The Hückel approximation is used to determine the energies and shapes of the π π molecular orbitals in conjugated systems. Within the Hückel approximation, the covalent bonding in these hydrocarbones can be separated into two independent "frameworks": the σ σ -bonding framework and the the σ σ -bonding framework. The wavefunctions used to describe the bonding orbitals in each framework results from different combinations of atomic orbitals. The method limits itself to addressing conjugated hydrocarbons and specifically only π π electron molecular orbitals are included because these determine the general properties of these molecules; the sigma electrons are ignored. This is referred to as sigma-pi separability and is justified by the orthogonality of σ σ and π π orbitals in planar molecules. For this reason, the Hückel method is limited to planar systems. Hückel approximation assumes that the electrons in the π π bonds “feel” an electrostatic potential due to the entire σ σ -bonding framework in the molecule (i.e. it focuses only on the formation of π π bonds, given that the σ σ bonding framework has already been formed).
Calcium oxide (CaO) or lime in solid form can be prepared from the decomposition of calcium hydroxide {Ca in to lime (CaO) and water ( at high temperature. The reaction is an endothermic reaction. That is heat is absorbed in this reaction process. One mole of calcium hydroxide decomposed into one mole of calcium oxide and one mole of water. The balanced reaction can be shown as-CaC (solid) → CaO (solid) + (liquid). The heat of the reaction is (+) 63.7 kJ/mole of CaO.
