CHM1046 Exam 3 Study Guide Chapter 15 Acids and Bases 15 2 The Nature of Acids and Bases Acid Characterized by Sour taste Ability to dissolve many metals Ability to turn blue litmus paper red Ability to neutralize bases Dissolve into ions when dissolved in water common acids HCl H2SO4 HNO3 HC3H3O2 Base Characterized by Bitter tate Ability to turn red litmus paper blue Ability to neutralize acids common bases NaOH KOH NaHCO3 Na2CO3 NH3 15 3 Definitions of Acids and Bases Arrhenius Definitions acid a substance that produces H ions in aqueous solutions HCl aq H aq Cl aq base a substance that produces OH ions in aqueous solutions NaOH aq Na aq OH aq Bronstedt Lowry Definitions more likely to be asked about on the exam acid a proton donor base a proton acceptor Amphoteric substance can act as an acid or a base depending on how it acts in the reaction Ex water is an amphoteric substance Conjugate Pairs The product that is formed when a proton is donated or accepted from the reactants Acids have conjugate Bases Bases have Conjugate Acids the stronger the acid the weaker the conjugate base and vice versa 15 4 Acid Strength and Acid Ionization Constant Ka A strong acid completely ionizes in solution HA aq H2O H3O aq A aq A weak acid partially ionizes in solution HA aq H2O H3O aq A aq note the difference in arrows Acids can be classified according to their number of protons Monoproctic acid HA Diproctic acid H2A Triproctic acid H3A Ka a measure of the strength of the acid The larger the Ka value the stronger the acid 15 5 Autoionization of Water and pH Autoionization When water acts as an acid and a base with itself H2O l H2O l H3O aq OH aq Kw Equilibrium Constant for H2O and the Disassociation constant for water the H3O OH will always 1 0 10 14 An acidic solution contains a base that creates additional H3O ions causing H3O to increase and OH to decrease A basic solution contains a base that creates additional OH ions causing OH to increase and H3O to decrease The pH Scale Ranges from 0 14 0 6 9999 is acidic 7 0 is neutral 7 1 14 is basic pH log H3O The pOH and other p scales pOH log OH pOH is the opposite of pH thus pH pOH 14 pKa log Ka pKb log Kb 15 6 Finding the H3O pH of Strong and Weak Acid Solutions Strong Acid complete ionization disassociation Weak Acid Only partially ionized must use ICE table to determine s to determine if you can disregard the x in the denominator of Ka equations step 1 multiply given Ka value by 100 step 2 compare this new number to the original concentration given to you step 3 if the concentration is greater than the Ka 100 you can disregard the x Note Pretty much every practice problem she s given us you can throw out the X so if you re not sure if you can or not odds are you can Percent Ionization of a Weak Acid ionization H3O aq HA initial 100 A Mixture of Two Weak Acids look for the strongest relative to the rest of the acids the acid with the larger Ka then use THAT Ka value to solve for the pH b c it is the one that will have the most effect 15 7 Base Solutions Strong Base completely disassociates in solution Weak Base partially disassociates in solution 15 8 The Acid and Base Proportion of Ions and Salts Anions as Weak Bases Depends on the strength of the acid it comes from Cl conjugate base of a weak acid HC2H3O2 weak acid an anion that is the conjugate base of a weak acid is itself a weak acid C2H3OH2 is a weak base and will cause an increase in pH of solution NO3 HNO3 strong acid NO3 is pH neutral because it s the conjugate base of a strong acid NO2 HNO3 weak acid Ka 4 6 10 4 NO2 is a weak base because it s the conjugate of a weak acid Ka Kb Kw Cations as Weak Acids Cations that are the counterions of strong bases are themselves pH neutral Cations that are conjugate acids of weak bases are weak acids Na NaOH strong base Na is the counter ion of a strong base so it s pH neutral H4 NH3 Kb 1 76 10 5 H4 is the conjugate acid of a weak base so it is a weak acid Classifying Salt Solutions as Acidic Basic or Neutral 1 The cation is pH neutral and the anion is pH neutral salt forms neutral solution 2 The cation is pH neutral and the anion is a weak base salt forms a basic solution 3 The cation is a weak acid and the anion is pH neutral salt forms an acidic solution 4 The cation is a weak acid and the anion is a weak base must compare the Ka and Kb values The stronger species determines the pH 15 9 Polyprotic Acids Ionize in successive steps each with its own Ka value Use the step with the strongest Ka value to determine the pH 15 10 Acid Strength and Molecular Structure Binary Acid H Y formed of hydrogen bonded to some other atom 2 factors that determine its strength bond polarity and bond strength 1 Bond Polarity Bond between H and atom should be polar for the hydrogen atom to be acidic 2 Bond Strength The weaker the H Y bond the stronger the acid is H F Bond energy 565 kJ mol weak acid H Cl 431 kJ mol strong acid as you go left right across the periodic table the acidity increases as you go down the columns of the periodic table the acidity increases Oxyacid Contains a hydrogen atom bonded to an oxygen atom which in turn is bonded to another atom Acidity of oxyacids depend on electronegativity of Y Y more electronegative stronger oxyacid The number of oxygen atoms bonded to Y more oxygen atoms stronger oxyacid 15 11 Lewis Acids and Bases Lewis Acid electron pair acceptor Lewis Base electron pair donor Chapter 16 Aqueous Ionic Equilibrium 16 2 Buffers Buffer solutions that resist pH change A buffer solution prepared from a weak acid and its conjugate base Acid Base pair is chosen with pKa value close to the required pH Calculate the pH of a buffer Henderson Hasselbach equation we had to use this for lab Example
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