Thermodynamic system illustrated brPage 2br Sign convention for heat flow in a thermodynamic system brPage 3br brPage 4br Energy changes in exothermic and endothermic reactions brPage 5br ID: 83176
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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. 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 U). This can be calculated from the values of initial and final state. The change in internal energy (U) is the heat change in a chemical reaction at constant volume because no external work is performed. Heat Heat Absorbed released Heat enters the system ; Heat leaves the system q is +ve q is ve Work is done on the system ; Work is done by the system w is +ve w is ve Few more examples are, 2(g)2(g) 2 NH3(g) HA reaction in which heat is absorbed when reactants are converted into products is called endothermic reaction.to form gaseous nitric oxide 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, 2(g) + heat 2NO Such reactions in which heat is absorbed by the system are called endothermic For endothermic reaction, so H is +ve The above reaction can be written as 2(g) In an exothermic reaction, the energy of and in an endothermic reactions the energy ofreactants. This is shown the figure. Energy ( k J ) 2(g) ofreaction ProgressofReaction Released Exothermic -ve Absorbed Endothermic +ve 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 In a thermochemical equation, the quantityindicates whether the reaction is exothermic or endothermic. 1. Hmole of oxygen gas to form 1 mole of liquid water. 2. H = +180.8 kJ reacts with 1 mole of oxygen gas to form 2 mole of nitric oxide gas . Energy Terms Used Sign Sign of H