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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 When two atoms form a bond, energy is stored in the molecule as part of the process When the bond is broken, energy must be injected into the molecule to disrupt the bond and separate the two atoms. This bond energy can be measured. It is equal and opposite ot the energy used to form the bond. Generically we refer to this energy as bond energy

Bond Energy Bond 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 Energy Bond Energy (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 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 Which of these bonds are most stable? – C=O, N≡N Least stable? - N-N, N-O

Energy level diagrams

Exothermic reaction The energy need to break the bonds is less than the energy released when new bonds are made C + 4H + 4O energy Energy needed to break bonds Energy released by forming bonds CH4(g) + 2O2(g) Energy released CO2(g) + 2H2O(l) “reaction path”

Endothermic reaction The energy need to break the bonds is more than the energy released when new bonds are made energy Energy released by forming bonds Energy needed to break bonds NH4NO3(l) Energy absorbed NH4NO3(s) + H2O (l) “reaction path”

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 When the net energy released is negative, the reaction is exothermic

Reactions and Energy If heat is generated during a reaction ≡ exothermic If heat is absorbed during a reaction ≡ endothermic

Calculating Energy of a Reaction H2 + 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 Heat of Reaction is 679 – 854 = -175kJ; energy is released during the reaction When the net energy released is negative, the reaction is exothermic Endothermic or exothermic?

Chemical Reactions and Energy Energy released This is an example on an exothermic reaction. In an exothermic reaction, the energy required to break the bonds is less than the energy required to make the new bonds. The excess energy is released as heat. Conversely, if more energy is needed to form the bonds in the product than is released with the reactants’ bonds are broken, the reaction absorbs heat. H2 + Cl2 Bonds are broken HCl

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 When the net energy value is positive, the reaction is endothermic. Endothermic or exothermic?

Chemical Reactions and Energy Energy absorbed This is an example on an endothermic reaction. In an endothermic reaction, the energy required to break the bonds is more than the energy required to make the new bonds. Energy is drawn in from the surroundings to complete the reaction. N2 + O2 Bonds are broken NO

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/mol When the net energy value is positive, the reaction is endothermic. 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/mol When the net energy value is positive, the reaction is endothermic.

FORMULA TO DETERMINE THE NUMBER OF RINGS AND/OR DOUBLE AND TRIPLE BONDS 2 + 2C + N – H – X 2 ρ = where C = number of carbon atoms N = number of nitrogen atoms H = number of hydrogen atoms X = number of halogen (Group 17) atoms