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Bond Energy or Bond Enthalpy Chemistry Tutorial

Key Concepts

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Bond Energy (Bond Enthalpy) Values

The amount of energy required to break particular bonds, values for bond energy or bond enthalpy, are often presented in a table like the one below:2

Single Bond Energies (kJ mol-1)
at 25°C
  S Si I Br Cl F O N C H
H 360 339 299 366 432 563 463 391 413 436
C 259 290 240 285 339 485 358 304 348  
N         200 270   163    
O   369     203 185 146      
F   541 258 237 254 153        
Cl 250 359 210 219 243          
Br   289 178 193            
I   213 151              
Si 227 177                
S 213                  

More compact tables of bond energies (bond enthalpies) are often provided in exams:

Bond Energies at 25°C (kJ mol-1)
H-H 436 C-C 348 C-H 413 O-H 463
H-Cl 432 C=C 610 C-O 358 O-O 146
H-Br 366 C≡C 835 C=O 736 O=O 498
H-I 299 benzene 518 C-Cl 339 N-H 391
Cl-Cl 243     C-Br 285 S-H 360
Br-Br 193     C-F 485 N-N 163
I-I 151     C-N 304 N≡N 945

When using these values it is important to remember that:

For example, from the table above we find the bond energy (bond enthalpy) for the H-H bond is 436 kJ mol-1.
If we have 1 mole of H-H bonds to break, we will need to supply 436 kJ of energy.

If we have 1 mole of gaseous molecular hydrogen, H2(g), then we need to supply 436 kJ of energy for the following reaction to take place:

H2(g) → 2H(g)
H:H → H. + H.

If we had 2 moles of gaseous molecular hydrogen, H2(g), then we need to supply 2 × 436 kJ = 872 kJ of energy to break all the H-H bonds and form 4 moles of gaseous hydrogen atoms, H(g).

Similarly, if we had ½ mole of H2(g), we need to supply ½ × 436 = 218 kJ of energy in order to break all the H-H bonds and produce 1 mole of H(g).

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Bond Energy and Chemical Stability

Breaking chemical bonds requires energy.

The more energy that is required to break a bond, the more chemically stable the compound will be.

When comparing two compounds, the compound containing the bond with the lowest bond energy will be the least stable compound, regardless of the strength of the other bonds present in the compound.

Consider these two molecules, water (H2O) and hydrogen peroxide (H2O2).

The electron dot diagram (Lewis Structure) for each molecule is shown below:

water
  ..  
: O:H
  ..
H
 
 
hydrogen
peroxide
  ..   ..  
H: O: O :H
  ..   ..  

Both molecules contain O-H bonds that need to be broken if a chemical reaction is to occur.
Each mole of O-H bonds requires 463 kJ of energy (from the table of bond energies, or bond enthalpies, above)

However, in order for hydrogen peroxide to decompose, or react, another bond, the O-O bond, must be broken.
From the table of bond energies (bond enthalpies) above, we see that breaking 1 mole of O-O bonds requires only 146 kJ of energy.

It is easier, requires less energy, to break the O-O bond compared to an O-H bond.
Therefore, hydrogen peroxide is less stable than water because it contains a bond that requires less energy to break.

Hydrogen peroxide is therefore more chemically reactive than water.

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Calculating Heat of Reaction (Enthalpy of Reaction) Using Bond Energy

Bond energies (bond enthalpies) can be used to estimate the heat of reaction (enthalpy of reaction).

For the general chemical reaction in which reactants form products:

chemical reaction: reactants products
bonds are broken made
energy is absorbed released

A chemical reaction will be endothermic if the energy absorbed to break bonds in the reactant molecules is greater than the energy released when bonds are formed in the product molecules.

Hbreak bonds > Hmake bonds     ΔHreaction is positive

A chemical reaction will be exothermic if the energy absorbed to break bonds in the reactant molecules is less than the energy released when bonds are formed in the product molecules.

Hbreak bonds < Hmake bonds     ΔHreaction is negative

That is, if we add together the bond energies of all the bonds that need to be broken in the reactant molecules, and, add together all the bond energies of all the bonds we need to make in order to produce product molecules, then we can subtract one from the other to arrive at an estimate of the enthalpy change for the overall chemical reaction:

ΔHo(reaction) = sum of the bond energies of bonds being broken - sum of the bond energies of the bonds being formed.
Or,
ΔHo(reaction) = ΣH(reactant bonds broken) - ΣH(product bonds formed)

For example, we could use the bond energies provided in the table above to calculate the heat of reaction (enthalpy change for the reaction), ΔHo, for the reaction:

CH4(g) + 4Cl2(g) → CCl4(g) + 4HCl(g)

The bonds that need to be broken in the reactant molecules are:

The bonds that need to be made when the products are formed are:

We will need to use published values of bond energies (bond enthalpies) to calculate the value of the heat of reaction (enthalpy change for the reaction).

Our solution to the problem is shown below:

  1. Write the balanced chemical equation, with all reactants and products in the gaseous state.

    CH4(g) + 4Cl2(g) → CCl4(g) + 4HCl(g)

  2. Write the general equation for the heat of reaction (enthalpy change for the reaction):

    ΔHo(reaction) = ΣH(reactant bonds broken) - ΣH(product bonds formed)

  3. Substitute bond energy values into the equation and solve for ΔHo(reaction)

    CH4(g) + 4Cl2(g)

    Energy absorbed to break bonds

    CCl4(g) + 4HCl(g)

    Energy released to form bonds

    bond type bond energy   bond type bond energy
    4 × C - H 4 × 413 = 1652   4 × C - Cl 4 × 339 = 1356
    4 × Cl - Cl 4 × 243 =   972   4 × H - Cl 4 × 432 = 1728

    ΣH(reactant bonds broken) = 2624   ΣH(product bonds formed) = 3084

    ΔHo(reaction) = 2624 - 3084 = -460 kJ mol-1

    Note that the reaction is exothermic, ΔH is negative (ΔH = -460 kJ mol-1).
    The energy absorbed by reactant molecules to break bonds (2624 kJ) is less than the energy released when bonds are formed to make product molecules (3084 kJ).

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Footnotes

1. The bonds must be broken homolytically, that is, half the electrons making up the bond go with one product fragment, and, the other half of the electrons making up the bond go to the other product fragment.

2. Do not be concerned if the values for bond energy that you have been given differ from those shown here.
Published bond energies are an "average energy" since the same bond in different molecules can require different amounts of energy to break.
Also note that the energy required to break bonds will be dependent on the conditions of the reaction, such as the temperature, so check the conditions for your values.
Techniques for establishing the energy required to break bonds change, so the listed values will change.
Just use whatever values you have been given. If no values have been provided, you can use the ones given here.