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Thermodynamics

A2 Unit 5. be able to construct Born–Haber cycles to calculate lattice enthalpies from experimental data be able to compare lattice enthalpies from Born–Haber cycles with those from calculations based on a perfect ionic model to provide evidence for covalent character in ionic compounds.

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Thermodynamics

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  1. A2 Unit 5 • be able to construct Born–Haber cycles to calculate lattice enthalpies from experimental data • be able to compare lattice enthalpies from Born–Haber cycles with those from calculations based on a perfect ionic model to provide evidence for covalent character in ionic compounds Thermodynamics

  2. Enthalpy change - definitions Definitions of enthalpy changes - must be learned! Enthalpy of formation (DHf): Enthalpy change when one mole of a substance is formed from its constituent elements with all substances in their standard states NB Enthalpy of formation of an element is zero EXO/ENDO Enthalpy of combustion (DHc): Enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in their standard states. EXO Ionisation enthalpy (DHi) The first ionisation energy is the enthalpy change when one mole of gaseous atoms loses one electron per atom to produce gaseous 1+ ions. The second ionisation energy is when one mole of gaseous 2+ ions is produced from one mole of 1+ ions. ENDO Electron affinity (DHea) The first electron affinity is the enthalpy change when one mole of gaseous atoms gains one electron per atom to produce gaseous 1- ions. The second electron affinity is the enthalpy change when one mole of gaseous 1- ions gains one electron per ion to produce gaseous 2- ions. EXO 1ST ENDO 2ND, etc

  3. Enthalpy change - definitions Definitions of enthalpy changes - must be learned! Enthalpy of atomisation (DHa) Enthalpy change when one mole of gaseous atoms is produced from an element in its normal state. ENDO Hydration enthalpy (DHhyd) Enthalpy change when one mole of gaseous ions become hydrated (dissolved in water). . EXO Enthalpy of solution (DHsol) Enthalpy change when one mole of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with each other. EXO/ENDO Bond dissociation enthalpy (DHhyd) Enthalpy change when one mole of covalent bonds is broken in the gaseous state. ENDO

  4. Enthalpy change - definitions Definitions of enthalpy changes - must be learned! Lattice enthalpy of disociation (DHL) Enthalpy change when one mole of a solid ionic compound is broken up into its constituent ions in the gas phase ENDO Lattice enthalpy of formation (DHL) Enthalpy change when one mole of a solid ionic compound is formed from into its constituent ions in the gas phase EXO

  5. Formation of sodium chloride Video

  6. Formation of sodium chloride Before Before 11p = 11+ 17p = 17+ Sodium atom Chlorine atom 11e = 11- 17e = 17- ɨ 0 0 ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ Cl Na ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ Na:2,8,1 Cl:2,8,7

  7. Formation of sodium chloride Before Before 11p = 11+ 17p = 17+ Sodium atom Chlorine atom 11e = 11- 17e = 17- ɨ - 0 0 + ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ Na Na ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ Cl:2,8,7 Na:2,8,1 After After 11p = 11+ 17p = 17+ Sodium ion Chloride ion 10e = 10- 18e = 18- 1+ 1-

  8. Formation of sodium chloride strong attractive force between ions of opposite charge: ɨ - + ɨ ɨ ɨ ɨ ɨ ɨ ɨ Na Na ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ ɨ Sodium ion Chloride ion An IONIC BOND

  9. Ionic bonding The + ions and - ions charges attract each other. Each + ion attracts all the - ions around it and each - ion attracts all the + charges around it. Enormous 3-d structures with + ions and - ions forming bonds with each other are formed; so many attractions between ions makes these very, very strong! The ions are regularly arranged forming a lattice with ions of opposite charge next to each other.

  10. Ionic bonding If 1g of sodium reacts with chlorine there will be: 26,086,956,521,739,100,000,000 sodium ions and chloride ions formed! That’s 200 times greater than all the seconds that have ticked by since the Earth came into existence

  11. Enthalpy changes on forming ionic compounds Sodium heated to atomise the solid sodium Enthalpy of atomisation (DHa) Na(s)  Na(g) ENDOTHERMIC Enthalpy change when one mole of gaseous atoms is produced from an element in its normal state. Chorine gas (chlorine molecules) dissociateinto chlorine atoms NB enthalpy change is for the production of two atoms of chlorine Bond dissociation enthalpy (DHhyd) Cl2(g)  2Cl(g) Enthalpy change when one mole of covalent bonds is broken in the gaseous state. ENDOTHERMIC NB enthalpy change is for the production of one atom of chlorine ½ Cl2(g)  Cl(g) Enthalpy of atomisation (DHa) ENDOTHERMIC Enthalpy change when one mole of gaseous atoms is produced from an element in its normal state.

  12. Enthalpy changes on forming ionic compounds Electron transfer takes place between gaseous sodium atoms and gaseous chorine atoms. This involves... Gaseous sodium atoms becoming sodium ions (IONISATION) Gaseous chlorine atoms becoming chloride ions Ionisation enthalpy (DHi) The first ionisation energy is the enthalpy change when one mole of gaseous atoms loses one electron per atom to produce gaseous 1+ ions. ENDOTHERMIC Na(g)  Na+(g) + 1e- Electron affinity (DHea) The first electron affinity is the enthalpy change when one mole of gaseous atoms gains one electron per atom to produce gaseous 1- ions. Cl(g) + 1e-  Cl-(g) EXOTHERMIC

  13. Enthalpy changes on forming ionic compounds Gaseous sodium ions and gaseous chloride ions are attracted and form a LATTICE Lattice enthalpy of formation (DHL) Enthalpy change when one mole of a solid ionic compound is formed from into its constituent ions in the gas phase Na+(g) + Cl-(g)  NaCl(s)

  14. Born-Haber Cycles Na+ (g) + e- + Cl (g) enthalpy H H first ionisation energy H first electron affinity Na (g) + Cl (g) Na+ (g) + Cl-(g) H atomisation/dissociation Na (g) + ½Cl2 (g) H lattice enthalpy H atomisation Na (s) + ½Cl2 (g) H formation NaCl (s) eg for sodium chloride:

  15. Na+ (g) + e- + Cl (g) enthalpy H Hfirst ionisation energy Hfirst electron affinity Na (g) + Cl (g) Na+ (g) + Cl-(g) Hatomisation Na (g) + ½Cl2 (g) Hlattice formation Hatomisation Na (s) + ½Cl2 (g) Hformation NaCl (s) Born-Haber Cycles : applying Hess’s Law There are two routes from elements to ionic compound Hformation HatmNa + HatmCl + H1st IE + H1st EA + Hlattice =

  16. Born-Haber Cycles: applying Hess’s Law Hformation HatmNa + HatmCl + H1st IE + H1st EA + Hlattice = Rearrange to find the lattice energy: Hlattice = Hformation - (HatmNa + HatmCl + H1st IE + H1st EA) So Born-Haber cycles can be used to calculate a measure of ionic bond strength based on experimental data.

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