1. Acid Base Indicators Experiment #6

2. What are acids and bases? There are many different definitions for classifying a substance as an acid or a base. Definitions are based on different theories Lewis’s theory of acids and bases Lowry-Bronsted theory of acids and bases Arrhenius theory of acids and bases

3. Proposed in 1884 by Svante Arrhenius An acid is a substance that contains hydrogen and produces H + in aqueous Solution. A base is a solution that contains the OH group and produces Hydroxide ions OH -, in aqueous solution Neutralization is defined as the combination of H + ions with OH - ions to form H 2 O molecules. H + (aq) + OH - (aq) H 2 O(l) Very Limited in Scope!!

4. Lowry-Bronsted theory Proposed in 1923 by N. Bronsted and T.M.Lowry An acid is defined as a proton donor, H +, and a base is Defined as a proton acceptor The complete ionization of hydrogen chloride, HCl in water is an acid-base reaction Step 2: H + (aq) + H 2 O(l) H 3 O + An acid-base reaction is the transfer of a proton from an acid to a base Overall: HCl(aq) + H 2 O(l) H 3 O + (aq) + Cl - (aq) (LB) Step 1: HCl(aq) H + (aq) + Cl - (aq) (Arrehnius)

5. Overall: HCl(aq) + H 2 O(l) H 3 O + (aq) + Cl - (aq) (LB) acid conjugate acid base conjugate base Overall: NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH - (aq) (LB) base conjugate acid conjugate base Whether water can act as an acid or base depends on the other species present

6. H 2 O (l) + H 2 O(l) H 3 O + (aq) + OH - (aq) Autoionization of water Simplified notation H 2 O(l) H + (aq) + OH - (aq) Water is said to be amphiprotic

7. Strength of acids Acids and bases are classified as strong or weak depending on the extent to which they can ionize. Higher ionization ≡ Strong Strong acid does not necessarily mean that it is the most corrosive acid. HF is a weak acid but it is one of the most corrosive acids. Strength of some acids: HClO 4 > HI > HBr > HCl > HNO 3 H 2 SO 4 > HNO 3 Strong acids and strong bases are completely ionized Weak acids and weak bases are partially ionized

8. Lewis theory An acid is any species that can accept a share in an electron pair. A base is any species that can make available or donate a share in an electron pair. A more broader definition. There is no need for the molecule to have hydrogens. Proposed in 1923 by G. N. Lewis

9. What happens when salts are dissolved in water? HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(aq) Acid Base Salt Water NaCl(s) + H 2 O NaOH(aq) + HCl(aq) Salt Water Base Acid Depending on the strength of the acid and base that are formed in solution, the medium will be either acidic or basic

10. How do I know if the resulting solution is acidic or basic? Measure pH of the solution What is pH? It is the measure of hydrogen ions in solution

11. What is pH? pH=1.0 pH=14.0 pH=7.0 Acidic Neutral Basic

12. What are some of the ways to measure pH? 1. Using an acid-base indicator 2. Using pH paper 3. Using pH meter 4. ……………..

13. What are acid-base indicators? How do they work? An organic dye, whose color depends on the concentration of H 3 O + or pH HIn(aq) + H 2 O(l) H 3 O + (aq) + In - (aq) acid conjugate acid base conjugate base When the indicator is added to the solution of interest, depending on the pH of the solution, the indicator stays in either its acid form or its conjugate form.

14. Indicator "Acidic" Color "Basic" color (MO)methyl orange Red below pH 4.0Yellow above pH 4.4 (BG)bromcresol green Yellow below pH 4.5Blue above pH 4.8 (BB)bromthymol blue Yellow below pH 6.9Blue above pH 7.3 (TB)thymol blue Yellow below pH 8.0Blue above pH 8.4 (P)Phenolphthalein Colorless below pH 9.3Pink above pH 9.7 (AY)alizarin yellow Yellow below pH 10.8Red above pH 11.2 If we are to pick a suitable indicator for following the pH change in the reaction, We have to pick one that is suitable over the pH range of the reaction. Indicators cannot give a particular numerical value for pH. They can only give a range for the pH of the solution.

15. Ionization of acids All acids undergo ionization. Strong acids ionize more than the weak ones. Strong acids are almost 100% ionized. Since the weak acids are partially ionized, the measure of the extent of ionization is given by an expression called acid ionization constant, K a. In our experiment today, we would like to measure the K a for a weak monoprotic acid.

16. What is a monoprotic acid? Usually acids have one or more hydrogen atoms (H) which they end up donating as protons (H + ). # of H atoms Name Example 1 monoprotic acid (HA) HCl – hydrochloric acid CH 3 COOH - acetic acid 2 diprotic acid (H 2 A) H 2 SO 4 – sulfuric acid 3 triprotic acid (H 3 A) H 3 PO 4 – Phosphoric acid

17. Ionization constant of an acid For a monoprotic weak acid (HA) dissolved in water, HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) acid conjugate acid base conjugate base

18. Ionization constant of an acid Ionization constant of the acid

19. Ionization constant of an acid Taking log of the equation on both sides,

20. Ionization constant of an acid Multiplying both sides of the equation by -1

21. Ionization constant of an acid Henderson-Hasselbach equation In the above equation, if we can make the [A - ] = [HA]

22. Ionization constant of an acid

23. 250mL V initial 0.2 M NaOH 250mL V final 100mL 1.0 g of HA In 100mL of water 50 mL of HA + 2 drops phenolphthalein V final - V initial =V used Add V used amount of water to the solution in the volumetric flask Mix the solution in volumetric flask to the Solution in the erlenmeyer flask Measure the pH of the final solution with indicators and pH paper