Unit 6: ACIDS BASES SALTS

Acids, Bases, and Salts When dissolved, these compounds ionize Example: HCl  H+ + Cl- These charged atoms (ions) are free to move, so the solutions conduct electricity Compounds that conduct in solution are called electrolytes Strong electrolytes ionize completely; in weak electrolytes, only some molecules ionize 100 100 HCl  H+ + Cl- HF  H+ + F- 0 100 100 97 3 3

Properties of Acids Properties of Bases -- dilute acids taste sour -- cause indicators to change color -- react with many metals to produce hydrogen gas Example: 2HCl + Zn  ZnCl2 + H2 -- react with bases to produce water and a salt neutralization Properties of Bases -- dilute bases taste bitter, and feel soapy -- cause indicators to change color -- can neutralize acids to produce water and a salt

Acid/Base Theories 1. Arrhenius a. acids produce H+ as the only + ion in solution (or H3O+ -- hydronium ion) Example: HCl  H+ + Cl- OR HCl + H2O  H3O+ + Cl- b. bases produce OH- as the only – ion in solution Example: Ca(OH)2  Ca+2 + 2OH- 2. Brönsted - Lowry a. acids can donate a proton to another molecule (H+) Example: HBr + H2O  H3O+ + Br- {HBr donates to H2O} b. bases can accept a proton (H+) from another molecule Example: NH3 + H2O  NH4+ + OH- {NH3 accepts from H2O} Some substances can act as acids or bases, like water (above)

Arrhenius Bronsted Summary: Acid Base H+ OH- proton donor proton acceptor (lose H+ ) (gain H+ )

Acid-Base Reactions Neutralization occurs when H+ + OH-  H2O acid base water Neutralization pattern: Acid + Base  Salt + Water Ex. HCl + NaOH  NaCl + H2O Ex. H2SO4 + Ca(OH)2  CaSO4 + 2H2O 1 mole of acid (H+) will neutralize 1 mole of base (OH-) Example: Suppose you spill 8.0L of 6.0M NaOH. How much 3.0M HCl is required to neutralize it? Solution: MA VA = MB VB (Table T) (3.0)(x) = (6.0)(8.0) x = 16L

The solution with the higher molarity will have the smaller volume You must adjust for the number of H+ or OH- by multiplying the molarity by the number of moles of ions that will be produced Example: Ca(OH)2  Ca+2 + 2OH- for 3M Ca(OH)2, use 6M OH- Example: H3PO4  3H+ + PO4-3 for 3M H3PO4, use 9M H+ Titration: a lab procedure that uses neutralization to find the molarity of an unknown acid or base obtain a measured amount of acid with known molarity 2. slowly add base with unknown molarity-- until the mixture is exactly neutralized: the end point

Example: Suppose you start with 15.0 mL of 2.0M HCl. To reach endpoint, you add 5.0 mL of unknown NaOH Solution: MA VA = MB VB (2.0)(15.0) = (x)(5.0) x = 6.0M H2O  H+ + OH- Water ionizes to a slight extent: Or, H2O + H2O  H3O+ + OH- In all aqueous solutions: [H+ ] x [OH-] = 1 x 10-14 [A] means “the molarity of A” or “the concentration of A”

Any solution: where [H+ ] is greater than [OH-] = 1 x 10-7 In pure water, [H+ ] = [OH-] (1 x 10-7 ) x (1 x 10-7) = 1 x 10-14 {to multiply powers of 10, add exponents} Example: in a 1 x 10-3M solution of HCl, what is [H+]? [OH-]? Solution: since HCl ionizes completely, [H+] = 1 x 10-3 [H+ ] x [OH-] = 1 x 10-14 (1 x 10-3) x [OH-] = 1 x 10-14 [OH-] = 1 x 10-11 Any solution: where [H+ ] is greater than [OH-] = Acid where [H+ ] is less than [OH-] = Base where [H+ ] is equal to [OH-] = Neutral

The pH scale -- measures the acidity/basicity of a solution -- ranges from 0 to 14 very acidic very basic -- pH 7 is neutral

pH is the negative logarithm of [H+] -- if [H+ ] = (1 x 10-4), pH = 4 -- if [H+ ] = (1 x 10-13) , pH = 13 Example: What is the pH of pure water? Solution: [H+ ] = 1 x 10-7 pH = 7 Example: What is the pH of a 1 x 10-2M HCl solution ? pH = 2 Solution: [H+ ] = 1 x 10-2 Example: What is the pH of a 1 x 10-4M NaOH solution? Solution: [OH-] = 1 x 10-4 pH = 10 [H+] = 1 x 10-10

Some pH values: lemon juice -- 2.3 vinegar -- 2.8 soft drinks -- 3.0 rain (clean) -- 6.0 rain (polluted) -- 3.0 sea water -- 8.4 Example: In a carbonated soft drink, what is [H+]? [OH-]? Solution: pH = 3, so [H+] = 1 x 10-3M [OH-] = 1 x 10-11M Example: How many times more acidic is acid rain, compared to clean? Solution: Acid, pH 3 Clean, pH 6 1 x 10-3M [H+] 1 x 10-6M .001M .000001 .001 .000001 = 1000 times as acidic

Since the pH scale is logarithmic; each unit of pH represents a 10x change in acidity [H+] pH times more acidic than pH 7 6 101 = 10 times 5 102 = 100 times 4 103 = 1000 times Table M can help predict indicator colors at different pH’s Example: What color would the following indicators be in a neutral solution? methyl orange = yellow thymol blue = yellow bromcresol green = blue litmus = purple?