Bond Energies covalent bonds. Non-metal elements combine to form molecules by sharing pairs of electrons in 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. absorbed. i.e. heat (energy) is
+ ………….………..continued For example, to break up the formula mass of hydrogen molecules, H2, (Mr=2 x 1=2),or 2 grams of hydrogen molecules requires 436 kJ (kilojoules) of energy. H = + 436 kJ Conversely, the forming of a covalent bond releasesenergy, and this process is said to be exothermic. i.e. heat (energy) is evolved. H H
+ ……………………..……continued For example, when the formula mass of chlorine molecules,(Mr = 2x35.5 = 71), or 71 grams of chlorine molecules are formed, 242 kJ of energy are released. H = - 242 kJ 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 x Energy 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 kJ 1 x Cl-Cl bond = 242 kJ Total = 678 kJ endothermic 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.
H H H H C C O H H ………..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 The structure of ethanol is: and it burns in oxygen to form carbon dioxide and water only H H H H C C O H H