Chapter 9 – Acids & Bases

Chapter 9 – Acids & Bases 9.1 Arrhenius Acids and Bases o Arrhenius Acid: substance that produces H+ ions in aqueous solutions. HCl (aq)  H+ (aq) + Cl- (aq) o Arrhenius Base: substance that produces OH- ions in aqueous solutions. NaOH (aq)  Na+ (aq) + OH- (aq) PROPERTIES OF ACIDS AND BASES Properties of Arrhenius Acids  Produce H+ ions in water  Taste sour  Act corrosive  Turn blue litmus turns red Properties of Arrhenius Bases  Produce OH- ions in water  Taste bitter  Feel slippery  Turn red litmus turn blue  Neutralization Reactions  Some acid base reactions look like this: ACID + BASE  SALT + H2O HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l) Salt - ionic compound formed during acid-base neutralization reaction. Example: For the reaction: 2HNO3 (aq) + Ca(OH)2 (aq)  2H2O (l) + Ca(NO3)2 (aq) ________ is the Arrhenius acid, and _________ is the Arrhenius base. (Answer: HNO3 is the acid, Ca(OH)2 is the base) 9.2 Bronsted-Lowry Acids and Bases Bronsted-Lowry acid-base reactions involve a transfer of a proton (H+) from an acid to a base. o Bronsted - Lowry Acid: proton donor (loses H+) o Bronsted - Lowry Base: proton acceptor (gains H+)  This definition allows for a broader range of bases to be included. Examples: 1) HCl (aq) + H2O (l)  H3O+ (aq) + Cl- (aq) 2) NH3 (aq) + HBr (aq)  NH4+ (aq) + Br- (aq) (Answers: 1. HCl is BLA, H2O is BLB; 2. NH3 is BLB, HBr is BLA) page 1 9.3 Strong Acids and Bases  Strong acids ionize almost completely (~100%) in water: HCl, HNO3, H2SO4 HCl (aq)  H+ (aq) + Cl- (aq) Almost all the HCl molecules break apart to form H+ ions and Cl- ions  100 % of acid molecules have ionized HCl = hydrochloric acid, HNO3 = nitric acid, H2SO4 = sulfuric acid  Strong bases dissociate almost completely (~100%) to form ions: KOH, NaOH NaOH (aq)  Na+(aq) + OH-(aq) Almost all the NaOH units break apart and dissociate completely 100% to form ions KOH = potassium hydroxide, NaOH = sodium hydroxide Examples: Cl- H+ OH- H+ OH- H+ Na+ Cl- Na+ OH- Cl- HCl (aq) Na+ NaOH (aq) 9.4 Weak Acids and Bases  Weak acids ionize very little (~1-5 %) in water: HF, H2CO3, H3PO4, HC2H3O2 HC2H3O2 (aq)  H+ (aq) + C2H3O2- (aq) Most of the HC2H3O2 molecules do not break apart to form ions  only ~ 1 % of the acid molecules have ionized HF = hydrofluoric acid, H2CO3 = carbonic acid, H3PO4 = phosphoric acid, HC2H3O2 = acetic acid  Weak bases dissociate very little (~1-5%) so just a few ions are formed: Mg(OH)2 Name is magnesium hydroxide Mg(OH)2 (s)  Mg2+ (aq) + 2 OH- (aq) Examples: HF HF HF HF HF H+ F- HF (aq) 9.5 pH Scale pH scale: expresses H+ concentrations on a scale that ranges from 0 -14. A pH value indicates how acidic or basic a solution is: page 2 Acidic: pH < 7 Neutral: pH= 7 Basic: pH > 7  The following formula gives the relationship between [H+] and pH: pH = - log[H+] or [H+] =10-pH [ ] means concentration or molarity in units of moles / liter  [H+] = the moles / Liter of H+ ions  [OH-] = the moles / Liter of OH- ions o In pure water, [H+] =[OH-] o In acidic solution [H+] >[OH-] o In basic solution [H+] <[OH-] Example: If [H+] = 10-5, then pH = 5. Example: If [H+] = 0.0001, then pH = ?  To find the pH make sure to convert the concentration to scientific notation: Express 0.0001 in scientific notation: [H+] = 10-4 so pH = 4 9.6 Buffers Buffer: A solution that resists changes in pH when a small amount of an acid or base is added.  Buffer systems are very important for maintaining the pH of biological fluids.  blood pH needs to stay around a pH value of 7.4 or death can result. 9.7 Solubility Rules Solubility: An amount of solute that can dissolve in a given amount of solvent.  Solubility rules are for an ionic solid placed in water at 25ºC.  Soluble: The ionic compound dissolves completely. o The ionic compound is given the designation aqueous (aq) as its physical state.  Insoluble: The ionic compound does not dissolve much - only a very small percent dissolves. o The ionic compound is given the designation solid (s) as its physical state. Solubility Rules Generally soluble compounds with: 1. Li+, Na+, K+, NH4+ (ALWAYS!) 2. acetate ion (C2H3O2-) 3. nitrate ion (NO3-) 4. halide ions (X): Cl-, Br-, and IBUT AgX, HgX2, PbX2 are all insoluble 5. sulfate ion (SO42-), BUT CaSO4, SrSO4, BaSO4, Ag2SO4, PbSO4 are all insoluble Generally insoluble compounds with: 6. carbonate ion (CO32-) 7. chromate ion (CrO42-) 8. phosphate ion (PO43-) 9. sulfide ion (S2-) BUT CaS, SrS, BaS are all soluble 10. hydroxide (OH-), BUT Ca(OH)2, Sr(OH)2, Ba(OH)2, are all soluble page 3 Example: Determine if the following are soluble or insoluble in water and write the correct physical state next to each: 1. Na2S _____ soluble insoluble 2. Al(OH)3 _____ soluble insoluble 3. AgBr _____ soluble insoluble 4. CaCO3 _____ soluble insoluble 5. KNO3 _____ soluble insoluble 9.8 Electrolytes Electrolyte: A substance that once dissolved in water, conducts electricity by the net movement of ions. Negative Electrode (-) (+) Positive Electrode + Negative Electrode (-) (+) Positive Electrode - + Electrolyte: Net movement of ions conducts electricity. Nonelectrolyte: No ions present. Electricity is not conducted. o Strong electrolyte - a solution that is a good conductor of electricity.  Substance that totally dissociates or ionizes in water. (All ions)  Soluble ionic compounds (e.g. NaCl, KBr, LiNO3, NaC2H3O2)  Strong acids (e.g. HCl, HBr, HNO3, HI)  Strong bases (e.g. NaOH, KOH, LiOH, Ca(OH)2) o Weak Electrolyte - solution that conducts electricity poorly.  Substance partially ionizes in water (few ions)  Insoluble ionic compounds (e.g. AgCl, Ca3(PO4)2)  Weak acids (e.g. HF, H2CO3)  Weak bases (e.g. NH3, Mg(OH)2) o Nonelectrolyte - solution that does not conduct electricity.  Neutral molecules are present in solution. (No ions!)  Molecular compounds such as H2O, C6H12O6 (sugar), I2 page 4 Examples: Weak Electrolyte Nonelectrolyte Strong Electrolyte BE ABLE to DRAW BEAKERS of WATER with ELECTROLYTES in THEM For Fun • Blood pH needs to be between 7.35 and 7.45 • Maintained by CO2 / HCO3- buffer system –Breathing can affect change in this system in seconds • Acidosis is excess acid. Results in heavy breathing, weakness, headache, coma, and pH < 6.8=death. • Alkalosis is excess base. Results in convulsions, muscular weakness, and pH>7.8 = death • Partial pressure of CO2 normal is 35-45 mmHg • High PCO2 means acidosis (lots of CO2 in blood) • Low PCO2 means alkalosis (little CO2 in blood) • Buffer rxn: CO2 + H2O  H2CO3  H+ + HCO3 CHAPTER 9 PRACTICE PROBLEMS 1. Are the following acidic, basic or neutral? pH = 3, pH = 7, pH = 11, pH = 1 2. Indicate the Arrhenius acid and base in each of the following reactions: a. 2 HNO3(aq) + Ca(OH)2(aq)  2 H2O(l) + Ca(NO3)2(aq) b. HI(aq) + KOH(aq)  H2O(l) + KI(aq) page 5 3. Indicate the Brønsted-Lowry acid and base in each of the following reactions: A) H2O (l) + NH3 (aq)  NH4+ (aq) + OH– (aq)  B) HSO3– (aq) + HNO3 (aq) C) H2PO4– (aq) + H2SO4 (aq)  4. A) [H+]=0.00001 M H2SO3 (aq) + NO3– (aq) H3PO4 (aq) + HSO4– (aq) means pH = ___________ B) [H+]=0.000001 M means pH = ___________ C) [H+]= 10-10 M means pH = ___________ 5. Classify these as non-, weak, or strong electrolytes and draw them in a beaker of water. LiBr, PbI2, C12H22O11, KCl Answers to Practice Problems 1. 3 is acidic, 7 is neutral, 11 is basic, 1 is acidic 2. a. acid = HNO3 and base = Ca(OH)2 3. A) acid = H2O, base = NH3, b. acid = HI and base = KOH B) acid = HNO3, base = HSO3– C) acid = H2SO4, base = H2PO4– 4. a) pH = 5, B) pH = 6, C) pH = 10 5. LiBr is strong, PbI2 is weak, C12H22O11 is non, KCl is strong page 6