bond enthalpies section 5.4. introduction more good stuff for h 2 the thermochemical equation...
TRANSCRIPT
Introduction
Average bond enthalpy: enthalpy change per mole when 1 mole of the same type of covalent bond is broken in the gas phase from many similar molecules
Bond breaking is always endothermic Bond formation is always exothermic
More Good Stuff For H
2 the thermochemical equation
describing the bond enthalpy is: H
2(g) → 2H
(g) ∆Hθ = +436 kJ mol-1
The larger the bond enthalpy, the stronger the covalent bond
Bond enthalpy is inversely proportional to bond length
Average Bond Enthalpies
Many bond enthalpies are average bond enthalpies, so some error occurs with calculations (won't be the exact experimental value)
See Table 10 in the data booklet for some average bond enthalpies
Use of Bond Enthalpies
Are used to determine the enthalpy change for a reaction
The molecules need to be in the gaseous state
Consider the combustion of methane CH
4(g) + 2O
2(g) → CO
2(g) + 2H
2O
(g)
Methane Combustion
The reaction can be thought of in 2 steps
1. all the bonds of the reactants have to be broken (endothermic)
2. bond formation for the products (exothermic)
Use the data booklet for the average bond enthalpies
Calculations for Bond Breaking
Bond breaking: 4 C-H bonds in CH
4 = 4 x 412 = 1648 kJ
2 O=O bonds in 2O2 = 2 x 496 = 992 kJ
Total amount of energy to break all these bonds = 1648 + 992 = 2640 kJ
Calculations for Bond Formation Bond making: Making 2 C=O bonds in CO
2 = 2 x 743 =
1486 kJ Making 4 O-H bonds in 2H
2O = 4 x 463 =
1852 kJ Total amount of energy released to the
surroundings when these bonds are formed = 1486 + 1852 = 3338 kJ
Calculating the Enthalpy Change of the Reaction
∆H = Σ (energy required to break bonds)
- Σ (energy released when bonds are formed)
For the combustion of methane: ∆H = 2640 – 3338 = -698 kJ mol-1
Note that the overall reaction is exothermic
Example Problem Using the average bond enthalpies in the
data booklet, calculate the enthalpy change for this reaction:
H2(g)
+ ½O2(g)
→ H2O
(g)
Breaking bonds: 436 + 496/2 = 684 Forming bonds: 2 x 463 = 926 Breaking – forming = 684 – 926 Enthalpy change = -242 kJ mol-1