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Bond Energy (Bond Enthalpy)

Key Concepts

Bond energy (enthalpy) is the energy (enthalpy) required per mole of gaseous compound to break a particular bond to produce gaseous fragments.
Bond energies (enthalpies) are positive.
H is positive.
Breaking bonds is an endothermic reaction.
Bond formation releases energy.
H is negative.
Bond formation is an exothermic reaction.
Bond energies (enthalpies) can be used to indicate how stable a compound is or how easy it is break a particular bond.
The more energy that is required to break a bond, the more stable the compound will be.
A larger bond energy implies the bond is harder to break, so the compound will be more stable.
Bond energies (enthalpies) can be used to estimate the heat (enthalpy) of a reaction.
H^o_(reaction) = sum of the bond energies of bonds being broken - sum of the bond energies of the bonds being formed.
H^o_(reaction) = H_{(reactant bonds broken)} - H_{(product bonds formed)}
Calculating heat (enthalpy) of reaction, H^o, from bond energies of reactants and products:

Write the balanced chemical equation, with all reactants and products in the gaseous state.
If a reactant or product is NOT in the gaseous state, you will need to use Hess's Law to include the relevant energy (enthalpy) for the change of state.
Write the general equation for the heat (enthalpy) of reaction:
H^o_(reaction) = H_{(reactant bonds broken)} - H_{(product bonds formed)}
Substitute bond energy values into the equation and solve for H^o_(reaction)

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 2 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.

Single Bond Energies (kJ mol^-1)

S Si I Br Cl F O N C H

H 339 339 299 366 432 563 463 391 413 436

C 259 290 240 276 328 441 351 292 348

N 200 270 161

O 369 203 185 139

F 541 258 237 254 153

Cl 250 359 210 219 243

Br 289 178 193

I 213 151

Si 227 177

S 213

Example of Bond Energy and Chemical Stability

The energy required to break the O - H bond in water, H₂O_(g), is 463 kJ mol^-1

The energy required to break the O - O bond in hydrogen peroxide, H₂O_2(g), is 139 kJ mol^-1.

Hydrogen peroxide is less stable than water because it contains the O - O bond which is easier to break.

Hydrogen peroxide will be more chemically reactive than water.

Calculating Heat (Enthalpy) of Reaction Using Bond Energy

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

H^o_(reaction) = sum of the bond energies of bonds being broken - sum of the bond energies of the bonds being formed.
Or,
H^o_(reaction) = H_{(reactant bonds broken)} - H_{(product bonds formed)}

Example of Using Bond Energies to Calculate Heat (enthalpy) of Reaction

Use the bond energies provided in the table above to calculate the heat (enthalpy) of reaction,

H^o, for the reaction:

CH_4(g) + 4Cl_2(g) -----> CCl_4(g) + 4HCl_(g)

Write the balanced chemical equation, with all reactants and products in the gaseous state.
CH_4(g) + 4Cl_2(g) -----> CCl_4(g) + 4HCl_(g)
Write the general equation for the heat (enthalpy) of reaction:
H^o_(reaction) = H_{(reactant bonds broken)} - H_{(product bonds formed)}

Substitute bond energy values into the equation and solve for

H^o_(reaction)

Bonds Broken			Bonds Formed
bond type	bond energy		bond type	bond energy
4 x C - H	4 x 413	= 1652	4 x C - Cl	4 x 328	= 1312
4 x Cl - Cl	4 x 243	= 972	4 x H - Cl	4 x 432	= 1728

H_{(reactant bonds broken)}		= 2624	H_{(product bonds formed)}		= 3040