THERMODYNAMICS || Thermodynamics Class 11 Notes || Notes ||
1. Introduction
Chemical energy stored by molecules can be released as heat during chemical reactions when a fuel like methane, cooking gas or coal burns in air. The chemical energy may also be used to do mechanical work when a fuel burns in an engine or to provide electrical energy through a galvanic cell like dry cell. Thus, various forms of energy are interrelated and under certain conditions, these may be transformed from one form into another. The study of these energy transformations forms the subject matter of thermodynamics. The laws of thermodynamics deal with energy changes of macroscopic system involving a large number of molecules rather than microscopic systems containing a few molecules. Thermodynamics is not concerned about how and at what rate these energy transformations are carried out, but is based on initial and final states of a system undergoing the
change. Laws of thermodynamics apply only when a system is in equilibrium or moves from one equilibrium state to another equilibrium state. Macroscopic properties like pressure and temperature do not change with time for a
system in equilibrium state.
We are interested in chemical reactions and the energy changes accompanying them. For this we need to know certain thermodynamic terms. These are discussed below:
1. The System and the Surroundings
A system in thermodynamics refers to that
part of universe in which observations are
made and remaining universe constitutes the surroundings. The surroundings include everything other than the system. System and the surroundings together constitute the universe .
The universe = The system + The surroundings. However, the entire universe other than the system is not affected by the changes taking place in the system. Therefore, for all practical purposes, the surroundings are that portion of the remaining universe which can interact with the system. Usually, the region of space in the
neighbourhood of the system constitutes its surroundings.
2. Types of the System
We, further classify the systems according to the movements of matter and energy in or out of the system.
(i). Open System
In an open system, there is exchange of m energy and matter between system and surrounding. The presence of reactants in an open beaker is an example of an open system. Here the boundary is an imaginary surface enclosing the beaker and reactants.
(i i) Closed System
In a closed system, there is no exchange of
matter, but exchange of energy is possible
between system and the surroundings
.The presence of reactants in a closed vessel made of conducting material e.g., copper or steel is an example of a closed system.
(i i i) Isolated System
In an isolated system, there is no exchange of energy or matter between the system and the surroundings. The presence of
reactants in a thermos flask or any other closed insulated vessel is an example of an isolated system.
The system must be described in order to make any useful calculations by specifying
quantitatively each of the properties such as its pressure (p), volume (V), and temperature (T ) as well as the composition of the system. We need to describe the system by specifying it before and after the change. You would recall from your Physics course that the state of a system in mechanics is completely specified at
a given instant of time, by the position and
velocity of each mass point of the system. In
thermodynamics, a different and much simpler concept of the state of a system is introduced. It does not need detailed knowledge of motion of each particle because, we deal with average measurable properties of the system. We specify
the state of the system by state functions or
state variables.
The state of a thermodynamic system is
described by its measurable or macroscopic
(bulk) properties. We can describe the state of a gas by quoting its pressure (p), volume (V), temperature (T ), amount (n) etc. Variables like p, V, T are called state variables or state functions because their values depend only on the state of the system and not on how it is
reached. In order to completely define the state of a system it is not necessary to define all the properties of the system; as only a certain number of properties can be varied independently. This number depends on the nature of the system. Once these minimum number of macroscopic properties are fixed, others automatically have definite values. The state of the surroundings can never be completely specified; fortunately it is not necessary to do so. THERMODYNAMICS
When we talk about our chemical system
losing or gaining energy, we need to introduce a quantity which represents the total energy of the system. It may be chemical, electrical, mechanical or any other type of energy you may think of, the sum of all these is the energy of the system. In thermodynamics, we call it the internal energy, U of the system, which may
change, when
• heat passes into or out of the system,
• work is done on or by the system,
• matter enters or leaves the system.
(a) Work
Let us first examine a change in internal
energy by doing work. We take a system
containing some quantity of water in a thermos flask or in an insulated beaker. This would not allow exchange of heat between the system and surroundings through its boundary and we call this type of system as adiabatic. The manner in which the state of such a system
may be changed will be called adiabatic
process. Adiabatic process is a process in
which there is no transfer of heat between the system and surroundings. Here, the wall separating the system and the surroundings is called the adiabatic wall.
(b) Heat
We can also change the internal energy of a
system by transfer of heat from the
surroundings to the system or vice-versa
without expenditure of work. This exchange of energy, which is a result of temperature difference is called heat, q. Let us consider bringing about the same change in temperature (the same initial and final states as before in section by transfer of heat through thermally conducting walls instead of adiabatic walls. THERMODYNAMICS
We take water at temperature, TA in a
container having thermally conducting walls, say made up of copper and enclose it in a huge heat reservoir at temperature, TB. The heat absorbed by the system (water), q can be measured in terms of temperature difference , TB – TA. In this case change in internal energy, ∆U= q, when no work is done at constant volume. By conventions of I U P A C in chemical thermodynamics. The q is positive, when heat is transferred from the surroundings to the system and the internal energy of the system increases and q is negative when heat is transferred from system to the surroundings resulting in decrease of the internal energy of the system. Himanshu swiz
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