Chapter 2 Acids and Bases Acid Base Rxn proton is transferred from an acid to a base Conjugate Base species formed when a proton is removed from an acid B Conjugate Acid species formed when a proton is added to a base HA Bronsted Lowry Acid proton donor HB Bronsted Lowry Base proton acceptor A HB aq A aq B aq HA aq Arrhenius Acid material that donates a proton H Arrhenius Base material that donates a hydroxide ion OH o Base alkali Lewis Acid accepts an electron pair from a LB Lewis Base donates an electron pair to a LA Acidic soln pH 7 Basic soln pH 7 Strength of an acid or base depends on the extent of ionization in HOH Strong Acids HCl HBr H2SO4 HNO3 HI HClO4 Strong Bases NaOH KOH LiOH CsOH Mg OH 2 Ca OH 2 Ba OH 2 o o o Weak acids and bases ionize to a lesser extent in HOH Weak Acids solutes that react reversibly w HOH to form H3O ions Weak Bases solutes that react w HOH to acquire a H ion and leave OH behind o Many org acids and bases are insoluble in HOH so a solvent other than HOH is required to put them into soln Solvents are typically another org compound Ex diethyl ether hexane dichloromethane A very weak acid can react w a particularly strong base and a very weak base can react w a particularly strong acid Mineral Acid any inorganic acid MA o Many of the strong MA s are insoluble in org solvents So mixes of HOH and org solvent are used The strength of an acid depends on the bases w which it reacts o Critical for est ing whether the equilibrium lies to the L or the R An acid reacts w a base regardless of solvent but the extent of ionization changes w the solvent used and w the base Ka Conjugate Acid Conjugate Base Acid Base o In gen chem the HOH base is removed from the equation to simplify equations however when HOH is removed from the K expression this also removes the base from the acid base equilibrium Therefore if there s a solvent other than HOH used then the base must be put back into the expression Acids react w bases in order to ionize What happens when HOH accepts a H to form H3O o A new sigma covalent bond is formed b w the H atom and the O atoms o One of the unshared electron pairs on O reacts w H to form the new bond to the proton o Blue curved arrow indicates O donates 2 electrons to H to form new O H bond The curved arrow formalism is used in chem rxns to indicate transfer electron density in order to form a new chem bond o A proton H has no electron density associated w it so it accepts the electron density from the electron rich O atom O base H acid o Resulting H3O has 3 bonds to O making it electron deficient so it takes on a o charge Further in this rxn one electron pair on O is donated to the electron deficient H to form a new bond O accepts H by donating 2 electrons to form a new bond Chapter 2 Acids and Bases Bases may be defined as 2 electron donors to an electron deficient center If the base donates electrons to a H it s a Bronsted Lowry base If the base donates electrons to an atom other than H it s formally a Lewis base o o o To donate electrons a base must have an excess of electron density Molecules containing O or N react as bases b c they have excess electron density The more easily an atom is able to donate electrons the more basic that atom should be o If all bases donate 2 electrons the electron flow is from the base to the acid rather than from the acid to the base An acid doesn t donate the proton but rather the H is attacked by the base to form a new bond to the proton Electron flow is always from a source of high electron density to a point of low electron density o A base is electron rich and will donate 2 electrons to an electron deficient atom such as H to form the CA There are no free protons they come from an acid such as HCl or HCOOH o A base doesn t react w a free proton but rather w an electron deficient H atom that s attached to another atom When the base donates 2 electrons to the H atom it literally pulls the base away The bond b w atoms will break as the 2 electrons from the base are used to form a new bond such as O H and the 2 electrons in the former bond will migrate toward the other atom like Cl from HCl to form the CB Sometimes the acid base rxn doesn t work very well b c the Ka favors the acid and base left rather than the CA and CB right o If the Ka for a given A B rxn is unfav changing the base may make Ka more fav Changing the base to one that s stronger for a given acid should shift Ka toward the CA and CB One base may be stronger than another and one must be able to predict relative basicity If one acid has a pKa 1 and the other pKa 8 the first is the stronger acid while the second in the stronger base o B c pKa corresponds to pH Why is HI more acidic than HF o HI is a stronger acid indicates Ka is larger If Ka is larger than pKa is smaller o The bond strength of HI is smaller The covalent radius of I is larger than F o The more stable ion will be less reactive So the bond distance b w H and I will be longer than the bond distance b w H and F A longer bond is a weaker bond If the H I bond is weaker then it should break more easily when attacked by the base o This pushes the rxn toward the right to favor the CA and CB A weaker bond is consistent w a more acidic compound Means than Ka is larger for the rxn w HI In the equilibrium rxn the CA and CB will react to give the acid and the base Reactivity refers to the reverse rxn of the A B equilibrium I is more less reactive b c it s larger and less able to donate electrons o I is weaker base I is more stable and therefore less reactive B c it s less reactive there s a higher concentration of I relative to F which means that the equilibrium in the HI rxn is pushed more to the right and the equilibrium for HF is pushed further to the left Chapter 2 Acids and Bases I is the CB so …