Download Thermochemistry: Enthalpy Changes in Chemical Reactions - Prof. Stefan Franzen and more Study notes Physical Chemistry in PDF only on Docsity! 1 Chemistry 331 Lecture 8 Thermochemistry NC State University Chemical Change In a chemical change the identity of substances is altered during the course of a reaction. One example is the hydrogenation of ethene: CH2=CH2(g) + H 2(g) CH3CH3(g) ∆H = -137 kJ The negative value of DH signifies that the enthalpy of the system decreases by 137 kJ and, if the reaction takes place at constant pressure, 137 kJ of heat is released into the surroundings, when 1 mol of CH2=CH2 combines with 1 mol of H2 at 25 oC. Standard Enthalpy Changes The reaction enthalpy depends on conditions (e.g. T and P). It is convenient to report and tabulate information under a standard set of conditions. Corrections can be made using heat capacity for variations in the temperature. Corrections can also be made for variations in the pressure. When we write ∆H in a thermochemical equation, we always mean the change in enthalpy that occurs when the reactants change into the products in their respective standard states. Standard Reaction Enthalpy The standard reaction enthalpy, ∆rH , is the difference between the standard molar enthalpies of the reactants and products, with each term weighted by the stoichiometric coefficient. The standard state is for reactants and products at 1 bar of pressure. The unit of energy used is kJ/mol. The temperature is not part of the standard state and it is possible to speak of the standard state of oxygen gas at 100 K, 200 K etc. It is conventional to report values at 298 K and unless otherwise specified all data will be reported at that temperature. ∆ rH ∅ = νHm ∅(products)Σ – νHm∅(reactants)Σ Enthalpies of Ionization The molar enthalpy of ionization is the enthalpy that accompanies the removal of an electron from a gas phase atom or ion: H(g) H+(g) + e-(g) ∆H = +1312 kJ For ions that are in higher charge states we must consider successive ionizations to reach that charge state. For example, for Mg we have: Mg(g) Mg+(g) + e -(g) ∆H = +738 kJ Mg+(g) Mg2+(g) + e-(g) ∆H = +1451 kJ We shall show that these are additive so that the overall enthlalpy change is 2189 kJ for the reaction: Mg(g) Mg2+(g) + 2e-(g) Electron Gain Enthalpy The reverse of ionization is electron gain. The corresponding enthalpy is called the electron gain enthalpy. For example: Cl(g) + e-(g) Cl- (g) ∆H = -349 kJ The sign can vary for electron gain. Sometimes, electron gain is endothermic. The combination of ionization and electron gain enthalpy can be used to determine the enthalpy of formation of salts. Other types of processes that are related include molecular dissociation reactions.
