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Bond energy calculations - Higher

Bond energy calculations - Higher

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Summary

Bond energy calculations

In a nutshell

Each type of bond has a bond energy. Bond energies can be used to work out the overall energy change of a reaction and determine if a reaction is endothermic or exothermic. Exothermic reactions have a negative overall energy change. Endothermic reactions have a positive overall energy change.


​

Breaking and forming bonds 

Chemical reactions involve breaking bonds in the reactants and forming new bonds to produce products. Energy transfers occur due to the breakage or formation of bonds.


Breaking an existing bond requires energy, therefore it is an endothermic process which absorbs energy from the surroundings. 


Forming new bonds releases energy into the surroundings and is an exothermic process.


A reaction is exothermic reaction if more energy is released when new bonds form than is absorbed to break old bonds. A reaction is endothermic if more energy is absorbed to break old bonds than is released when new bonds form.



Bond energies 

Definition

Every type of bond has a bond energy. Bond energy refers to the amount of energy required to break one mole of a specific bond. The bond energy of a particular bond will not be the same across all compounds; bond energies will vary slightly depending on the compound.


The overall energy change of a reaction can be calculated using bond energies.


​Overall energy change=Energy needed to break bonds − Energy released when bonds formOverall \space energy \space change = Energy \space needed \space to \space break \space bonds \space - \space Energy \space released \space when \space bonds \space formOverall energy change=Energy needed to break bonds − Energy released when bonds form


procedure 

1.
First work out the energy needed to break all the bonds present. This is done by adding the bond energies of all the bonds in the reactants.
2.
Work out the energy released when new bonds form. This is done by adding the bond energies of all the bonds in the products.
3.
To get the overall energy change of a reaction, subtract the energy released when new bonds formed from the energy required to break old bonds.

​​

The unit for energy change is kJ mol−1kJ \space mol^{-1}kJ mol−1.​


Bond energy for exothermic reactions

​If the overall energy change of a reaction is negative, more energy is released than absorbed, which means that the reaction is exothermic.


Example

Hydrogen reacts with oxygen to form water.


Bond​​​​
​H−HH-HH−H                          
​H−OH-OH−O                     
​O=OO=OO=O      
Bond energy (kJ mol−1)(kJ \space mol^{-1})(kJ mol−1)​(KJ \space mol^{-1})
(KJ \space mol^{-1})
436436436​
464464464​
498498498​


H−HH−H + O=O⟶ H−O−H  +  H−O−H_{H-H }^{H-H} \space + \space O=O \longrightarrow \space H-O-H \space \space+ \space\space H-O-HH−HH−H​ + O=O⟶ H−O−H  +  H−O−H


First, calculate the amount of energy needed to break all the bonds present in the reactants. There are two hydrogen molecules and one oxygen molecule:


​Energy needed to break bonds=2(H−H)+1(O=O)= 2(436) + 498=1370 kJ mol−1Energy \space needed \space to \space break \space bonds= 2(H-H) + 1(O=O)=\space 2(436) \space + \space 498 =1370 \, kJ \space mol^{-1}Energy needed to break bonds=2(H−H)+1(O=O)= 2(436) + 498=1370kJ mol−1


Then calculate the energy released when the products form. There are two water molecules:

​

Energy released when bonds form=4(O−H)= 4(464)=1856 kJ mol−1Energy \space released \space when \space bonds \space form= 4(O-H)=\space 4(464) =1856 \, kJ \space mol^{-1}Energy released when bonds form=4(O−H)= 4(464)=1856kJ mol−1


Finally, subtract the energy released when products form from the energy required to break the bonds in the reactants:

​

Overall \space energy \space change = Energy \space needed \space to \space break \space bonds \space - \space Energy \space released \space when \space bonds \space formOverall \space energy \space change = Energy \space needed \space to \space break \space bonds \space - \space Energy \space released \space when \space bonds \space for

Overall energy change=1370 − 1856= −486 kJ mol−1Overall \space energy \space change= 1370 \space - \space 1856 = \space -486 \, kJ \space mol^{-1}Overall energy change=1370 − 1856= −486kJ mol−1


The overall energy change for this reaction is −486 kJ mol−1‾\underline{-486 \,kJ \space mol^{-1}}−486kJ mol−1​ therefore this reaction is exothermic.

​

Bond energy for exothermic reaction

​If the overall energy change of a reaction is positive,  more energy is absorbed than is released, which means that the reaction is endothermic.​


Example

​​Nitrogen reacts with oxygen to form nitric oxide.

​

​Bond​​​​​
​​N≡NN\equiv NN≡N                   
​​O=OO=OO=O​                            ​
​​N=ON=ON=O​              ​
​Bond energy (kJ mol−1)(kJ \space mol^{-1})(kJ mol−1)​
945945945​
498498498​
630630630​


630\Large N\equiv N \space + \space O=O \longrightarrow \space 2N=O\Large N\equiv N \space + \space O=O \longrightarrow \space 2N=O​​

N≡N + O=O⟶ 2N=ON\equiv N \space + \space O=O \longrightarrow \space 2N=ON≡N + O=O⟶ 2N=O


First, calculate the amount of energy needed to break all the bonds present in the reactants. There is one nitrogen molecule and one oxygen molecule: ​

​

Energy needed to break bonds=(N≡N) +(O=O) = 945 + 498=1443 kJ mol−1Energy \space needed \space to \space break \space bonds= (N \equiv N) \space + (O=O) \space = \space 945 \space + \space 498 =1443 \, kJ \space mol^{-1}Energy needed to break bonds=(N≡N) +(O=O) = 945 + 498=1443kJ mol−1


Overall \space energy \space change = Energy \space needed \space to \space break \space bonds \space - \space Energy \space released \space when \space bonds \space formOverall \space energy \space change = Energy \space needed \space to \space break \space bonds \space - \space Energy \space released \space when \space bonds \space for


Then calculate the energy released when the products form. There are two nitric oxide molecules:

​

Energy released when bonds form=2(N=O)=2(630)=1260 kJ mol−1Energy \space released \space when \space bonds \space form= 2(N=O) = 2(630)=1260 \space kJ \space mol^{-1}Energy released when bonds form=2(N=O)=2(630)=1260 kJ mol−1


Finally, subtract the energy released when products form from the energy required to break the bonds in the reactants:

​


Overall \space energy \space change = Energy \space needed \space to \space break \space bonds \space - \space Energy \space released \space when \space bonds \space formOverall \space energy \space change = Energy \space needed \space to \space break \space bonds \space - \space Energy \space released \space when \space bonds \space for

Overall energy change=1443 − 1260= 183 kJ mol−1Overall \space energy \space change= 1443 \space - \space 1260 = \space 183 \space kJ \space mol^{-1}Overall energy change=1443 − 1260= 183 kJ mol−1


The overall energy change of this reaction is 183 kJ mol−1‾\underline{183 \,kJ \space mol^{-1}}183kJ mol−1​ therefore this reaction is endothermic.


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Learn with Basics

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Length:
An introduction to chemical reactions

Unit 1

An introduction to chemical reactions

Exothermic and endothermic reactions and catalysts

Unit 2

Exothermic and endothermic reactions and catalysts

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Bond energy calculations - Higher

Unit 3

Bond energy calculations - Higher

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FAQs - Frequently Asked Questions

How do you work out the overall energy change of a reaction?

The overall energy change of a reaction can be calculated using bond energies. Overall energy change = Energy needed to break bonds − Energy released when bonds form.

What is bond energy?

Bond energy refers to the amount of energy required to break one mole of a specific bond. The bond energy of a particular bond will not be the same across all compounds; bond energies will vary slightly depending on the compound.

How can you tell if a reaction is exothermic or endothermic from the overall energy change?

​If the overall energy change of a reaction is negative, more energy is released than absorbed, which means that the reaction is exothermic. ​If the overall energy change of a reaction is positive, more energy is absorbed than is released, which means that the reaction is endothermic.

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