Acids and Bases

Properties of acids and bases Get 8 test tubes. Rinse all tubes well with water. Add acid to four tubes, base to the other four. Touch a drop of base to your finger. Record the feel in the chart (on the next slide). Wash your hands with water. Repeat for acid. Use a stirring rod, add base to the litmus and pH papers (for pH paper use a colour key to find a number). Record results. Repeat for acid. Into the four base tubes add: a) two drops of phenolphthalein, b) 2 drops of bromothymol, c) a piece of Mg, d) a small scoop of baking soda. Record results. Repeat for acid. Clean up (wash tubes, pH/litmus paper in trash).

Observations NaOH(aq) HCl(aq) Taste Bitter Sour *Usually, but not always NaOH(aq) HCl(aq) Taste Bitter Sour Feel (choose slippery or not slippery) Slippery Not slippery pH (# from the key) 14 1 Litmus (blue or red) Blue Red Phenolphthalein *Pink *Cloudy/ white Bromothymol *Blue *Yellow Magnesium NR Bubbles Baking soda NR Bubbles

pH There are many ways to consider acids and bases. One of these is pH. Read pg. 368-70. [H+] is critical in many chemical reactions. A quick method of denoting [H+] is via pH. By definition pH = – log [H+], [H+] = 10-pH The pH scale, similar to the Richter scale, describes a wide range of values An earthquake of “6” is 10 as violent as a “5” Thus, the pH scale condenses possible values of [H+] to a 14 point scale (fig. 2, p370) Also, it is easier to say pH = 7 vs. [H+] = 1 x 10–7

Calculations with pH Q: What is the pH if [H+]= 6.3 x 10–5? pH = – log [H+] (‘6.3’, ‘exp’ or ‘EE’, ‘5’, ‘+/-’, ‘log’, ‘+/-’) (‘-’, ‘log’, ‘6.3’, ‘exp’ or ‘EE’, ‘-’, ‘5’) Ans: 4.2 Q: What is the [H+] if pH = 7.4? [H+] = 10–pH mol/L (’10’, ‘xy’, ‘7.4’, ‘+/-’, ‘=‘) (’10’, ‘^’, ‘-’, ‘7.4’, ‘=‘) 3.98 x 10–8 M Try questions 2 and 6 (a-b) on page 375

Pg. 375 2 a) pH = – log [H+] = – log [1x10–8] = 8.0 b) pH = – log [H+] = – log [1x10–7] = 7.0 c) pH = – log [H+] = – log [2.5x10–6] = 5.60 d) pH = – log [H+] = – log [1.3x10–4] = 3.89 6 a) [H+] = 10–pH = 10–5.4 = 4 x 10–6 mol/L b) [H+] = 10–pH = 10–5.72 = 1.9 x 10–6 mol/L

Historical views on acids O (e.g. H2SO4) was originally thought to cause acidic properties. Later, H was implicated, but it was still not clear why CH4 was neutral. Arrhenius made the revolutionary suggestion that some solutions contain ions & that acids produce H3O+ (hydronium) ions in solution. Ionization + H O H O Cl Cl H + + The more recent Bronsted-Lowry concept is that acids are H+ (proton) donors and bases are proton acceptors

The Bronsted-Lowry concept In this idea, the ionization of an acid by water is just one example of an acid-base reaction. + Cl H O acid base conjugate acid conjugate base conjugate acid-base pairs Acids and bases are identified based on whether they donate or accept H+. “Conjugate” acids and bases are found on the products side of the equation. A conjugate base is the same as the starting acid minus H+.

Practice problems Identify the acid, base, conjugate acid, conjugate base, and conjugate acid-base pairs: HC2H3O2(aq) + H2O(l)  C2H3O2–(aq) + H3O+(aq) acid base conjugate base conjugate acid conjugate acid-base pairs OH –(aq) + HCO3–(aq)  CO32–(aq) + H2O(l) base acid conjugate base conjugate acid conjugate acid-base pairs Reference: pg. 386 – 387 Try Q18 (p389), Q 8 & 11 (p392): do as above

Answers: question 18 (a) HF(aq) + SO32–(aq)  F–(aq) + HSO3–(aq) acid base conjugate base conjugate acid conjugate acid-base pairs (b) CO32–(aq) + HC2H3O2(aq)  C2H3O2–(aq) + HCO3–(aq) base acid conjugate base conjugate acid (c) conjugate acid-base pairs H3PO4(aq) + OCl –(aq)  H2PO4–(aq) + HOCl(aq) acid base conjugate base conjugate acid conjugate acid-base pairs

HCO3–(aq) + S2–(aq)  HS–(aq) + CO32–(aq) acid base conjugate acid conjugate base conjugate acid-base pairs 8b) H2CO3(aq) + OH –(aq)  HCO3–(aq) + H2O(l) acid base conjugate base conjugate acid conjugate acid-base pairs 11a) H3O+(aq) + HSO3–(aq)  H2O(l) + H2SO3(aq) acid base conjugate base conjugate acid conjugate acid-base pairs 11b) OH –(aq) + HSO3–(aq)  H2O(l) + SO32–(aq) base acid conjugate acid conjugate base conjugate acid-base pairs For more lessons, visit www.chalkbored.com