Enthalpy
15.2 Born-Haber Cycle
Our calculations of enthalpies so far have involved covalent substances. Now we need to look at the enthalpy changes involved in the formation of giant ionic lattices.
Lattice enthalpy is defined as either the: 'the enthalpy change that occurs when one mole of a solid ionic crystal is broken into its ions in the gaseous state, at standard temperature and pressure. Because all the bonds in the ionic lattice are broken, it is an endothermic process, ∆H is positive. ' The IB uses this definition.
M+ (g)
MX (s)
+
X- (g)
or 'the enthalpy change that occurs when one mole of a solid ionic crystal is formed form its ions in the gaseous state, at standard temperature and pressure. Because all the bonds in the ionic lattice are made, it is an exothermic process, ∆H is negative. '
M+ (g)
+
X- (g)
MX (s)
[Remember that bond breaking is endothermic and bond making is exothermic]
Standard lattice enthalpy values for ionic solids are found in the IB Chemistry data booklet.
Sodium Chloride ionic lattice
Born-Haber Cycles (named after Fritz Haber and Max Born) are simply energy / enthalpy cycles that show how ionic compounds are formed from their constituent elements. They are useful because they allow lattice enthalpies to be calculated theoretically using empirical data (observations and data collected from experiments).
1|P a g e
Definitions involved in calculation of lattice enthalpy of NaCl
∆Hf° standard enthalpy of formation of sodium chloride. The enthalpy change when one mole of a
NaCl is formed from its elements under standard conditions. They are exothermic because new bonds are being made between the atoms of the elements sodium and chlorine.
Na(s) + ½ Cl2 (g)
NaCl(s)
∆Hf° (NaCl) = - 411 kJmol-1
∆H1° standard enthalpy of atomization of chlorine. The energy required to change one mole of gaseous chlorine molecules into one mole of gaseous chlorine atoms. Endothermic because the
Our calculations of enthalpies so far have involved covalent substances. Now we need to look at the enthalpy changes involved in the formation of giant ionic lattices.
Lattice enthalpy is defined as either the: 'the enthalpy change that occurs when one mole of a solid ionic crystal is broken into its ions in the gaseous state, at standard temperature and pressure. Because all the bonds in the ionic lattice are broken, it is an endothermic process, ∆H is positive. ' The IB uses this definition.
M+ (g)
MX (s)
+
X- (g)
or 'the enthalpy change that occurs when one mole of a solid ionic crystal is formed form its ions in the gaseous state, at standard temperature and pressure. Because all the bonds in the ionic lattice are made, it is an exothermic process, ∆H is negative. '
M+ (g)
+
X- (g)
MX (s)
[Remember that bond breaking is endothermic and bond making is exothermic]
Standard lattice enthalpy values for ionic solids are found in the IB Chemistry data booklet.
Sodium Chloride ionic lattice
Born-Haber Cycles (named after Fritz Haber and Max Born) are simply energy / enthalpy cycles that show how ionic compounds are formed from their constituent elements. They are useful because they allow lattice enthalpies to be calculated theoretically using empirical data (observations and data collected from experiments).
1|P a g e
Definitions involved in calculation of lattice enthalpy of NaCl
∆Hf° standard enthalpy of formation of sodium chloride. The enthalpy change when one mole of a
NaCl is formed from its elements under standard conditions. They are exothermic because new bonds are being made between the atoms of the elements sodium and chlorine.
Na(s) + ½ Cl2 (g)
NaCl(s)
∆Hf° (NaCl) = - 411 kJmol-1
∆H1° standard enthalpy of atomization of chlorine. The energy required to change one mole of gaseous chlorine molecules into one mole of gaseous chlorine atoms. Endothermic because the
Bibliography: Clark, Jim. Chem Guide. 2008. . Clugston, Michael and Rosalind Flemming. Advanced Chemistry. Oxford: Oxford University Press, 2000. Derry, Lanna, et al. Chemistry for use with the IB Diploma Programme Higher Level. Melbourne: Pearson Heinemann, 2009. Melbourne: IBID Press, 2007. Neuss, Geoffrey. IB Diploma Programme Chemistry Course Companion. Oxford: Oxford University Press, 2007. —. IB Study Guides, Chemistry for the IB Diploma. Oxford: Oxford University Press, 2007. —. "IB Chemistry Examination Papers ." Cardiff: International Baccalaureate Organisation, 19992008.