Bond+Enthalpies

Chemical reactions involve the making and breaking of bonds. //Bond enthalpy// is the energy change when one mole of gaseous atoms form one mole of a covalent bond in the gaseous state.

X (g) + Y (g) ---> X-Y (g) X and Y are the gaseous atoms and X-Y is the gaseous molecule.

For some bonds the value of the bond enthalpy is to some extend affected by the surrounding bonds and therefore slightly different in different compounds. Thus we calculate the //average bond enthalpy//: the average of the enthalpy required to break a particular covalent bond in a range of molecules.

Using average bond enthalpies, the approximate enthalpy change for a reaction can be calculated. A chemical reaction involves both **breaking** and **making bonds**:
 * **Bond breaking** is an **endothermic** process
 * **Bond making** is an ** exothermic ** process

Thus the enthalpy change of the reaction (ΔH) can be calculated as follows:

ΔH reaction = energy taken in to break bonds – energy given out to make new bonds If the energy taken in to break bonds is smaller than energy produced in forming new bonds, the reaction is exothermic (ΔH is negative) and vice versa.

//Find the enthalpy change of the reaction: // 3F 2 (g) + NH 3 (g) ---> 3HF (g) + NF 3 (g)

Given the average bond enthalpy values: F-F (g) 158 KJ mol -1 N-H (g) 388 KJ mol -1 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">H-F (g) 562 KJ mol -1 N-F 272 KJ mol -1

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Energy in to break bonds = (3x F-F) + (3x N-H) = (3x 158) + (3x288) = 1638 KJ mol -1 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Energy out to form bonds= (3x H-F) + (3x N-F) = (3x 562) + (3x 272) = 2502 KJ mol -1 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ΔH=1638 - 2502= -864 KJ mol -1 __<span style="color: #800080; font-family: Tahoma,sans-serif; font-size: 14pt;">NOTE: __ <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Going from single to double to triple covalent bonds, the value of the bond enthalpy increases. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Bonds between two very electronegative elements tend to be unusually weak (hence a lower bond enthalpy).

By the end of this lesson you should be able to:
 * Define the term average bond enthalpy
 * Explain in terms of average bond enthalpies why a reaction is exothermic or endothermic
 * Know that when the energy taken in to break bonds is greater than energy given out when making new bonds the reaction is endothermic
 * Know that when the energy taken in to break bonds is less than energy given out when making new bonds the reaction in exothermic


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