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Bond energies

Combustion of methane

One molecule of methane and two molecules of oxygen

Combustion of methane

CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

Combustion of methane

CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

All reactions involve bond breaking and bond making as the atoms “swap partners”

Bond breaking - endothermic

• Energy is always required to be inputted to break a bond. Bond breaking is always endothermic.

Bond making - exothermic

• Energy is always released when a bond is formed. Bond making is always exothermic.

Chemical Energy

• When chemical reactions occur, bonds are first broken and then formed.

• The amount of energy released / absorbed during a chemical reaction can be measured and calculated from the bond energies

Bond Energy

• Energy is absorbed by atoms when their bonds break.

(+∆H) - bond dissociation

• Energy is released when bonds form between atoms.

(-∆H) - bond formation

Bond EnergyBond

Energy (kJ/mol)

H - H 436

C - H 413

N - H 393

436 kJ is required to disrupt H-H

436 kJ is released when H-H is formed

The higher the bond energy, the more work is required to break the bond, the more stable it is

Bond EnergyBond

Energy (kJ/mol)

BondEnergy (kJ/mol)

H - H 436 N - N 160

C - H 413 N = O 631

N - H 393 N triple N 941

P - H 297 N - O 201

C - C 347 N - P 297

C - O 358 O - H 464

C - N 305 O - S 265

C - Cl 397 O - Cl 269

C = C 607 O - O 204

C = O 805 C - F 552

O = O 498 C - S 259

Energy level diagrams

Exothermic reaction

The energy need to break the bonds is less than the energy released when new bonds are made

“reaction path”

ener

gy

CH4(g) + 2O2(g)

C + 4H + 4O

CO2(g) + 2H2O(l)

Energy needed to break bonds Energy released by

forming bonds

Energy released

Endothermic reaction

The energy need to break the bonds is more than the energy released when new bonds are made

“reaction path”

ener

gy

NH4NO3(s) + H2O (l)

Energy needed to break bonds

Energy released by forming bonds

NH4NO3(l)

Energy absorbed

ΔH – Energy change in a complete reaction

If heat is given out, the reaction has lost energy so ΔH is negative

ΔH – Energy change in a complete reaction

If heat is absorbed (reaction gets colder), the reaction has gained energy so ΔH is positive

Calculating ΔH

CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

Calculating ΔH

CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

Bonds broken = 4 x (C-H) + 2 x (O=O)

= 4 x 413 + 2 x 498

= 1662 + 996 = 2658 KJ/mol

Calculating ΔH

CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

Bonds broken = 4 x (C-H) + 2 x (O=O)

= 4 x 413 + 2 x 498

= 1662 + 996 = 2658 KJ/mol

Bonds made = 4 x (O-H) + 2 x (C=O)

= 4 x -464 + 2 x -805

= -1856 + -1610 = -3466 KJ/mol

Calculating ΔH

CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

Bonds broken = 4 x (C-H) + 2 x (O=O)

= 4 x 413 + 2 x 498

= 1662 + 996 = 2658 KJ/mol

Bonds made = 4 x (O-H) + 2 x (C=O)

= 4 x -464 + 2 x -805

= -1856 + -1610 = -3466 KJ/mol

Overall Energy change = 2658 + -3466 = -808 KJ/mol

(Exothermic)

Calculating Energy of a Reaction

H2 + Cl2 2HCl

Breaking H bond = 436 kJ/mol

Breaking Cl bond = 242 kJ/mol

H-Cl bond forming = -431 kJ/mol

Reactions and Energy

If heat is generated during a reaction ≡ exothermic

If heat is absorbed during a reaction ≡ endothermic

Calculating Energy of a ReactionH2 + Cl2 2HCl

Moles of Bonds Broken

Energy absorbed (kJ)

Moles of Bonds Formed

Energy released (kJ)

1 H-H @ 431.2 kJ

436 2 H-Cl @427 kJ 854

1 Cl-Cl @ 243

679 854

Heat of Reaction is 679 – 854 = -175kJ; energy is released during the reaction

Endothermic or exothermic?

Chemical Reactions and Energy

H2 + Cl2 Bonds are broken HCl

Energy releasedEnergy released

Calculating chemical energy

N2 + O2 2NO

Breaking one N bond = 946 kJ/mol

Breaking one O bond = 498 kJ/mol

Forming 2 N-O bonds = 2 x 631 kJ/mol = 1262 kJ/mol

Net energy released = (946 + 498) + 2(-631) = +182 kJ/mol

Endothermic or exothermic?

Chemical Reactions and Energy

N2 + O2 Bonds are broken NO

Energy absorbedEnergy absorbed

Calculating chemical energy

2H2 + O2 2H2O

Breaking H bond = 436 kJ/mol

Breaking O bond = 498 kJ/mol

O-H bond forming = -464 kJ/mol

Net energy released = ((2*436) + 498) + 4(-464) = -442 kJ/molWhen the net energy value is positive, the reaction is endothermic.