In the previous chapter, we completed a discussion on states of matter. In this chapter we will see thermodynamics.
■ Let us first see an example in which energy is converted from one form to another. It can be written in 5 steps:
1. We know that, when fuels like methane and coal burn, heat is released.
• When fuels burn, the chemical energy stored in the molecules is converted into heat energy.
2. This heat energy can be supplied to a gas inside a cylinder.
3. The heated gas will expand and push the piston of the cylinder.
4. The moving piston will rotate the wheel of an automobile.
5. Thus chemical energy is converted into mechanical energy.
■ Another example is the galvanic cell in which chemical energy is converted into electrical energy.
• There are many situations where we will need to transform energy from one form to another. So some questions arise:
♦ How much energy can be obtained from a chemical reaction or process ?
♦ What all conditions need to be satisfied to carry out the reaction or process ?
♦ How much materials will be required to obtain the desired quantity of energy ?
♦ Will it be economical to carry out such a reaction or process ?
• The principles of thermodynamics will help us to find the answers to such questions.
Now we will see some thermodynamic terms. They can be explained in 12 steps:
1. Consider the chemical reaction between two reactants X and Y.
• We will be observing the changes happening to the molecules of X and molecules of Y.
2. Imagine a group consisting of the two items:
♦ All the molecules of X.
♦ All the molecules of Y.
• Then this group is called the system.
3. Objects and particles other than those mentioned in (2) will constitute the surroundings.
4. System and surroundings together constitute the universe.
• So we can write: Universe = System + Surroundings.
5. The green region in fig.6.1(a) below is the universe.
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| Fig.6.1 |
6. In this universe, a container is placed on a table top.
• Inside the container, there are some magenta molecules and some cyan molecules.
♦ All those molecules (magenta and cyan) together constitute the system.
7. Usually, the ‘neighborhood of the system’ is affected by the 'reactions or processes' undergone by the system.
• For example, if the system releases some heat, the neighborhood will become hot.
• The ‘neighborhood of the system’ is shown separately in fig.b.
♦ This neighborhood is the surroundings.
8. So if we add the molecules into fig.b. we will get fig.c, which is the universe.
9. Movement of matter
• Sometimes matter may move from the system into the surroundings.
♦ For example, if the system is water and if that water is heated, some water molecules will escape in the form of steam.
• The reverse can also happen. That is., matter may move from the surroundings into the system.
♦ For example, while a reaction is going on, we may add small quantities of one of the reactants.
10. Movement of energy
• Sometimes energy may flow from the surroundings into the system.
♦ For example, the gas inside a cylinder may absorb heat from the surroundings.
• The reverse can also happen. That is., energy may flow from the system into the surroundings.
♦ For example, if the reaction is exothermic, the heat liberated may flow into the surroundings.
11. To estimate the exact quantities of matter and energy flowing in either direction, we must define a boundary for the system.
♦ Every thing inside that boundary will be the system.
♦ Every thing outside that boundary will be the surroundings.
12. The boundary can be a thin imaginary layer on the inner surface of the container.
• In fig.6.2(a) below, this layer is shown in green color.
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| Fig.6.2 |
• In fig. (b), the container is closed with a lid.
♦ The thin layer at the bottom surface of the lid is shown in pink color.
♦ It is given another color to indicate that, it can be moved away from the green layer.
• In fig. (c), the container is a cylinder-piston arrangement.
♦ The layer at the bottom surface of the piston is given a pink color.
♦ It is given a different color to indicate that, it can be moved relative to the green layer.
Different types of systems
• Systems can be classified according to:
♦ the movements of matter into or out of the system.
♦ the movements of energy into or out of the system.
• The three types of systems are: Open system, Closed system and Isolated system.
• We will now see each of them in detail.
Open system
This can be explained in 3 steps:
1. In this system, there is exchange of matter and energy between the system and the surroundings. This is shown in fig.6.3(a) below:
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| Fig.6.3 |
2. The reactants are placed in an open container.
• The imaginary green layer allows the passage of both matter and energy.
• So matter can move in or out through the top.
3. Energy can move in or out through the top as well as the sides.
• For example, heat produced inside is allowed to pass through the green layer.
• It reaches the walls of the container. If the walls are made of materials like copper, heat reaches the outer surface of the container and from there, dissipates into the surroundings.
Closed system
This can be explained in 3 steps:
1. In this system, there is exchange of energy, but no exchange of matter. This is shown in fig.6.3(b) above.
2. The reactants are placed in a closed container.
• Heat produced inside is allowed to pass through the green and pink layers.
• The heat reaches the walls and lid of the container and from there, dissipates into the surroundings.
3. But matter can not move in this way.
• We assume that, the green and pink layers in this closed system do not allow matter to pass through.
• But the layers allow the passage of energy.
Isolated system
This can be explained in 2 steps:
1. In this system, neither matter nor energy is exchanged. This is shown in fig.6.3(c) above.
2. We can assume that, the green and pink layers have insulating properties.
• Reaction carried out inside a thermos flask is an example for an isolated system.
State of the system
This can be explained in steps:
1. Consider a system such as the one shown in fig.6.4(a) below:
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| Fig.6.4 |
• It will have a certain pressure, volume, temperature and mass.
• Let us see each of these properties in detail.
■ Pressure:
• The molecules in the system will be exerting certain pressure on the walls of the container.
♦ This pressure can be measured using a pressure gauge.
♦ It is just like measuring the pressure of air inside the wheel of an automobile.
■ Volume:
• The system will be having a certain volume.
♦ In fig.6.4(a), it is the volume of the container.
■ Temperature:
The system will be having a certain temperature.
♦ This temperature can be easily measured using a thermometer.
• In our previous classes, we have see that, temperature represents the average kinetic energy of the molecules in the system.
■ Mass:
• The system will be having a certain mass.
♦ Usually, instead of mass, we use the number of moles, n.
2. If there is no flow of matter and energy into or out of the system, P, V, T and n will remain constant.
• In such a condition, we measure them and record them as: PA, VA, TA and nA
♦ The subscript 'A' indicates the initial state.
• After recording them, we start the reaction or the process.
3. A reaction can be started in different ways. Here we mention two most common methods:
(i) Add desired quantity of another reactant into the system.
(ii) Add heat into the system. For example, some decomposition reactions require heat energy.
4. A process can also be started in different ways. Here we mention two most common methods:
(i) Add heat into the system. A gaseous system will expand by absorbing heat.
(ii) Compress the system using a piston.
5. In any case, when the reaction/process is complete, the system will be having new values for P, V and T. (If any matter was added, n will also change)
• Record the new values as PB, VB, TB and nB
♦ The subscript 'B' indicates the final state.
• So we can write:
♦ PA, VA, TA and nA help us to describe the initial state of the system.
♦ PB, VB, TB and nB help us to describe the final state of the system.
6. P, V, T and n are called state variables.
State variables do not depend on the path. This can be explained using some examples:
Example 1:
• This can be written in 4 steps:
(i) Let VA and VB be the initial and final volumes.
(ii) We can heat the system and increase the volume directly from VA to VB
♦ This is shown in fig.6.4 (b) above.
(iii) We can first cool the system and decrease the volume to a lower value VA1
♦ Then we can heat the system and increase the volume from VA1 to VB
♦ This is shown in fig. 6.4(c) above.
(iv) In either case, the final volume VB does not depend on the path.
Example 2:
• This can be written in 4 steps:
(i) Let TA and TB be the initial and final temperatures.
(ii) We can heat the system and increase the temperature directly from TA to TB
(iii) We
can first cool the system and decrease the temperature to a lower value TA1
♦ Then we can heat the system and increase the temperature from TA1 to TB
(iv) In either case, the final temperature TB does not depend on the path.
7. So we have seen variables which do not depend on path. Then a question arises:
■ Are there variables which depend on the path?
• The answer is: Yes.
• Heat is a quantity that depends on the path.
This can be explained using an example. It can be written in 4 steps:
(i) Suppose that, a copper piece is to be heated from 25 °C to 175 °C
(ii) One person may give an excess heat and bring it to 200 °C first. Then he will cool it to 175 °C
• The heat given out during cooling is lost to the surroundings. That heat is not available any more.
• So overall, an excess heat is spent to bring the copper piece to the required 175 °C
(iii) Another person may give the exact amount of heat to bring the copper piece to 175 °C
• So in this case, no heat is lost.
(iv) Thus we see that, heat is path dependent.
■ Another quantity which is path dependent is: Work
We will see it’s details in later sections.
Macro properties and Micro properties
This can be explained in 5 steps:
1. Properties like pressure, volume, temperature, number of moles etc., are called macro properties of the system.
• This is because, they are related to the bulk of the system.
■ We can insert a pressure gauge at any point of the system.
• Whichever be the point of insertion, the reading in the pressure gauge will be the same.
■ We can insert a thermometer at any point in the system.
• Whichever be the point of insertion, the reading in the thermometer will be the same.
■ Volume of a system is the overall volume.
• We will never separate a portion of the system and record it’s volume.
■ n is the overall number of moles.
• We will never separate a portion of the system and record the number of moles in it.
2. We can write:
P, V, T and n are related to the total bulk of the system. So they are called macro properties
• We will see more such macro properties in later sections.
3. Consider the individual molecules of the system. Let us write some properties of those molecules:
■ Each molecule will have it’s own speed and direction.
• In other words, each molecule will have it’s own velocity.
• Some molecules will be moving towards left, some others towards right, some others upwards so on . . .
• Let us number the molecules as 1, 2, 3, . . . , n
♦ Then we can write the velocities as: v1, v2, v3, . . . , vn
■ Since, each molecule has it’s own velocity, each of them will be having it’s own kinetic energy also.
♦ We can write the kinetic energies as: KE1, KE2, KE3, . . . , KEn
■ Since the molecules are always in motion, they will be striking against the walls of the cylinder.
• While striking the walls, each molecule will be exerting a force on the walls.
♦ We can write the forces as: F1, F2, F3, . . . , Fn
4. The three properties mentioned in the above step, are related to individual molecules. They are called micro properties of the system.
♦ We are considering such properties at the micro level.
5. The macro properties and micro properties are related to each other.
• But for our present discussion in thermodynamics, we do not need the micro properties. We need the macro properties only.
• We can arrive at the required results with out knowing the micro properties.
•
In fact, the discoveries in thermodynamics were made long before
individual particles like molecules, atoms, electrons, protons etc.,
were discovered.
In the next section, we will see internal energy of a system
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