2 ChemCatalystYou have a beaker containing 0.10 M HCl, hydrochloric acid. When you test the conductivity of this solution, the light bulb shines brightly. How do you explain this observation?You have a second beaker containing 0.10 M CH3COOH, acetic acid. When you test the conductivity of this solution, the light bulb shines, but very dimly. How do you explain this observation?Unit IV • Investigation IV-X2
3 Purpose: This activity allows you to compare acid solutions from a molecular point of view. Unit IV • Investigation IV-X
4 Making senseSome acids on the Handout are labeled “strong” and others labeled “weak”. What is the difference between them?Unit IV • Investigation IV-X
5 Acids dissociate into ions in solution. 21Flask 1: M HClhydrochloric acid -strongFlask 2: M HClhydrochloric acid - strongpH = 2.7pH = 2Unit IV • Investigation IV-X
6 Some acids do not dissociate completely in solution.
7 Strong and weak acidsAcids that dissociate completely into ions are called strong acidsHCl, HNO3, HBrAcids that do not dissociate completely in solution are called weak acidsHF, CH3COOHThe pH of an acid solution is determined by:the molarity of the solutionthe identity of the acid in solutionUnit IV • Investigation IV-X
8 Check-InA solution of hydrocyanic acid has a molarity of M and a pH of 5.7. Do you think it is a strong or a weak acid? Explain your thinking.Unit IV • Investigation IV-X
9 What factors affect the strength of an acid? Not all acids dissociate to the same extent in solution. Some acids ionize completely, while others ionize partially. Both molarity and the identity of the dissolved acid affect the pH of a solution. Strong acids are acids that dissociate completely in solution. They have higher H+ concentrations as a result. Weak acids are acids that do not dissociate completely in solution. They have lower H+ concentrations as a result.Unit IV • Investigation IV-X
10 Some background Acid = proton donor Base = proton acceptor Conjugate pairCH3OOH +H2O H3O+ + CH3OO-conjugate pair
11 Water does the same thing! Lets look at the “equation” for water ionizingH2O + H2O H3O+ + OH-[H3O+][OH-]/[H2O]2 =K = constant value (55.6 M no mater what), so lets focus on the stuff that changes[H3O+][OH-] = Kw = 1.0 * (have you seen this before?)
12 We already knew… Neutral means [H3O+]= [OH-]= 1.0 * 10-7 Acidic means [H3O+] > [OH-]Basic means [H3O+]< [OH-]
14 Strong vs. weak HCl H+ + Cl- [H+][Cl-]/[HCl] ~ 1.0 In other words, we have (negligibly) little HCl left after the system has come to equilibrium[H+] = [HCl]
15 Strong vs. weak CH3COOH +H2O H3O+ + CH3COO- OR HC2H3O2 H+ + C2H3O2-Based upon the demonstration, there is lots of CH3COOH left after the system has come to equilibriumHow do we know?So [H+] ≠ [HC2H3O2]
16 Just how weak?A 1.0 M solution of acetic acid has a pH of What percentage of the acetic acid is ionized in the solution?= [H+] = M([H+]/ [HC2H3O2])*100 = % ionization = (0.0043/1.0)*100 = 0.43%
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