added, so it is known as additive property. eg. mass, volume, surface area, energy, number of moles, enthalpy, free energy, entropy, etc.

2. Intensive property

The thermodynamic property whose magnitude doesn’t depend upon the amount of substance present in that system is called intensive property. This property cannot be added, so it is known as non-additive property. eg. temperature, pressure, density, viscosity, refractive index, surface tension, pH, emf, boiling point, etc.

Thermodynamic equilibrium

A system is said to be in thermodynamic equilibrium when temperature, pressure and concentration remain the same.

1. Thermal equilibrium: A system is in thermal equilibrium if the temperature remains same.
2. Mechanical equilibrium: A system is in mechanical equilibrium if the pressure remains same.
3. Chemical equilibrium: A system is in chemical equilibrium if the chemical composition or concentration remains same.

A system is said to be in non-equilibrium if temperature, pressure and concentration are different in different parts of the system.

Thermodynamic process

1. Isothermal process

A process is said to be isothermal if the temperature of the system remains constant. For an isothermal process, change in temperature is zero i.e. dT = 0.

2. Adiabatic process

A process is said to be adiabatic if no heat enters or leave the system. For an adiabatic process, change in heat is zero i.e. dq =0.

3. Isobaric process

A process is said to be isobaric if the pressure of the system remains constant. For an isothermal process, change in pressure is zero i.e. dP = 0.

4. Isochoric process

A process is said to be isothermal if the volume of the system remains constant. For an isothermal process, change in volume is zero i.e. dV = 0.

5. Cyclic process

A process is said to be cyclic if the system returns to its original state after completing a series of changes. In the cyclic process, changes in energy and heat are zero.

6. Reversible and irreversible process

A process carried out infinitesimally slowly such that the system can be reversed back is called a reversible process and a process that doesn’t take place infinitesimally slowly such that the system cannot be reversed back is called an irreversible process. Almost all processes occurring in nature or laboratory are irreversible processes.

State of the system and state function

A thermodynamic system is said to be in a particular state if the thermodynamic variables are fixed. Generally, there are four types of thermodynamic variables which are commonly adopted to signify the thermodynamic system in order to make a state. They are temperature, pressure, volume and mass.
Any change in the magnitude of these properties alters the state of the system. Therefore, these properties are referred to as state variables or state functions or thermodynamic parameters. A change in any of these state functions is measured as the difference between the final and initial state.

Internal energy

It is defined as the total sum of all kinds of possible forms of energy possessed by the substance present in the system under the particular state. It is denoted by E.

E = K.E. + P.E. + ...

Quantitatively, it is defined as the thermodynamic function which measures the total heat content of the system at constant volume.

\Delta E = q_{v}

It is a state function. So absolute value of it cannot be measured but the change in value between two states is practically measured.
Let E₁ and E₂ are the internal energies of two states, then change in internal energy is:

\Delta E = E_{1} - E_{2}

Exchange of energy between the system and surroundings

Energy is exchanged between the system and surroundings mainly in two ways:

1. As heat

If the system and surroundings are at two different temperatures, energy is transferred or exchanged between the system and surroundings in the form of heat. Suppose the system is at a higher temperature than the surroundings, the energy is transferred from the system to the surroundings as heat till both attain the same temperature.
On the other hand, if the surroundings are at a higher temperature than the system then the heat energy flows from the surroundings to the system till again the temperature is equalised.

2. As work

If the system and surroundings are at two different pressures, then energy is exchanged between the system and surroundings in the form of work. Suppose there is a gaseous system enclosed in a cylinder fitted with a piston. If the pressure of the gas is higher than that of the surroundings, the gas will expand, push up the piston and do the work against the surroundings. Here, energy is transferred from the system to the surroundings in the form of work. If the surrounding is at high pressure, the gas contracts and work is done on the system by the surrounding. In this case, energy is transferred from the surroundings to the system in the form of work.
In both cases of expansion and contraction, the mechanical work done by (on) the system is given by:

W=P\Delta V\\where,\ P = Pressure\ \&\\ \Delta V = Change\ in\ volume = V_{2} - V_{1}

First law of thermodynamics

The first law of thermodynamics can be defined in different ways:

• The total energy of a system and its surroundings remains constant.
• Energy can neither be created nor be destroyed but can be converted from one form to another.
• When a particular type of energy disappears, an equivalent amount of another form of energy must appear.
• It is impossible to construct a perpectual motion machine that can produce work without spending energy on it.

Mathematical formulation of the first law of thermodynamics

First law of thermodynamics is an application of the law of conservation of energy to the thermodynamic system. Suppose a thermodynamic system possesses internal energy E₁ initially. Let q quantity of heat is supplied to it. This supplied heat is used to increase the internal energy of the system. Let E₂ be the internal energy in the final state of the system.

So, increase in internal energy = E₂ – E₁ —–(i)
Heat = Increase in internal energy + Work done
or, q = (E₂ – E₁) + w
or, q = ΔE + w —–(i)
This work is PV work. So w = PΔV
Now, equation (ii) becomes,

q = \Delta E + P\Delta V

This is the mathematical formulation of the first law of thermodynamics.

Special cases

i. In the isochoric process:

V = constant\ \ So,\ V = 0\\ Then,\ q = E + 0\ \ or,\ q = E\ \ So,\ E = q_{v}

This shows that the total heat absorbed by a system is utilized to raise the internal energy at constant volume.

ii. In adiabatic process

\begin{align*} q &= 0\\ so,\ q &= \Delta E + w \\or,\ 0 &= \Delta E + w \\or,\ \Delta E &= -w \\ or,\ w &= -\Delta E \end{align*}

This shows that a decrease in internal energy is equal to the work done by the system.

iii. In cyclic process

\begin{align*} E = constant,\ &\&\ \Delta E = 0\\ Then, q &= 0 + W\\ so, q &= W \end{align*}

This shows that the total heat supplied to the system is utilized to perform the work done by the system.

Limitations of first law of thermodynamics

• It doesn’t tell about the direction of the change of state of the system.
• It doesn’t tell about the direction of the flow of energy.
• It doesn’t explain about feasibility and spontaneity of a process i. e. whether the reaction will occur or not under a given condition.

Sign convention of heat and work

• If heat is absorbed by the system, q is positive.
• If heat is evolved by the system, q is negative.
• If work is done on the system, w is positive.
• If work is done by the system, w is negative.