1. Acid Base Titrations Chemistry 12◊ Chapter 14

2. Titration: A titration is a technique for finding an unknown concentration of one chemical from the known concentration of a second chemical. A solution of known concentration (called a standard solution) is added gradually to another solution of unknown concentration until the chemical reaction between the two solutions is at equivalence point: The equivalence point is the exact number of moles required for the reaction moles of H+ = moles of OH- detected by a color change in an acid-base indicator or electronically

3. How it works:

4. Explained….. Step 1: The NaOH is placed in the buret. (The is the solution of known concentration that is always added from the buret.) Step 2: The unknown conc of HCl is placed in the Erlenmeyer flask along with approximately 20.00 mL of distilled water and 2-3 drops of phenolphthalein indicator. (Since the solution in the flask is acidic, phenolphthalein is colourless) Step 3: NaOH is added to the HCl in the flask. When the NaOH comes in contact with the solution in the flask, it turns pink and then the pink colour quickly disappears. This is because the OH- from the NaOH interact with the phenolphthalein to change the phenolphthalein from colourless to pink

5. About indicators: Substances that change colour when reacted with acids and bases are called indicators. They are usually organic compounds that establish an equilibrium between their molecular form (colour 1) and their ionic form (colour 2) in solution: HIn (aq)  H + (aq) + In - (aq) colour 1 colour 2 The pH at which the colour change occurs is called the end point. It refers to the colour transition when the two colours are present in equal amounts. [HIn] = [In-] Indicators are useful only in colourless solutions and over a narrow pH range. Therefore, they must be chosen to match the pH of the solution being formed

7. Colour Ranges: Can you see a problem with using phenolphthalein for a acid base neutralization ?

8. Indicators don't change colour sharply at one particular pH. Instead, they change over a narrow range of pH. More about pH curves later!

9. Stoich again….. Standard stoichiometry steps can be used to calculate the unknown concentration. Three-step stoichiometry solution:  M = mol/L  mol A  mol A  mol B (mole to mole ratio)  mol B  M = mol/L

10. Example: In a titration, 16.80 mL of NaOH (aq), of unknown concentration is neutralized by 25.00 mL of a 0.190 M solution of sulfuric acid, H 2 SO 4. What is the concentration of the sodium hydroxide solution?

11. Balance EQ: 2 NaOH + H 2 SO 4 → Na 2 SO 4 + 2 H 2 O Step 1: M = mol/L 0.190 M = x/0.02500 L = 0.00475 mol H 2 SO 4 Step 2: mole to mole ratio x mol NaOH = 0.00475 mol H2SO4 x 2 mol NaOH = 0.00950 mol NaOH 1 mol H2SO4 Step 3:M = mol = 0.00950 mol = 0.565 M NaOH L 0.01680 L

12. Try it Page 602 # 17-19