Acid-Base Indicator Useful Indicators

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1 Chemistry H Acids and Bases This presentation was created by Professor Carl H. Snyder Chemistry Department University of Miami Coral Gables, FL CSnyder@miami.edu Chapter 10 - Acids and Bases Copyright 2004 by Carl H. Snyder, University of Miami. All rights reserved. Acid-Base Indicator Useful Indicators Anthocyanins - Found in red cabbage. Various colors, depending on strength of acid or base. Litmus - From small plants indigenous to the Netherlands. Red in acid; blue in base The anthocyanins that give red cabbage its characteristic color serve as acid-base indicators. They change color when ammonia (a base) is added to the cabbage extract. Common Acids and Bases Vinegar, vitamin C, lemon juice Drain cleaner, household ammonia, lye, milk of magnesia Characteristics of Acids and Bases Water solutions of acids taste sour turn litmus red react with certain metals to release H 2 Water solutions of bases taste bitter turn litmus blue feel slippery or oily 1

2 Neutralization : Acid + Base = Salt (+ Water) Typical Neutralization Reactions Acidity At The Molecular Level Lavoisier, 1778 (Wrong!) Oxy-Gen -- Generating Acidity Antoine Lavoisier, French, 1778 Svante August Arrhenius, Swedish, 1887 Johannes Bronsted, Danish; Thomas M. Lowry, English; 1923 (And others, especially Gilbert N. Lewis, American, early 1920s). Arrhenius, 1887 (Limited to H 2 O) Protons in Water Arrhenius: An acid is anything that produces protons (H + ) in water. A base is anything that produces hydroxide ions (OH - ) in water. The Arrhenius definiton is limited to aqueous (water-based) systems. Bronsted-Lowry (Good) Proton Transfers An acid is a proton-donor A base is a protonacceptor Water is not a necessary component. 2

3 Bronsted-Lowry In Action HCl vapors and NH 3 vapors react to form a cloud of solid NH 4 Cl particles. Acids, Bases, Indicators A Summary This neutralization takes places in the atmosphere. No water is present. The Hydronium Ion, H 3 O + Amphoteric Water Isolated protons barely exist in water. In water, almost all protons bond to H 2 O molecules to form H 3 O +, hydronium ions. Water ionizes to form both hydronium ions (acidic) and hydroxide ions (basic) at the same time. In pure water their concentrations are equal. Whether an aqueous solution is acidic or basic depends on whether it has a higher concentration of hydronium ions or hydroxide ions. [H 3 O + ] Placing square brackets around a chemical species means that we are representing the molar concentration of that species. The molar concentration of the hydronium ion in water at 25 o C is M, or 1 x 10-7 M. ph 3

4 The ph of Neutral Water, 25 o C The Ion Product Constant of Water The ionization of pure water produces equal concentrations of H + and OH -. The arithmetic product of these two concentrations is a constant at any given temperature: ph -- Generalizations ph -- Details The ph of Everday Things The Carboxyl Group Molecules of almost all organic acids contain this structural unit. 4

5 Some Common Organic Acids Part I Some Common Organic Acids Part II formic acid, the sting of red ants carbonic acid, carbonated drinks acetic acid, vinegar citric acid, citrus fruit propionic acid; cheeses (calcium salt: a food preservative) butyric acid; rancid butter oxalic acid; rhubarb and spinach benzoic acid; sodium salt: preservative in canned and bottled fruit drinks Some Common Organic Acids Part III Organic Acids in Our Foods Bacterial fermentation converts lactose (milk sugar) into lactic acid, the odorous acid of sour milk. Human metabolism generates lactic acid in our muscles as we work or exercise. Citrus fruits contain citric acid Rhubarb contains oxalic acid Cheeses contain propionic acid Canned and bottled fruit drinks contain the sodium salt of benzoic acid (benzoate of soda) Bases Used As Antacids Commercial Antacids These bases are strong enough to neutralize stomach acid, but too weak to injure mucous membranes or the stomach lining. 5

6 The Chemistry of Alka-Seltzer Le Châtelier s Principle, Alka- Seltzer, and Red Cabbage Alka-Seltzer contains sodium bicarbonate (NaHCO 3 ), a small amount of potassium bicarbonate, and citric acid, which is a weaker acid than HCl, and thus gentler to the body. The citric acid reacts with bicarbonate to produce carbonic acid, which decomposes to water and carbon dioxide. With Alka-Seltzer, the top reaction -- the decomposition of H 2 CO 3 -- predominates. In the red-cabbage experiment, the bottom reaction -- the formation of H 2 CO 3 -- predominates. To explain this difference, we turn to Le Châtelier s Principle. The Constant Ratio Le Châtelier s Principle Red Cabbage Le Châtelier s Principle rests on the constancy of this ratio. Increase the concentration of CO 2 and the equilibrium shifts toward the H 2 CO 3 Increase the concentration of H 2 CO 3 and the equilibrium shifts toward the CO 2 Le Chatelier s Principle Alka-Seltzer End - Chapter 10 6