Bond Energies Nonmetal elements combine to form molecules

Bond Energies Non-metal elements combine to form molecules by sharing pairs of electrons in covalent bonds. Every covalent bond requires a certain amount of energy to break it. This is known as the bond energy. Since the breaking of a covalent bond requires energy, the process is said to be endothermic. i. e. heat (energy) is absorbed.

…………. . continued For example, to break up the formula mass of hydrogen molecules, H 2, (Mr=2 x 1=2), or 2 grams of hydrogen molecules requires 436 k. J (kilojoules) of energy. + H = + 436 k. J Conversely, the forming of a covalent bond releases energy, and this process is said to be exothermic. i. e. heat (energy) is evolved.

…………. . ……continued For example, when the formula mass of chlorine molecules, (Mr = 2 x 35. 5 = 71), or 71 grams of chlorine molecules are formed, 242 k. J of energy are released. + H = - 242 k. J Hence, bond energies can be used to calculate the overall heat change for a chemical reaction. That is, we can calculate to what extent a chemical reaction is exothermic or endothermic.

separate atoms Energy x reactants y z products Progress of reaction x = Total energy needed to break the bonds in the reactants. y = Total energy released when the bonds are formed in the products. z = Overall energy change for the reaction; i. e. exothermic

…………. summary You must start any calculation by writing a balanced symbols equation; – preferably showing all the covalent bonds H H + Cl Cl 2 H Cl Carry out your calculations in the from of a ‘balance sheet’ 1 x H-H bond = 436 k. J 1 x Cl-Cl bond = 242 k. J Total = 678 k. J endothermic 2 x H-Cl bond = 2 x 431 = 862 k. J exothermic …continued

………………. . continued Overall, more heat is evolved in forming the new covalent bonds than is required to break the original covalent bonds: Heat change for the reaction is: 862 – 678 = 184 k. J Since this is heat evolved, we say: H = -184 k. J (exothermic)

Why should we study bond energies? • Bond energies can be used in industry to predict the heat change for untried chemical reactions. • This allows the chemical engineer to modify the process to allow for excessive amounts of heat being evolved; e. g. introduce cooling at appropriate stages. • Also, if it is known that large amounts of heat (energy!) are to be released, it may be appropriate to ‘trap’ this energy for use elsewhere; i. e. to improve the economics of the process.

………. . continued • Bond energies are also useful in determining the efficiency of fuels; i. e. by calculating the heat (energy) evolved per gram of fuel. See the homework question in the workbook! Tips for HWK H H The structure of ethanol is: H C C H H and it burns in oxygen to form carbon dioxide and water only O H