The superscript ‘’o’’ indicates a ‘’standard’’ given temperature and 1 atomosphere pressure. The symbol to indicate the standard enthalpy of formation is: This is another value that is available in a table of known values. The final term you will need to know to calculate lattice energy using the Born-Haber Cycle is the standard enthalpy of formation. The sign reflects the direction of the reaction depending on whether energy (heat) was added or removed. If it is defined as the energy released when the gaseous ions combine to form the solid lattice, then the lattice energy will be a negative number. In your text book if lattice energy is defined as the amount of energy required to heat a solid to the gaseous ions, the lattice energy will be a positive number. Δ H = + 787 kJ/mol Positive means heat was used (endothermic) to move to a higher energy state To indicate that heat is added, the lattice energy would be expressed as a positive 787 kJ/mol: To convert the NaCl solid back to the gaseous ion state, enough heat would have to be added to break the lattice bond. Δ H = – 787 kJ/mol Negative means heat was lost (exothermic) and the reaction resulted in a lower energy state Symbolically the lattice energy is expressed as the change in enthalpy associated with forming the solid lattice: The number is negative and energy is lost as heat. In the case of reacting gaseous sodium cation and gaseous chloride anion to form a lattice of sodium chloride solid, the reaction is expressed symbolically as:īy looking at a table of known values we determine the change in enthalpy or lattice energy to be negative 787 kJ/mol. Lattice Energy is the enthalpy change (Δ H) that accompanies the formation of 1 mole of an ionic solid from its separated gaseous ions. The next term you will need to understand to calculate lattice energy using the Born-Haber Cycle is Lattice Energy itself.ĥ. We discussed the energy required to break the Cl-Cl bond in the discussion of dissociation energy. Remember that Cl normally exists as a diatomic molecule, Cl 2 gas. A free radical is an atom with at least one lone electron in its outer shell. That means that the chloride anion, Cl- (g) is more stable than the chlorine free radical, Cl (g). Since heat is released, the reaction is exothermic and the Δ H is a negative number. The electron affinity for chlorine can be found on a table of known electron affinities and is expressed as the delta H: This increased stability is confirmed by the change in enthalpy. Chlorine needs an electron to complete its octet. Part II begins with a continuation of the discussion on EA: Electron Affinity.Ĭhlorine is happier (more stable) as a charged gaseous ion (g) than a gaseous free radical (g) with an unshared electron. Connect to WiFi to avoid cellular charges for video streaming.īe sure to watch video’s Lattice Energy Part I and Part II before reviewing the transcription.
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