A. Lattice Enthalpies Combining equations for the first ionization energy and first electron affinity:

Transcription

1 [15.1B Energy Cycles Lattice Enthalpy] pg. 1 f 5 CURRICULUM Representative equatins (eg M+(g) M+(aq)) can be used fr enthalpy/energy f hydratin, inizatin, atmizatin, electrn affinity, lattice, cvalent bnd and slutin. Enthalpy f slutin, hydratin enthalpy and lattice enthalpy are related in an energy cycle. Cnstructin f Brn-Haber cycles fr grup 1 and 2 xides and chlrides. Calculatin f enthalpy changes frm Brn-Haber r disslutin energy cycles. Relate size and charge f ins t lattice and hydratin enthalpies. A. Lattice Enthalpies Cmbining equatins fr the first inizatin energy and first electrn affinity: ΔH ϴ = = +147 kj ml -1 Electrn transfer prcess verall Energetically Even thugh it leads t stable with cnfiguratins charged ins cme tgether t frm the inic because f the strng between ins Offsets electrn transfer prcess Ex. Na + (g) + Cl - (g) NaCl(s) ΔH ϴ =-790 kj ml -1 This step f the prcess explains the readiness f sdium and chlrine t frm an Lattice Enthalpy ( ) relates t the frmatin f ins frm ne mle f a crystal breaking int gaseus ins. Ex. NaCl(s) Na + (g) + Cl - (g) H ϴ lat = +790 kj ml 1 lattice energies cannt be determined directly B. Frmatin f Inic Cmpunds frmatin f an cmpund frm its takes place in a number f steps includes frmatin f the frm its cnstituent ins. Frm Hess s law, the enthalpy change fr the verall frmatin f the slid must be equal t the f the enthalpy changes accmpanying the individual steps. Enthalpy f (ΔHatm )- Energy change when ne mle f gaseus atms are frmed frm the element in its standard states

2 [15.1B Energy Cycles Lattice Enthalpy] pg. 2 f 5 C. Frmatin f Inic Cmpunds : Determining Values Ex. Frmatin f sdium chlride: Na(s) + ½Cl2(g) NaCl(s) H f (NaCl) = 411 kj ml 1 Step Descriptin H /kj ml 1 Na(s) Na(g) ½Cl2(g) Cl(g) Sdium ne mle gaseus ins Enthalpy change f atmizatin. ½ mle f Cl Cl bnds break 1 mle Cl atms E = enthalpy H atm(na) = +107 ½E(Cl Cl) = ½(+242) Na(g) Na + (g) + e One remved frm uter shell f gaseus sdium atm energy f sdium Cl(g) + e Cl (g) One added t uter shell f gaseus chlrine atm Electrn f chlrine Na + (g) + Cl (g) NaCl(s) Gaseus ins cme tgether t frm ne mle f slid sdium chlride f sdium chlride H i(na) = +496 H e (Cl) = 349 H lat =??? D. Frmatin f Inic Cmpunds: Bn Haber Cycles 1) Sdium Chlride 1 Energy cycle based n Hess s law Start by drawing a lng vertical line, and put a 0 alng it First equatin is the prducts in the frmatin reactin 1

3 [15.1B Energy Cycles Lattice Enthalpy] pg. 3 f 5 C. Bn Haber Cycles 2) Magnesium Oxide 2 Calculate the enthalpy change fr the reactin Mg (s) + ½ O 2 (g) MgO (s) What kind f enthalpy change is this? standard enthalpy f frmatin f MgO Hatm (O) = +249 kj/ml Hatm (Mg) = +148 kj/ml Hlattice energy (MgO) = kj/ml H1st inizatin energy(mg) = +738 kj/ml H2nd inizatin energy (Mg) = kj/ml H1st electrn affinity (O) = -141 kj/ml H2nd electrn affinity (O) = +798 kj/ml 2

4 [15.1B Energy Cycles Lattice Enthalpy] pg. 4 f 5 Bn Haber Cycles 3) Use A Brn-Haber Cycle T Calculate The Electrn Affinity Of Chlrine 3 Hatm (Cl) = +122 kj/ml H1st inizatin energy(mg) = +738 kj/ml Hfrmatin (MgCl2) = -641 kj/ml Hatm (Mg) = +148 kj/ml H2nd inizatin energy (Mg) = kj/ml Hlattice energy (MgCl2) = kj/ml 3

5 [15.1B Energy Cycles Lattice Enthalpy] pg. 5 f 5 D. The Inic Mdel Assumptin fr theretical lattice enthalpy: crystal is made up frm perfectly ins. Inic mdel assumes that the nly interactin is due t frces between the ins. An in the inic radius f ne f the ins the attractin between the ins. An in the inic charge the inic attractin between the ins. T calculate the lattice energy fr ne mle, mre in need t be cnsidered as a slid crystal frms Overall attractin between the psitive and negative ins ver the repulsin f ins with the same charge as ins are generally surrunded by neighburing ins f charge. This leads t the general expressin: K : cnstant, depends n gemetry f the lattice n,m: magnitude f n the ins. R: f the in frm X-ray diffractin measurements f the crystal, theretical values can be calculated nce the f the slid lattice is knwn. E. Patterns in Lattice Enthalpy Lattice enthalpy decreases with increasing in radius and increases with increasing in charge. F- Cl- Br- I- Li Na K Rb Cs Lattice enthalpies decrease as the size f the catin r anin increases cntains the ins with the smallest inic radii and has the highest lattice enthalpy cntains the largest ins and the smallest lattice enthalpy Lattice enthalpies are greater when inic cmpunds frm between smaller, mre highly charged ins, Ie. thse with greatest. H lat (kj/ml) H lat (kj/ml) Explanatin f Difference NaCl 1029 MgCl Mg 2+ has charge & inic radius CaF CaO 3401 O 2- has charge & inic radius H lat wuld be higher fr CaO if xygen was NaI 705 AgI 892 Ag+ has inic radius, lattice energy H lat desn t fllw trend AgI has strnger bnding due t character, based n