Ch. 23 - Acids & Bases I. Intro to Acids & Bases Definitions Properties Uses A. Definitions Acids • Substances that donate hydrogen ions (H+) form hydronium ions (H3O+) in water HCl + H2O + H3O + – Cl Properties of Acids sour taste corrosive electrolytes turn litmus red react with metals to form H2 gas Strength of acid depends on how well they form H+ in water. pH less than 7 Some examples of Acids HNO3 + H2O H3O+ + NO3Nitric acid water hydronium ion nitrate ion Some Examples of Acids Digesting Your Food Your stomach contains two strong acids: HCl (hydrochloric acid and pepsin). More examples of Acids H3PO4 - soft drinks, fertilizer, detergents H2SO4 - fertilizer, car batteries HCl - gastric juice HC2H3O2 - vinegar Bases A base is a substance that either contains hydroxide ions-OH-, or reacts with water to form hydroxide ions. NH3 + H2O + NH4 + OH Hydroxide Properties of Bases bitter taste corrosive electrolytes turn litmus blue slippery pH feel greater than 7 Some examples of Bases •KOH K+ + OH•potassium hydroxide -> potassium ion plus hydroxide Many soaps contain bases! Some Common Bases & Uses NaOH - lye, drain and oven cleaner, hair relaxers Mg(OH)2 - laxative, antacid- “MOM” Milk of magnesia NH3 - cleaners, fertilizer Al(OH)3 aluminum hydroxide Maalox (antacid) Strengths of Acids and Bases Strong Acid/Base • 100% ions in water • strong electrolyte • HCl, HNO3, NaOH, LiOH Weak Acid/Base • few ions in water • weak electrolyte • HC2H3O2, NH3 - + - + Concept Test! Which of the following "molecular" pictures best represents a concentrated solution of the weak acid HA? A. B. pH Scale- measuring the strength of acids and bases pH • a measure of the concentration of H3O+ ions in solution • measured with a pH meter or an indicator with a wide color range 14 0 7 INCREASING ACIDITY NEUTRAL INCREASING BASICITY The pH scale is a way of expressing the strength of acids and bases. Instead of using very small numbers, we just use the NEGATIVE power of 10 on the Molarity of the H+ (or OH-) ion. Under 7 = acid 7 = neutral Over 7 = base pH of Common Substances 0-3 strong acids 4-6 weak acids--8-11 weak bases 12-14 strong bases pH pH pH pH pH Testing Indicator • substance that changes color in an acid or base Examples: • Litmus paper - red/blue • phenolphthalein - colorless/pink • goldenrod - yellow/red • red cabbage juice (natural indicator)pink/green Neutralization Reactions When an acid and a base react, they produce a salt and water. Acid + Base salt + water HCl + NaOH NaCl + H2O The pH of the products will be nearly neutral or 7.0. Neutralization to the rescue! When a strong acid is spilled in the lab, it can be neutralized using a base – like baking soda. When a strong base is spilled, it can be neutralized using an acid-like vinegar. Both will produce salt-water with a pH of near 7.0. A common neutralization reaction! People use antacids such as Tums or Rolaids to neutralize stomach acid (HCl) from overeating and drinking. Antacids are weak bases. Baking soda and water can also be used. What do these weak antacids do to the excess stomach acid?