System. System, Boundary and surroundings: Nature of heat and work: Sign convention of heat: Unit-7 Thermodynamics

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1 Unit-7 Thermodynamics Introduction: The term Thermo means heat and dynamics means flow or movement.. So thermodynamics is concerned with the flow of heat. The different forms of the energy are interconvertible from one form to another. For example, chemical energy can be converted in to electric energy and electrical energy can be converted into chemical energy and so on. During the process of transformation, the changes are accompanied by different physical and chemical processes. The study of flow of heat or any other form of energy accompanying the physical or chemical transformations is called thermodynamics. System, Boundary and surroundings: Surroundings System Boundary Fig 1. Thermodynamic system illustrated A system is that part of universe which is under thermodynamic study and the rest of universe is surroundings. The real or imaginary surface separating the system form surroundings is called boundary. Nature of heat and work: When a change in the state of a system occurs energy is transferred to the surroundings or from the surroundings. This energy may be transferred as heat or mechanical work. Sign convention of heat: The symbol of heat is q. If heat flows from the surroundings into the system, the energy of the system is raised. So it is considered as positive, +q If heat flows from the system into the surroundings, the energy of the system is lowered. So it is considered as negative, -q

2 Heat Absorbed Heat released q is (+) q is (-) Fig 2.Sign convention for heat flow in a thermodynamic system. Sign Convention of work: The symbol of work is w If work is done on a system (known as work of compression) the energy of the system is increased. So it is considered as positive, +w. If work is done by the system (known as work of expansion) the energy of the system is decreased. Summary : System So it is considered as negative w. Heat enters the system ; q is +ve Work is done on the system ; w is +ve Heat leaves the system q is ve Work is done by the system w is ve Thermochemistry Internal Energy (I.E) : It is the energy associated with the given mass of a substance. Internal energy is represented by the symbol U. The absolute value of internal energy can not be measured. When a system changes from initial state (reactant) to the final state (product) there is change in internal energy ( U). This can be calculated from the values of initial and final state. U = U 2 -U 1 = U product - U reactant The change in internal energy ( U) is the heat change in a chemical reaction at constant volume because no external work is performed.

3 Enthalpy: In the laboratory most of the chemical reactions are carried out at constant pressure (atmospheric pressure) rather than at constant volume. In order to study the heat changes for reactions taking place at constant pressure and at constant temperature a new term enthalpy was introduced. It is represented by the symbol H and is also called heat content. The enthalpy of a system defined as the sum of the internal energy and the product of pressure and volume. Thus, H = U + PV Where U = Internal energy P = Pressure V = Volume. Just like internal energy, absolute value of enthalpy (H) can not be measured. However a change in enthalpy ( H) can be measured accurately and is given by H = H product - H reactant H = H P H R Exothermic Reaction: A reaction in which heat is evolved or liberated when reactants are converted into products is called exothermic reaction. Consider the combustion reaction between carbon (graphite) and oxygen (air) to form gaseous carbon dioxide. In this reaction heat is released. The reaction is represented as, C (s) + O 2(g) CO 2(g) + heat In the above reaction the mixture of carbon, Oxygen and carbon dioxide constitutes the system and the rest is the surroundings. The heat energy is transferred from the system to the surroundings. Several chemical reactions are exothermic in nature. Ex: C (s) + O 2(g) CO 2(g) kJ For exothermic reaction. H P < H R so H is ve So the above reactions can be written as C (s) + O 2(g) CO 2(g) H = kJ

4 Few more examples are, N 2(g) + 3H 2(g) H 2(g) + ½ O 2(g) 2 NH 3(g) H = kJ H 2 O (2) H = kj Endothermic Reaction: A reaction in which heat is absorbed when reactants are converted into products is called endothermic reaction. Consider the reaction between gaseous nitrogen. N 2(g) and gaseous oxygen O 2(g) to form gaseous nitric oxide NO (g). This reaction is carried out at high temperature. So during the reaction heat is absorbed, by the system from the surroundings. The reaction is represented as, N 2(g) + O 2(g) + heat 2NO (g) Such reactions in which heat is absorbed by the system are called endothermic reactions. For endothermic reaction, H P >H R so H is +ve The above reaction can be written as N 2(g) + O 2(g) 2NO (g) H = kj Note: In an exothermic reaction, the energy of the products is less than that of reactants and in an endothermic reactions the energy of the products is greater than that of the reactants. This is shown the figure. Fig 3.Energy changes in exothermic and endothermic reactions Energy ( k J ) C (s) + O 2(g) CO 2 (g) Heat Given Out Energy ( k J ) N 2(g) +O 2(g) 2NO (g) Heat Taken In Progress of reaction Progress of Reaction

5 Sign of H Energy Terms Used Sign Released Exothermic -ve Absorbed Endothermic +ve Thermochemical Equation: A thermochemical equation is a balanced chemical equation which shows the quantity of heat liberated or absorbed during the reaction and physical states of reactants and products. In a thermochemical equation, the quantity of heat liberated or absorbed is represented by writing the value of H (heat change) at the right side of the equation. Example: The sign of H indicates whether the reaction is exothermic or endothermic. 1. H 2(g) + ½ O 2(g) H 2 O (l) H = kj It tells that kj of heat is liberated when 1 mole of hydrogen gas combines with half mole of oxygen gas to form 1 mole of liquid water. 2. N 2(g) + O 2(g) 2NO (g) H = kj It tells that 180.8kJ of heat is absorbed when 1 mole of nitrogen gas reacts with 1 mole of oxygen gas to form 2 mole of nitric oxide gas.

6 Exercises: 1. Define thermodynamics. 2. What is a system? 3. If work is done on a system, what happens to the energy of the system? 4. What happens to the energy of a system during the work of expansion? 5. What happens to the energy of a system when heat enters and leaves the system? 6. What is internal energy? 7. How is enthalpy related to internal energy? 8. What is an exothermic reaction? Give an example. 9. What is an endothermic reaction? Give an example. 10. Define a thermochemical equation. 11. Write the thermochemical equation for the combustion of carbon. 12. The value of H is +ve for an endothermic reaction. Why? 13. The value of H is -ve for an exothermic reaction. Give reason. 14. Mention the sign of H, if energy is released during a chemical reaction. 15. Mention the sign of H, if energy is absorbed during a chemical reaction. 16. Explain the following thermochemical equation. H 2(g) + ½ O 2(g) H 2 O (l) H = kj 17. Write the thermochemical equation for the formation of nitric oxide. 18. Mention the sign of work w during the work of expansion. 19. Why is the sign of work w positive during the work of compression? 20. The sign of q is negative, when heat leaves the system. Give reason.

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