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Unit 17 Acids and Bases Chapter 14

Unit 17 Acids and Bases Chapter 14. Section 14.1: The Nature of Acids and Bases. Arrhenius Model Acids produce H + in aqueous solution while bases produce OH - Bronsted -Lowry Model Acid is a proton (H+) donor. Base is proton acceptor Show how water can act as an acid or a base

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Unit 17 Acids and Bases Chapter 14

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  1. Unit 17 Acids and Bases Chapter 14

  2. Section 14.1: The Nature of Acids and Bases Arrhenius Model Acids produce H+ in aqueous solution while bases produce OH- Bronsted-Lowry Model Acid is a proton (H+) donor. Base is proton acceptor Show how water can act as an acid or a base Hydronium ion- H3O+

  3. Conjugate acid/base pair- two substances related to each other by the donating and accepting of a single proton • Think think of the reaction between an acid and a base as competition for the H+between the two bases (the original base and the CB)

  4. Acid Dissociation in Water: HA +H2O ↔ H3O+ + A- or HA + H2O ↔ H+ + A- Acid dissociation constant (Ka): Ka = [H3O+][A-]/[HA] or Ka = [H+][A-]/[HA] *Remember that pure solids or pure liquids are not included in equilibrium expressions- this is the reason that water is not present in the equation *Although water is not included in the equilibrium expression, it does play a role in ionization of the acid Example: pg. 673 (#28 both CB/CA and Ka)

  5. Section 14.2: Acid Strength • Strong acid- almost all the original HA is dissociated at equilibrium (→) • strong acid yields a weak CB • The CB is much weaker than water for the affinity for H+ • HA + H2O→H3O+ + A- • The equilibrium lies far to the right for a strong acid • Ka is large because the equilibrium lies to the right (product favored) • All [HA]o = [H+]eq and [HA]eq=0 because we assume 100% dissociation

  6. Weak acid- acid for which very little dissociates • (↔ or ←) • weak acid yields a strong CB • The CB is much stronger than water for the affinity for H+ • HA + H2O←H3O+ + A- • The equilibrium lies far to the left for a weak acid • Ka is small because the equilibrium lies to the left (reactant favored) • All [HA]eq > [H+]eq because we assume little dissociation

  7. Diprotic acid- acid that has 2 acidic protons • H2SO4 + H2O → HSO4-1 + H3O+ (strong for 1st H+) • HSO4-1 + H2O ↔ SO42- + H3O+ (weak acid-HSO4+) • Monoprotic acids- acids with one proton • Oxyacids- acidic proton is attached to an oxygen atom- most acids (HNO3, H3PO4, etc) • Organic Acids- acids with a carbon atom backbone • Contain carboxyl group (COOH)- this is the acidic proton • usually these acids are weak- acetic acid • Amphoteric- substances that behave either as acids or as bases (H2O and NH3 are the most common)

  8. Autoionization- transfer of a proton from one water molecule to another to produce a hydroxide ion and a hydronium ion • H2O + H2O  H3O+ + OH- • NH3 + NH3 ↔ NH4+ + NH2- • Ion-product constant (dissociation constant or equilibrium constant for water) • Kw= [H+][OH-] = 1.0 x 10-14 @ 25°C • (WEAK acid b/c small Ka)

  9. In ANY aqueous solution at 25 degrees Celsius, no matter what it contains, the value of Kw is always 1.0 x 10-14 • There are three possibilities: • Neutral solution where [H+] = [OH-] • Acidic Solution where [H+] > [OH-] • Basic Solution where [H+] < [OH-] • 1.0 x 10-14 = [H+] [OH-] • Reminder: You can compare the strengths of acids based on their Ka values!! • Examples: pgs. 673-674 (#32, 36, 38) • HW: 14.1 (pg. 673 # 27, 29) and 14.2 (pg. 674 #33-37 odd)

  10. Section 14.3: The pH Scale Sig Figs for logarithms- the number of decimal places in the log is equal to the number of significant figures in the original number Ex. [H+]= 1.0 x 10-9 (2 sig figs) then the log[H+]=2.00 (2 decimals) Equations: pH = -log[H+] pOH = -log[OH-] the pH changes by 1 for every power of 10 change Also, the pH decreases as [H+] increases pK= -log K Kw = [H+] [OH-] [H+] = 10-pH [OH-] = 10-pOH pH + pOH = 14 (at 25°C) Example pg. 674 (#40) HW pg. 674 (#39-45 odd)

  11. Section 14.4: Calculating the pH of Strong Acid Solutions • 2 Things to Focus on for Acid-Base Calculations • -solution components and their chemistry. • -which components are significant and which can be ignored • Major species- solution components present in relatively large amounts • -For strong acids, it is IONS and WATER (because the acid has dissociated) • If the [Strong Acid] > 1.0 x 10-7, then [H+] from autoionization of water can be ignored because the [H+] produced is negligible. • - so 0.10 M HCl = [H+] = 0.10 M, therefore the pH = -log (0.10) = 1.00 • If the [Strong Acid] < 1.0 x 10-7, then [H+] from the strong acid can be ignored because the amount dissociated is negligible. • -so [H+] = 1.0 x 10-7 (from water) and pH = 7.00 • Example pg. 674 (#48, 50, 52) • HW pg. 674-675 (#47-51 odd)

  12. Section 14.5: Calculating the pH of Weak Acid Solutions • What are the steps for calculating the pH of a weak acid solution? • Find the major species in the solution. • Determine which of the major species supplies the hydrogen ions. • IF Ka> Kw, the H+ comes from the weak acid • List the initial concentrations. • Initials for the IONS will usually be 0 • Let x be the change in the concentration of the substance required to reach equilibrium. • “-x” for WEAK ACID and “+x” for the IONS • Assume “-x” is negligible for the weak acid • Solve • “x” will usually be the amount of H+ dissociated • Use the 5% rule to verify whether approximations are valid • x/[HA]o * 100% < 5% (if not, you can’t assume “-x” is negligible for the weak acid

  13. Percent dissociation- also percent ionization amount dissociated initial concentration x 100% What happens to percent dissociation when a weak acid is diluted? the percent dissociation increases When water is added, Q is less than Ka, so the system must adjust to the right to reach the new equilibrium position, so percent dissociation increases when the acid is diluted. Example: What is the pH of 1.00 M HF? What is the percent dissociation? HW: pg. 675 (#53, 55, 59-69 odd)

  14. Section 14.6: Bases Strong bases- complete dissociation of OH compounds (100% soluble) - Group 1, calcium, strontium, and barium -KOH → K+ + OH- -Ca(OH)2 → Ca2+ + OH- Many bases do not contain the hydroxide ion. How do these substances act as bases? they increase the concentration of hydroxide ion because of their reaction with water. NH3 +H2O ↔NH4+ + OH- What do many of these bases without hydroxide ions have in common? A lone pair of electrons is located on a nitrogen atom in many cases

  15. Kb– refers to the reaction of a base with water to form the conjugate acid and the hydroxide ion Weak bases- bases with small Kb values- where the base competes with the OH- for the H+ion - B + HOH ↔ BH+ +OH- -Calculations are similar to those for acids Equations: Kb = [BH+][OH-] / [B] Kb x Ka = Kw HW: pg 675-676 (#71-83 odd, 87-91 odd)

  16. Section 14.7 Polyprotic Acids • Polyprotic acids- acids that furnish more than one proton- dissociates in a stepwise manner- one proton at a time • Triprotic acids- acids with 3 protons- like phosphoric acid. • Describe the relative sizes of dissociation constants for polyprotic acids. (Ka1 > Ka2> Ka3) (see pg. 650-651) • This means that the first proton is easiest to remove and that it is more difficult to remove successive protons- this happens because the negative charge on the acid increases, making it more difficult to remove a positively charged proton • only the first dissociation step makes an important contribution to [H+] this simplifies pH calculations

  17. Why is sulfuric acid unique among strong acids? It is the only diprotic strong acid - its first dissociation step and a weak acid in its second step. General Rules: For concentrated solutions, 1M or higher, only use Ka1for calculations. For dilute concentrations- the second step does make a significant contribution and the quadratic equation needs to be used -Use all K values for each successive step HW: pg. 676 (# 93-97)

  18. Section 14.8: Acid-Base Properties of Salts Salt- an ionic compound What type of salts produce neutral solutions when dissolved in water? -salts that consist of the cations of strong bases and the anions of strong acids -KCl, NaNO3, NaCl, KNO3

  19. What kind of salt yields a basic solution? A salt that consist of a cation of a strong base and the anion of a weak acid A solution of sodium acetate contains sodium and acetate and water. NaC2H3O2 (aq) + H2O (l) →NaOH (aq) + HC2H3O2 (aq) Weak Acid Na+ + C2H3O2- + H2O (l) → Na+ + OH- + HC2H3O2 (aq) C2H3O2- + H2O (l) → OH- + HC2H3O2 (aq) BASIC

  20. What kind of salt yields a acidicsolution? A salt that consist of a cation of a weak base and the anion of a strong acid NH4Cl (aq) + H2O (l) → NH4OH (aq) + HCl (aq) NH4+ + Cl- + H2O (l) → NH3(g) + HOH(l) + H+ + Cl- NH4+ → NH3(g) + H+ Acidic Salts with highly charged metal ions also produce acidic solutions. (Al+3, W+6, etc) The high charge on the metal ion polarizes the O-H bonds in the attached water molecules, making the hydrogens in these water molecules more acidic than those in free water molecules. The higher the charge on the metal ion, the stronger the acidity of the hydrated ion.

  21. How do you determine whether a solution will be acidic, basic, or neutral when there is a salt made up of 2 ions that can affect the pH of aqueous solutions? Compare the Ka value for the acidic ion with the Kb value for the basic ion. If the Ka> Kb, the solution will be acidic. If the Kb> Kavalue, the solution will be basic. Equal Ka and Kb values mean that the solution is neutral Equations: Ka x Kb=Kw Example: pg. 676 (# 105) HW: pg. 676 (#99, 101, 109, 111 (ab, de))

  22. What are the two main factors that determine whether a molecule containing a bond between an atom and hydrogen will behave as a Bronsted-Lowry acid? 1. strength of the bond (weak bonds, more acidic) 2. polarity of the bond (more polar, more acidic) C-H bonds are strong and nonpolar, so there is no tendency to donate protons. What is the trend in strength of oxyacids? -The strength of the acid grows as the # of oxygen atoms increases. -This occurs because very electronegative oxygen atoms are used to draw electrons away , which polarizes and weakens the O-H bond -A proton is easiest to remove from a molecule with the largest number of attached oxygen atoms. -This is the same reason that hydrated metal ions become acidic. -The greater the electronegativity of the attached ion, the stronger the acid because the EN strong atom pulls electrons away from others and weakens bonds. Section 14.9: The effect of Structure on Acid-Base Properties

  23. Section 14.10: Acid-Base Properties of Oxides Substances that contain H-O-X bonds can behave as acids or bases. What determines which behavior will occur? -If X has a high electronegativity, the O-X bond will be covalent and strong, meaning that a proton will be released. -If X has a low electronegativity, the O-X bond will be ionic and can be broken in polar water and OH- is released Acidic oxides: when a covalent oxide dissolves in water, an acidic solution forms SO3 + H2O → H2SO4 Basic oxides: when an ionic oxide (metal oxide) dissolves in water and a basic solution results CaO + H2O → Ca(OH)2 HW: pg.677 (113-118)

  24. Table 14.9 Comparison of Electronegativity of X and Ka Value for a Series of Oxyacids

  25. Section 14.11: The Lewis Acid-Base Model • Lewis acid- electron-pair acceptor OR it has an empty atomic orbital that it can use to accept or share an electron pair from a molecule that has a lone pair of electrons (usually BORON) • Lewis base- electron-pair donor OR molecule that has a lone pair of electrons(usually oxygen, nitrogen, or fluorine) • Lewis model for acids and bases is beneficial because it covers many reactions that don’t involve Bronsted-Lowry aids • HW: pg. 677 (#119, 123)

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