THERMODYNAMICS Introduction Thermodynamics is a study of physical and chemical phenomena which involve heat and temperature. Practically Thermodynamics is the theory of converting heat to work and understanding the role of energy and other properties of matter in this conversion process. Interestingly, ordinary experiences of our daily lives such as heat, temperature, work, energy, and properties of matter have deeply rooted with thermodynamics phenomena. Historically in 1824, Nicholas Leonard Sadi Carnot an French Engineer in his famous thesis “Reflections on the motive power of fire” showed that the work produced by a steam engine is proportional to the heat transferred from the boiler to the condenser, and that in general work could only be gained from heat by a transfer from a warmer to a colder body. Generally thermodynamics contains four laws; 1. Zeroth law: deals with thermal equilibrium and establishes a concept of temperature. 2. The First law: throws light on concept of internal energy. 3. The Second law: indicates the limit of converting heat into work and introduces the principle of increase of entropy. 4. Third law: defines the absolute zero of entropy. Basic Concepts and Definitions System: In thermodynamics, system is the specified matter under investigation or it is a region of space which is under consideration in the analysis of a problem. In simpler way a system may be as large as ocean and as small as a test tube in the chemistry laboratory. By dictating certain conditions and laws of thermodynamics System further known as; open system, closed system and isolated system. Open System: An open system is a system that freely exchanges energy and matter with its surroundings. For example, when you are boiling vegetables in an open pan on a stove, energy and matter are being transferred to the surroundings through steam. The pan is an open system because it allows for the transfer of matter (for example adding spices/salt to the pan or tasting what is being cooked) and for the transfer of energy (for example heating the pan and allowing steam to leave the pan). Boilers, turbines, heat exchangers. Fluid flow through them and heat or work is taken out or Supplied to them. Most of the engineering machines and equipment are open systems. Fig. Open system Closed System: A closed system is a system that exchanges only energy with its surroundings, not matter. For example, when a lid is put a beaker and contents in the beaker are boiled, the sides of the beaker will start getting foggy and misty. This fog and mist is the steam which confirms that the beaker allows for energy transfer. Thus, even though a closed system cannot allow matter transfer, it can still allow energy transfer. Car battery, Electric supply takes place from and to the battery but there is no material transfer. Fig. Closed system Worked example: 1. Classify each of the following systems into open or closed systems. (1) Kitchen refrigerator, (2) Ceiling fan (3) Thermometer in the mouth (4) Air compressor (5) Pressure Cooker (6) Carburetor (7) Radiator of an automobile. Solution: Kitchen refrigerator: Closed system. No mass flow. Electricity is supplied to compressor motor and heat is lost to atmosphere. Ceiling fan: Open system. Air flows through the fan. Electricity is supplied to the fan. Thermometer in the mouth: Closed system. No mass flow. Heat is supplied from mouth to Thermometer bulb. Air compressor: Open system. Low pressure air enters and high pressure air leaves the compressor, electrical energy is supplied to drive the compressor motor. Pressure Cooker: Closed system. There is no mass exchange (neglecting small steam leakage). Heat is supplied to the cooker. Carburetor: Open system. Petrol and air enter and mixture of petrol and air leaves the carburetor. There is no change of energy. Radiator of an automobile: Open system. Hot water enters and cooled water leaves the radiator. Heat energy is extracted by air flowing over the outer surface of radiator tubes. Surroundings: Anything outside the thermodynamic system is called the surroundings. To be more perfect anything outside the thermodynamic system which affects the behaviour of the system is known as surrounding. Energy: is the ability to do work. Work is when an object moves against a force and is defined by the following equation: W = F x D ………….(1) ‘W’ represents work, ‘F’ represents force, and ‘D’ represents distance. It can be as simple as picking up a cricket ball or as complicated as pushing a bus. When you are moving an object against a force (i.e. gravity), you are doing work on that object. There are many different types of energy, but the two that will be discussed here are potential energy and kinetic energy. Potential energy is "stored energy," energy that contains the potential to do work when released. Any object that is stationary contains potential energy. For example, if someone is standing in a cricket ground holding a ball in their hand, the ball has potential energy. Note that the ball is stationary. Kinetic energy on the other hand, is known as the energy created by movement. Now imagine that someone is still holding that same ball in their hand. By throwing the ball, potential energy is transformed to kinetic energy, because the ball is now moving and is not stationary anymore. Heat: A form of energy associated with the motion of atoms or molecules and capable of being transmitted through solid and fluid media by conduction, through fluid media by convection, and through empty space by radiation. Temperature: Heat is energy transferred between the system and the surrounding. It is a property which is used to determine the degree of coldness or hotness or level of heat intensity of a body or Temperature is the property of matter which reflects the quantity of energy of motion of the component particles. It is also termed as a measure of the intensity of heat, i.e. the hotness or coldness of a sample or object. Work: Work is defined as the energy transferred (without transfer of mass) across the boundary of a system and surrounding. In thermodynamics, the term work denotes a means for transferring energy. Work is an effect of one system on another that is identified and measured. Work done by a system is considered negative: W > 0 i.e., for example if you throw an object. Work done on a system is considered positive: W < 0 i.e., for example if you hit by a truck. Work = Force x Distance …………….(3) The Internal Energy (E): of a system is the total energy content of the system. It is the sum of the kinetic, potential, chemical, electrical, and all other forms of energy possessed by the atoms and molecules of the system. E is path independent, but Q (heat content) and W (work done) are path dependent. For an ideal gas, the internal energy depends only on temperature. Enthalpy: Imagine heating or cooling of water as shown in below Fig. 1.4.; in both the case there will be change in heat content that means while heating, heat energy will be used for boiling water (precisely temperature was enters into the system). On the other hand while cooling water, heat is released to the surrounding (precisely temperature was going out of the system). To summarize in both the reaction (cooling or heating or any other reaction to say) there will be change in the heat content. Thermodynamically this change in the heat content was known as Enthalpy (∆H) ( in Greek “heat inside”). The heat released or absorbed in the constant pressure process is called the enthalpy change for the reaction. Fig. 1.4. Enthalpy of the process. The following conventions are used for enthalpy changes. ∆H < 0 A reaction is exothermic when heat is given out by the system and enters into the surroundings. ∆H > 0 A reaction is endothermic when heat is absorbed by the system. Enthalpy is a very important concept in thermodynamics it can be related to other parameter like as follows; for example, if we perform a chemical reaction at constant pressure (P) and during that reaction if change in volume (∆V) takes place then enthalpy of that reaction will be equal to ∆H =∆E + P∆V ………….(4) Where ∆U is the change in the internal energy for the process. Specific Heats: The specific heat of a substance is the heat required in calories to raise the temperature of 1 gram by 1 degree Celsius. There are two kinds of specific heats: Specific heat at constant volume, Cv (the energy required when the volume is maintained constant). Specific heat at constant pressure, Cp (the energy required when the pressure is maintained constant) The specific heat at constant pressure Cp is always higher than Cv because at constant pressure the system is allowed to expand and energy for this expansion must also be supplied to the system. Types of thermodynamic processes: We say that a thermodynamic process has occurred when the system changes from one state (initial) to another state (final). Isothermal process: When the temperature of a system remains constant during a process, we call it isothermal. Heat may flow in or out of the system during an isothermal process. An isothermal process in one in which the initial and final temperatures are the same. dT = 0 Adiabatic process: No heat can flow from the system to the surroundings or vice versa. dq = 0 It can be also defined as “The process in which neither heat enters into nor goes out of the system is called adiabatic process.” In an adiabatic process, compression always results in warming and expansion in cooling.