1. Acid Strength and Structure Acid-Base Properties of Salts AP Chemistry

2. Structural Considerations of Acids Acid strength is based on its ability to release a proton (H + ) Acid strength is based on its ability to release a proton (H + ) Any factors that influence this release is important in determining relative acid strength. Any factors that influence this release is important in determining relative acid strength. Two main factors of strength are: Two main factors of strength are: –Strength of the H-X bond –The polarity of the H-X bond

3. Bond Strength HF HF HCl HCl HBr HBr HI HI HI has a long bond length  strongest acid HF has a short bond length  weakest acid Strongest Bond Weakest Bond Weakest Acid Strongest Acid

4. Bond Polarity and Electronegativity Increasing electronegativity of associated atoms increase polarity around the proton. Increasing electronegativity of associated atoms increase polarity around the proton. This make it more likely for the proton to dissociate. This make it more likely for the proton to dissociate. Increasing number of oxygen atoms in oxy-acids also increases polarity of the molecule. Increasing number of oxygen atoms in oxy-acids also increases polarity of the molecule.

5. HClO 4 HClO 3 HCLO 2 HClO H 2 SO 4 H 2 SO 3 HNO 3 HNO 2 More oxygens = stronger acid More oxygens means more electrons are pulled away from the proton thereby weakening that bond. Strongest Acid Weakest acid

6. Which will be the strongest acid and why? HClO 3 HBrO 3 HIO 3  Strongest Most electronegative halogen

7. Acid-Base Properties of Salts A salt is technically defined as a substance containing a cation other than H+ and an anion other than OH-. A salt is technically defined as a substance containing a cation other than H+ and an anion other than OH-. When an acid and base react in a neutralization reaction, a salt and water are always produced. When an acid and base react in a neutralization reaction, a salt and water are always produced. When a salt dissolves in water it breaks into its ions. When a salt dissolves in water it breaks into its ions. Under certain conditions (depending on the cation and anion in the salt) a salt solution can be acidic or basic. Under certain conditions (depending on the cation and anion in the salt) a salt solution can be acidic or basic.

8. 1.) The salt produced by the reaction of a strong acid and a strong base is always neutral. (NaCl, KCl, NaNO 3 ) NaCl  Na+ + Cl- The cation comes from the base The cation comes from the base The anion comes from the acid The anion comes from the acid Cl- is the conjugate base of a strong acid HCl. It wants to stay as an ion! Cl- is the conjugate base of a strong acid HCl. It wants to stay as an ion! Na+ is the conjugate acid of a strong base NaOH. It also wants to stay an ion Na+ is the conjugate acid of a strong base NaOH. It also wants to stay an ion It will not attack water (hydrolysis) and change the pH It will not attack water (hydrolysis) and change the pH

9. 2) The salt produced by the reaction of a weak acid and a strong base is basic. (NaC 2 H 3 O 2, NaF) F- is the conjugate base of a weak acid HF F- is the conjugate base of a weak acid HF This makes F- as strong base! This makes F- as strong base! It will attack water (hydrolyze) as follows: It will attack water (hydrolyze) as follows: F- + H 2 O  HF + OH- F- + H 2 O  HF + OH- This increases [OH-] making the solution basic! This increases [OH-] making the solution basic!

10. 3.) The salt produced by the reaction of a strong acid and a weak base is acidic. (NH 4 Cl, NH 4 NO 3 ) NH 4 + is the conjugate acid of a weak base, NH 3 NH 4 + is the conjugate acid of a weak base, NH 3 This makes NH 4 + a strong acid. This makes NH 4 + a strong acid. It will give water its H+ ion in solution as follows: It will give water its H+ ion in solution as follows: NH 4 + + H 2 O  H 3 O + + NH 3 This increases [H + ] making the solution acidic! This increases [H + ] making the solution acidic!

11. 4.) A salt made up of a highly charge metal ion and a strong acid will be acidic.(FeCl 3, Al(NO 3 ) 3 ) H Fe 3+ ---- O H When the Fe 3+ is solvated by water the high charge pulls electrons away from oxygen, which then in turn pulls electrons away from the hydrogens with more force. This weakens the H-O bond and allows the H+ to dissociate more easily.

12. 5.)The salt produced by the reaction of a weak acid and a weak base depends of the Ka of the acid: (NH 4 CN, NH 4 C 2 H 3 O 2 ) If Ka>Kb, an acidic salt results. If Ka>Kb, an acidic salt results. If Ka<Kb, a basic salt results. If Ka<Kb, a basic salt results. If Ka = Kb, a neutral salt results. If Ka = Kb, a neutral salt results.

13. An important relationship at 25 o C: An important relationship at 25 o C: Ka x Kb = Kw Where Ka is the dissociation constant for the acid and Kb is the dissociation constant for the conjugate base. Where Ka is the dissociation constant for the acid and Kb is the dissociation constant for the conjugate base.

14. Determining pH of Salt Solutions Determining pH of Salt Solutions What is the pH of a 0.5M NaF solution. The Ka of HF is 7.2 x 10 -4. What is the pH of a 0.5M NaF solution. The Ka of HF is 7.2 x 10 -4. In an aqueous solution, NaF will completely dissociate (its an ionic compound) and the following equilibrium exists: In an aqueous solution, NaF will completely dissociate (its an ionic compound) and the following equilibrium exists: F - (aq)+ H 2 O(l) ⇄ HF(aq)+OH-(aq) F - (aq)+ H 2 O(l) ⇄ HF(aq)+OH-(aq) The Na+ has no acid or base properties and therefore is not considered in the equilibrium, Since HF is a weak acid, its conjugate base, F- should be considered a strong base. Therefore the Kb expression will be important: The Na+ has no acid or base properties and therefore is not considered in the equilibrium, Since HF is a weak acid, its conjugate base, F- should be considered a strong base. Therefore the Kb expression will be important: KaxKb=Kwso: KaxKb=Kwso: Kb=Kw/Ka=1.39x10 -11 Kb=Kw/Ka=1.39x10 -11

15. F- (aq)+ H 2 O(l) ⇄ HF(aq) + OH-(aq) F- (aq)+ H 2 O(l) ⇄ HF(aq) + OH-(aq) Initial: 0.5M -----0 0 Equil. : 0.5 – x -----x x 1.39 x 10 -11 = x 2 or 1.39 x 10 -11 = x 2 1.39 x 10 -11 = x 2 or 1.39 x 10 -11 = x 2 0.5 – x 0.5 x = 5.3 x 10 -6 M x = 5.3 x 10 -6 M x = [OH-], so pOH = 5.28 and pH = 8.72 x = [OH-], so pOH = 5.28 and pH = 8.72 As expected, the solution is basic. As expected, the solution is basic.