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