Sec on 3 7 Forces and poten al energy in Molecules Forma on of chemical bonds Lets start Last week we looked at IE and EA as prototypic chemical reac ons This week de ne the energe cs of general bond forma on Determining factors in ionic covalent polar covalent bonds Stability of chemical bonds Why is H2 more stable than 2 H atoms Coordinates of H2 like Li atom Nuclei xed at d RAB e1 is at r1A r1B e2 is at r2A r2B e1 to e1 is r12 1 AUrac ve and 2 repulsive terms Need some e ec ve poten al Ve Added complica on RAB is a variable Coordinates of H2 like Li atom Ve holds the molecule together For large RAB Ve 0 As RAB decreases Ve ve smaller RAB Ve ve due to proton proton repulsion 1 AUrac ve and 2 repulsive terms Need some e ec ve poten al Ve Added complica on RAB is a variable Ve vs RAB for a diatomic molecule Energy to dissociate molecule Measure of molecular stability Equilibrium bond length Virial Theorem Is poten al energy a suitable criterion for chemical bond stability Virial theorem states average kine c energy KE and average poten al energy PE of a group of par cles electrosta cally interac ng only are related KEav 1 2 PEav KEav 1 2 PEav Consequence kine c energy only increases by half the poten al energy in ionic bonding So decrease in PE is driving force in bond forma on Total average energy Eav KEav PEav The change in Eav KEav PEav Eav 1 2 PEav Consequence change in total energy has same sign as change in PE meaning net decrease in total energy in bond forma on is always accompanied by a net crease in PE Note covalent bonds and polar covalent bonds have comparable importance They both decreases in PE and KE contribute Nota on of anions ve and ca ons ve COMBINE IE AND EA REACTIONS LEWIS DOT DIAGRAMS OF ION PAIRS REACTION ENERGIES Smallest IE is Cesium is 376 kJ mol 1 Largest EA is Cl is 349 kJ mol 1 remember to take the ve of the EA Consequently the crea ng the ion pair is always endothermic endo in exo out Addi onally we need to include the Coulomb stabiliza on Energy A and B governed by rela ve contribu ons t to experimental data is determines the distance at which repulsion is important from data Ion Ion repulsion at short r coloumbic aTrac on at larger distance Essen al to know Poten al energy diagram Reasons for discrepancies in calculated vs experimental data Ionic bonds have some covalent character Point charge assump on ignores polariza on of F by Na Conclusions Ionic bonding in KF shows dissocia on energy Ed to dissociate molecule to neutral atoms star ng from equilibrium bond length Re of 549 kJ mol 1 is predominantly ionic Columbic reduc on in PE P E of K and F atomic separa on set to zero at R12 large Cross over point between K and F atoms with poten al energy and Forma on of ionic bond at closer internuclear distances Mechanism for forma on of ionic bond from gas phase atoms Harpoon E ect John Polanyi Metal atoms send electron by harpooning to halogen electron tunneling at longer distance than normally expected e Harpoon E ect Covalent and polar covalent bonding How to atoms of similar EN bind CH4 C 2 55 and H 2 20 Forms a molecular gas not an ionic solid H2 one electron molecular ion Remember F propor onal to 1 r2 Internuclear repulsion Electron helps bond nuclei Electron tends to pull nuclei apart H2 molecular ion one electron molecule HA HB Internuclear distance RAB Electron nuclear aUrac ons at distances rAe rBe nuclear nuclear opposes bond forma on Forces between 3 par cles are FAB ZAe ZBe R2AB repulsive FAe e ZAe RAB 2 FAe e ZBe RAB2 Bonding and an bonding in a homonuclear diatomic molecule STABILITY IN SHARING Hyperbola of revolu on 1 06 Ed 255 5 kJ mol 1 Typical of covalent bond H2 Ed 432 kJ mol 1 Stable species Electron in internuclear region pulls nuclei together Electron in outer regions pulls nuclei apart Creates bonding and an bonding regions The classical descriptors of chemical bonds Bond length energy order and polarity Bond length energy order polarity Bond Length and energy show trends Bond lengths usually increase with At No Z Bond Lengths of typical groups are similar within a few percent measured by rota onal spectroscopy Bond Energies Molecular stability is determined by energy to dissociate it into cons tuent atoms bond energy bond dissocia on energy Ed i e H2 g 2H g E Ed Measured using thermochemical techniques Ed measured in kJ mol 1 Bond energies Ed measured in kJ mol 1 generally get weaker with increasing Z HF HCl HBr HI Bond energies Ed measured in kJ mol 1 F2 is anomalously weak Bond energies decreases drama cally in diatomic N2 O2 F2 Bond strengths Bond energies to within 10 are similar from one compond to another Allows for tabula on of average bond energies