Transcribed By Angela Vivien Dela Cruz Lorraine Claire Lanuza Topics Hard Soft Acid Base Theory HARD SOFT ACID BASE THEORY Arrhenius acids forms hydronium ions in water bases forms hydroxide ions in water only applied in aqueous solutions water as a solvent Bronsted and Lowry acids as proton donors bases as proton acceptors levelling effect limitation of the bronsted and lowry States that in aqueous solutions we cannot tell which is the strongest acid because they have the same level of acidity and will undergo complete dissociation example HCl HBr H2SO4 HClO4 in water to determine the strongest acid use different Acetic acid can be used as a solvent to type of solvent determine the strongest acid because when we put the inorganic acids in acetic acids it will not 100 completely dissociate Thus Ka can be computed inorganic compound and can determine the strongest acid for each Conjugate acid base pairs weak acid will produce strong conjugate base vice strong acid will produce weak conjugate base vice versa versa Lewis Acid and Bases Base is and electron pair donor acid is an electron pair acceptor The two combine to form an adduct the product of used for hard and soft acid base concept Said to be the most complete covers the definitions Also includes the reactions of metal ions or of Arrhenius and Bronsted Lowry concepts Lewis acid and base atoms with ligands to form coordination compounds DELA CRUZ LANUZA BATCH 2024 Course Professor s Mr Luisito Salting aq For example Ag aq 2NH3 Ag NH3 2 Ag will be Mixed Orbital Ag has an empty orbital positive electrophilic central atom while the NH3 will be the base ligands Lewis Acid Central Metal Lewis Base Ligands the acid metal Lone fair from ammonia will fill the orbital Electron deficient Compounds Trivalent boron is considered as a Lewis acid HOMO and LOMO Highest Occupied Molecular Orbital HOMO Occupied electron pair donor Base Lowest Unoccupied Molecular Orbital LOMO Unoccupied electron pair acceptor Acid The electron pair of HOMO will be donated to the unoccupied or empty level in LUMO Resulting Molecular Orbital Net lowering of energy the adduct is stable LUMO and HOMO are called frontier orbitals If there is a net lowering of energy the adduct is stable large energy separation of HOMO and LUMO will lead to no reaction HOMO can be promoted using radiation for the electron to go up will be an easier reaction for HOMO and LUMO because of the smaller energy separation Page 1 of 5 Transcribed By Angela Vivien Dela Cruz Lorraine Claire Lanuza Topics Hard Soft Acid Base Theory Lewis Acids and Bases cont There is the possibility of competing reaction pathways depending upon which reactants are present and the relative energies of possible products Because acids can have a complex structure to address the problem introduce structural As a result a compound such as water may effect steric effect putting a large group to make the acid unreactive to determine which LUMO will react to HOMO and a variety of LUMO the HOMO does not know where to donate its lone pair serve as an acid a base and an oxidizing agent or a reducing agent A Lewis base has an electron pair in its highest occupied molecular orbital HOMO of suitable symmetry to interact with the LUMO of the Lewis acid The closer the two orbitals are in energy the stronger the bond in the adduct and more stable the closer in energy of HOMO and LUMO will make the HOMO and LUMO is based in energy separation Lewis Acid BAse rxn much easier Structural Effect To make others unreactive Surround by bulky groups Introduce steric effects to properly manage HOMO interactions Hard and Soft Acids and Bases polarizable Hard Acids and Bases small compact non Soft Acids and Bases larger more diffuse distribution of electrons can easily disturb their electron cloud the polarizability of an acid or base plays a role in its reactivity polarizability increases as the size of the atom polarizability is the ability of an element to High Polarizability soft acid base low polarizability hard acid base increases F Cl Br I disturb an electron cloud Course Professor s Mr Luisito Salting In addition to their intrinsic strength Hard acids react preferentially with hard Hard Soft reaction is not preferred bases and soft acids react preferentially with soft bases Aqueous Solubility of Silver Halides Compound Solubility Product AgF fluorine hard base 205 AgCl chlorine hard base 1 8 x 10 10 AgBr bromine soft base 5 2 x 10 13 AgI iodine soft base 8 3 x 10 17 It should be known that Ag is a soft acid AgF and AgCl have the higher solubility product due to AgBr and AgI have the lower solubility product to to a a Soft Acid Hard Base interaction Soft Acid Soft Base interaction Soft Hard high solubility Hard Hard low solubility Soft Soft low solubility Solubility Of Lithium Halides an exemption to the rule LiBr LiCl LiI LiF Li is the smallest metal LiF has the lowest solubility due to their compact atomic radius it s hard acid hard base interaction is very strong Thiocyanate Bonding SCN displays linkage isomerism as ligand coordinates to metals via the sulfur or the nitrogen Mercury II ion bonds to the sulfur a soft soft interaction whereas zinc ion binds to nitrogen atom Linkage isomerism will happen when the ligand is the ambidentate DELA CRUZ LANUZA BATCH 2024 Page 2 of 5 Transcribed By Angela Vivien Dela Cruz Lorraine Claire Lanuza Topics Hard Soft Acid Base Theory ambidentate has 2 donor atoms but cannot be used both at the same time to form a bond with central atom K for Ligand Exchange Reactions K favorability of rxn The comparison between MeHg H2O HCL MeHgCl H3O K 1 8 x 1012 large K favorable MeHg H2O HF MeHgF H3O K 4 5 x 10 2 low K unfavorable base product side must be observed The central metal and the exchanged ligand in the First equation Hg is a soft acid while Cl is a hard Second equation Hg is a soft acid while F is a hard The difference is the atomic size of Cl and F Cl is is bigger compared to F Hg will interact more with Cl and will create a more favorable product formation in terms of softness Cl is softer than F Thus Cl will base act as a soft base K formation of product products are favored Ksp related to the solubility of a substance Note R G Pearson using Kf values categorized acids and bases as either hard or soft Hard base order F Cl Br I Compact with the electrons held fairly tightly Not very polarizable Metal ions such as Li is a hard acid by the nucleus Soft base order I Br Cl F Course Professor s