3.8A POTENTIAL ENERGY AND COVALENT BONDS
- Formation of covalent bonds:
H• + H• H− H 'H° = −435 kJ mol−^1
- The potential energy of the atoms decreases by 435 kJ mol−^1 as the covalent
bond forms.
Figure 3.6 The relative potential energies of hydrogen atoms and a hydrogen
molecule.
- Enthalpies (heat contents), H: (Enthalpy comes from en + Gk: thalpein to heat)
- Enthalpy change, 'H°: the difference in relative enthalpies of reactants and
products in a chemical change.
- Exothermic reactions have negative 'H°.
- Endothermic reactions have positive 'H°.
3.9 THE RELATIONSHIP BETWEEN THE EQUILIBRIUM CONSTANT
AND THE STANDARD FREE-ENERGY CHANGE, 'G°
3.9A Gibbs Free-energy
- Standard free-energy change ('G°):
'G° = −2.303 RT log Keq
1) The unit of energy in SI units is the joule, J, and 1 cal = 4.184 J.
2) A kcal is the amount of energy in the form of heat required to raise the