Week 1: Acid Base Chemistry

Acid & Base Acid & Base Initially, acids were identified as substances with a sour taste, like vinegar or lemon juice, and with the ability to change the color of certain plant-derived dyes Acids are how defined as substances that dissolve in water to produce H+ ions, while bases are substances that produce OH- ions in water An acidic solution has a higher concentration of hydronium ions (H3O+) than hydroxide ions A basic solution has a higher concentration of hydroxide ions (OH-) than hydronium ions A neutral solution has equal concentrations of hydronium ions (H3O+) and hydroxide ions (OH-) pH The pH of a solution indicates how acidic it is and is calculated as the negative logarithm (log) of its hydrogen ion concentration (pH = -log[H+]) An acid increases the concentration of H+ in a solution by donating one of its hydrogen atoms through dissociation On the other hand, a base raises pH by providing hydroxide (OH-) or another ion or molecule that removes hydrogen ions from the solution pH scale 0. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. Battery acid Stomach acid Vinegar Orange Juice Tomato Black Coffee Urine Water Seawater Baking Soda Antacid tablet Ammonia solution Soapy water Bleach Drain cleaner Buffers Buffers A buffer is a solution of weak acid and conjugate base or weak base and conjugate acid used to resist pH change with added solute. A buffer solution resists changes in its pH when a small amount of acid or base is added to it Buffer solutions are resistant t0 pH change because of the presence of an equilibrium between the acid (HA) and its conjugate base (A-) When some strong acid is added to a buffer, the equilibrium is shifted to the left, and the hydrogen ion concentration increases by less than expected for the amount of strong acid added Buffer solutions are necessary in biology for keeping the correct pH for proteins to work Buffers can be prepared in multiple ways by creating a solution of an acid and its conjugate base Ideal buffer components control pH and ionic strength without interacting in other ways with the system being studied Henderson-Hasslebalch equation Because of the commonly observed deviations from ideal behavior, buffers are typically prepared by adding the buffer components to a container, adjusting the solution to the desired pH by adding an acid or a base, and then adding sufficient water to reach the expected final volume pH = pKa + log10 ([conjugate base]/[weak acid]) Since weak acids and bases ionize to a minimal extent, the Henderson-Hasslebalch equation is used to estimate the pH of these solutions weak acids weak acids && weak weakbases bases Weak acids and bases are only partially ionized when dissolved in water, while strong acids and bases are completely ionized Compounds that are weaker acids than water (those found below water in the column of acids) do not exhibit observable acidic behavior when dissolved in water Ionization of weak acids and Ka Acetic acid, CH3COOH, is a typical weak acid and it is the ingredient of vinegar, it is partially ionized in its solution CH3COOH CH3COO- + H+ The acid dissociation constant (Ka) i's a measure of the extent to which an acid dissociates in solution and therefore its strength. The less an acid dissociates, the smaller the value of Ka General Ka expressions take the form Ka = [H3O+][A-] / [HA] Definition of pKa and Ka and relationship Strong acids completely dissociate into their ions in aqueous solutions, while weak acids only partially dissociate, resulting in an equilibrium between the acid and its conjugate base HA A- + H+ A higher pKa value indicates a weaker acid because it suggests a lower Ka value