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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. - PowerPoint PPT Presentation
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Bond EnergiesNon-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 beexothermic. i.e. heat (energy) is evolved.
+
For example, to break up the formula mass of hydrogen molecules, H2, (Mr=2 x 1=2),or 2 gramsof hydrogen molecules requires 436 kJ (kilojoules) of energy.
H = + 436 kJ
……………………..……continued
of chlorine molecules are formed, 242 kJ of energy are released.
For example, when the formula mass of chlorine molecules,(Mr = 2x35.5 = 71), or
71 grams
Hence, bond energies can be used to calculate the overall heat change for a chemical reaction.
H = - 242 kJ+
That is, we can calculate to what extent a chemical reaction is exothermic or endothermic.
reactants
separate atoms
products
x
y
z
Ene
rgy
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
…………………….summaryYou must start any calculation by writing a balanced symbols equation; – preferably showing all the covalent bonds
1 x H-H bond = 436 kJ1 x Cl-Cl bond = 242 kJTotal = 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
………………..continuedOverall, 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 kJSince 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:
H H
C
H
H C
H
O H
Tips for HWK
and it burns in oxygen
to form carbon dioxide and water only