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

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

……………………..……continued of chlorine molecules are formed, 242 kJ of energy are released. For example, when the formula mass of chlorine molecules,(M r = 2x35.5 = 71), or 71 grams Hence, bond energies can be used to calculate the overall heat change for a chemical reaction.  H = kJ + That is, we can calculate to what extent a chemical reaction is exothermic or endothermic.

reactants separate atoms products x y z Energy 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 1 x H-H bond = 436 kJ 1 x Cl-Cl bond = 242 kJ Total = 678 kJ endothermic H H + Cl Cl 2 H Cl Carry out your calculations in the from of a ‘balance sheet’ 2 x H-Cl bond = 2 x 431 = 862 kJ 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 kJ Since this is heat evolved, we say:  H = -184 kJ (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! The structure of ethanol is: C H H HC H H O H HH C H HC H O H Tips for HWK and it burns in oxygen to form carbon dioxide and water only