Module 5: Combustion Technology. Lecture 32: Fundamentals of thermochemistry

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1 1 P age Module 5: Combustion Technology Lecture 32: Fundamentals of thermochemistry

2 2 P age Keywords : Heat of formation, enthalpy change, stoichiometric coefficients, exothermic reaction. Thermochemistry describes the thermal behaviour of a reacting system. The transfer of heat energy between a system and its surroundings occurs by the difference of temperature. The transfer of this energy takes place from a hotter body to a colder body until the temperatures of the two bodies become equal or the molecular energies of the two bodies become the same. The change of states also may happen due to this heat transfer. Combustion is a chemical reaction between a fuel and oxygen. The heat energy generates from the combustion of a fuel with air where the internal energy of the fuel species is the chemical energy. This energy is associated with the chemical bonds and intermolecular attractions. During chemical reaction, some chemical bonds are broken with the formation of new compounds with the transfer of some heat energy between system and its surroundings. The heat of reaction is the quantity of heat exchanged between a system and its surroundings during a chemical reaction at a specified temperature. The heat of combustion is defined as the quantity of heat released in combustion of a fuel with oxygen. Some important properties such as, pressure, volume, temperature, internal energy, enthalpy, entropy and free energy are required for quantitative analysis. The properties, depending only on the final and initial conditions of that system, are called state functions and they are independent of the path connecting with the initial and the final state. The most important state function related with a chemical reaction system is enthalpy. Enthalpy is usually evaluated at standard state, 1 atm pressure and 298 K of temperature. This property is used to evaluate the heat or energy evolved or absorbed in a reaction. The enthalpy of an element at standard state is assumed to be zero. The absolute value of the enthalpy of a process cannot be determined, rather the enthalpy change of the process can be determined. The enthalpy required to form a molecule from its constituent s elements at

3 3 P age standard state is defined as the heat of formation,. The standard enthalpy of formation of any substance is defined as the enthalpy of that substance compared to the enthalpies of the elements from which it is formed. Fig. 1 Heat of reaction from standard heat of formatiom Thus, the standard enthalpy change of formation of carbon dioxide by the reaction of carbon and oxygen from their elemental forms is called the standard enthalpy of formation of carbon dioxide or the heat of reaction for combustion. The standard heat of formation of each element of a fuel are required for determination of Heat of reaction for the combustion. This can be explained by the diagram as shown in Fig.1. This can be expressed by the equation Where, and are the stoichiometric coefficients in the chemical reaction of reactants and products respectively. Thus, the heat of reaction can be derived from the available heats of formation data. The reaction may be written as,

4 4 P age (i.e. the enthalpy change) at 298 K is kj mol -1. The knowledge of thermochemistry is essential to determine the molar heat of reaction. The amount of heat energy released during reaction will be obtained from the difference of internal energy between products and reactants. The reaction where heat is released by the reaction is called exothermic reaction. The combustion reactions are exothermic reactions. The thermochemical calculation are performed either at constant volume or constant pressure. The reactions are generally carried out at constant pressure. We have where is the enthalpy, is the internal energy and is pressurevolume work. The enthalpy change of a process is given by. From first law of thermodynamics,. In the present system, the work done () in the system is the pressure volume work. However, because the system remains at a constant volume, the work done will be zero. At constant pressure process, assuming ideal gas law and, we have. is the number of mole change in a reaction or, The heat change at constant pressure process is and, at constant volume is. In liquid phase reaction the volume change is also negligible and is same as. Let us consider a reaction in a general form

5 5 P age The heat change or the enthalpy change of the reaction is equal to the difference in enthalpies of the products and reactants. The change in enthalpy of complete combustion of one mole of a substance for burning in excess air is called the heat of combustion / / Carbon monoxide also may oxidized as / The quantity of heat is released depends on the enthalpy of the fuel and oxidant relative to that of the enthalpy of the combustion products. Hydrocarbon fuels on combustion in oxygen produce carbon dioxide and water with the release of heat. The most relevant law for the evaluation of the energy of the reaction system is the Hess' Law. This law states that the total amount of heat evolved or absorbed in a chemical reaction is the same whether the process takes place in one or more steps. This is also known as the law of constant heat summation. In order to explain the Hess' Law, the burning of carbon to carbon dioxide reaction is used. The reaction may take place in two different ways: 1. Direct burning of carbon to carbon dioxide /

6 6 P age 2. Carbon first burnt to CO and CO is further oxidized to CO / / Overall change of enthalpy= /. Hess s law applied for these reactions may be explained in a diagram as shown in Fig.2. Fig. 2 Application of Hess s Law for the burning of pure carbon Example1 : Determine the heat of reaction for 2 using the following heat of reactions of the following reactions, / (1), / (2) 2 2, / (3) Eqn. (2) is multiplied by 2 and added to Eqn. (1), (4) By subtracting Eqn. (3) from Eqn. (4), /

7 7 P age Reference 1. Essentials of Physical Chemistry, B.S Bahl, G. D. Tuli and A. Bahl, S. Chand & Co. Ltd, Fuels and Combustion, S. Sarkar, 3 rd Edition, University Press, India, Physical Chemistry, P. C. Rakshit, 6 th Edition, Sarat Book Distributers, India, 2001

Types of Energy Calorimetry q = mc T Thermochemical Equations Hess s Law Spontaneity, Entropy, Gibb s Free energy

Unit 7: Energy Outline Types of Energy Calorimetry q = mc T Thermochemical Equations Hess s Law Spontaneity, Entropy, Gibb s Free energy Energy Energy is the ability to do work or produce heat. The energy