Lattice Dissociation Enthalpy

1 Lattice Enthalpy WARNING There can be two definitions - one is the opposite of the other!Make sure you know which one is being Dissociation EnthalpyDefinitionThe Enthalpy change when ONE MOLE of an ionic Lattice dissociatesnto isolated gaseous highly endothermic - there is a strong electrostatic attraction between ions of opposite charge a lot of energy must be put in to overcome the attractionExample Na+ Cl (g) - > Na+(g) + Cl (g) ENDOTHERMIC EXOTHERMICL attice Formation EnthalpyDefinitionThe Enthalpy change when ONE MOLE of an ionic crystal Lattice is formed from itsisolated gaseous highly exothermic - strong electrostatic attraction between ions of opposite charge a lot of energy is released as the bond is formed relative values are governed by the charge density of the Na+(g) + Cl (g) - > Na+ Cl (s)Notes one cannot measure this value directly.

2 It is found using a Born-Haber cycle the greater the charge densities of the ions, the more they attract each otherand the larger the Lattice Enthalpy . the more exothermic the Lattice Enthalpy , the higher the melting pointLattice Enthalpy 1F325 KNOCKHARDY PUBLISHING 2009M X +(s)(g)(g)+M + X LATTICEDISSOCIATIONENTHALPYM X +(s)(g)(g)+M + X LATTICEFORMATIONENTHALPYC onsequencesHIGH CHARGE DENSITY IONSLOWER CHARGE DENSITY IONS GREATER ATTRACTION LESS ATTRACTIONLARGE Lattice Enthalpy SMALLER Lattice ENTHALPYT hermal stability and Lattice EnthalpyOxides thermal stability of Group II oxides decreases down the groupMg2+ O2- Ca2+ O2- Sr2+ O2- Ba2+O2- Lattice Enthalpy (kJ mol-1)-3889 -3513 -3310-3152 Melting Point ( C)

3 2853 decreasing values > MgO magnesium oxide is used to line furnaces - REFRACTORY LINING this is because of its high melting point (2853 C) the high melting point is a result of the large (highly exothermic) Lattice Enthalpy high Lattice Enthalpy due to the attraction between ions of high charge densityCarbonates thermal stability of Group II carbonates increases down the group MgCO3 decomposes much easier than BaCO3 BUT the Lattice Enthalpy of MgCO3 is HIGHER!MgCO3 CaCO3 SrCO3 BaCO3 Decomposes at350 C 832 C 1340 C 1450 C Lattice Enthalpy (kJ mol-1)

4 -3123 > -2556 Mg2+ ions are SMALLER and have a HIGHER CHARGE DENSITY this makes them MORE HIGHLY POLARISING they DISTORT THE CO32- ion this WEAKENS THE ATTRACTION BETWEEN IONS the Lattice IS NOT AS STRONG2 Lattice EnthalpyF325 KNOCKHARDY PUBLISHING 2009Mg2+O2-Na+Cl substance in the the following pairs has the larger Lattice Enthalpy ?a) NaCl or KClb) NaF or NaClc) MgCl2 or NaCld) MgO or MgCl2 Calculating Lattice EnthalpyIntroduction you cannot measure Lattice Enthalpy directly values are found using a Born-Haber cycle Born-Haber cycles use Hess s Law The following Enthalpy changes are part of a Born-Haber cycle.

5 Standard Enthalpy Change of Formation ( f )DefinitionThe Enthalpy change when ONE MOLE of a compound is formed in its standardstate from its elements in their standard , but not exclusively, exothermicExample(s)Mg(s) + Cl2(g) > MgCl2(s)Na(s) + Cl2(g) > NaCl(s)Notes Elements In their standard states have zero Enthalpy of Enthalpy Change of Atomisation ( at , 298 )DefinitionThe Enthalpy change when ONE MOLE of gaseous atoms is formed from anelement in its standard endothermic - you have to break the bonds holding the atoms togetherExample(s) Cl2(g) > Cl(g) and Na(s) > Na(g)(see note)

6 Note for solid elements, the change is known as Enthalpy OF SUBLIMATIONL attice Enthalpy 3F325 KNOCKHARDY PUBLISHING equations representing the standard Enthalpy changes of atomisation / sublimation equations representing the standard Enthalpy changes of formation ofmagnesium oxidesodium oxidepotassium bromideFirst Ionisation EnergyDefinitionThe energy required to remove one mole of electrons (to infinity) from one mole ofgaseous atoms to form one mole of gaseous positive endothermic need to overcome the pull of the nucleus on the electronExample(s)Na(g) - > Na+(g) + e andMg(g) - > Mg+(g) + e Notes There is an ionisation energy for each successive electron removed.

7 SECOND IONISATION ENERGYMg+(g) - > Mg2+(g) + e Look back in your notes to refresh your memory about the trends in sElectron AffinityDefinitionThe Enthalpy change when ONE MOLE of gaseous atoms acquires ONE MOLE ofelectrons (from infinity) to form ONE MOLE of gaseous negative exothermic - a favourable process due to the nucleus attracting theelectronExampleCl(g) + e - > Cl (g) Notes Do not confuse electron affinity with Lattice EnthalpyF325 KNOCKHARDY PUBLISHING equations representing the following electron affinity (EA) changes;1st EA of bromine1st EA of oxygen2nd EA of equations representing the following ionisation energy changes.

8 1st IE of calcium2nd IE of calcium1st IE of lithium1st IE of aluminiumBORN-HABER CYCLEST heory involve the application of Hess s Law used to outline the thermodynamic changes during the formation of ionic salts used to calculate Lattice Enthalpy Lattice Enthalpy cannot be determined directly by experimentBORN-HABER CYCLE FOR SODIUM CHLORIDEA ccording to Hess s Law, the Enthalpy change is independent of the path taken. 6 = - (STEP 5) - (STEP 4) - (STEP 3) - (STEP 2) + (STEP 1) - (-364) - (+500) - (+121) - (+108) + (-411) = - 776 kJ mol-1 Lattice Enthalpy 5F325 KNOCKHARDY PUBLISHING 2009123645Na(s) + Cl2(g)NaCl(s)Na(g)+ Cl2(g)Na(g) + Cl(g)Na+(g) + Cl(g)Na+(g) + Cl (g)STEPS (values are in kJ mol-1) Enthalpy change of formation of NaClNa(s) + Cl2(g) > NaCl(s) 411 Enthalpy change of sublimation of sodiumNa(s) > Na(g)

9 + 108 Enthalpy change of atomisation of chlorine Cl2(g) > Cl(g)+ 121 Ist Ionisation Energy of sodiumNa(g) > Na+(g) + e + 500 Electron Affinity of chlorineCl(g) + e > Cl (g) 364 Lattice Enthalpy of NaCl Na+(g) + Cl (g ) > NaCl(s) a similar Born-Haber cycle for NaCl2 .If the Lattice Enthalpy of NaCl2 is -3360 kJ mol -1, what is its Enthalpy of formation ?What does this tell you about the stability of NaCl2 ?BORN-HABER CYCLE FOR MAGNESIUM CHLORIDE6 Lattice EnthalpyF325 KNOCKHARDY PUBLISHING 20091236454aMg(s) + Cl2(g)MgCl2(s)Mg(g) + Cl2(g)Mg(g) + 2Cl(g)Mg+(g) + 2Cl(g)Mg2+(g) + 2Cl(g)Mg2+(g) + 2Cl (g)

10 STEPS1 Enthalpy of formation of MgCl22 Enthalpy of sublimation of Mg3 Enthalpy of atomisation of Cl x 24 Ist Ionisation Energy of Mg4a 2nd Ionisation Energy of Mg5 Electron Affinity of Cl x 26 Lattice Enthalpy of the Lattice Enthalpy of MgCl2 is -2493 kJ mol -1, what is its Enthalpy of formation ? a Born-Haber cycle for magnesium oxide, an ionic salt dissolve in water?IntroductionIf a pair of oppositely charged gaseous ions are placed together, they will attracteach other. The energy change ( Lattice Enthalpy ) is highly the ions were put in water, they would be attracted to polar water resulting energy change (HYDRATION Enthalpy ) is highly both; the greater charge density of the ions = a more exothermic reactionThe missing stage of the cycle is knownas the Enthalpy OF size and value of the Enthalpy ofsolution depends on the relative valuesof the Lattice Enthalpy and the hydrationenthalpy.