Acids, Bases, & Salts

1. Acids, Bases, & Salts

2. Acids & Bases Bracken Cave, near San Antonio, Texas, is home of 20-40 million bats, which is probably the largest colony of mammals in the world. Visitors to the cave must wear protective goggles and respirators to protect themselves from the dangerous levels of ammonia in the cave. Ammonia is a by-product of the bats’ urine. Ammonia is considered a base… What is the difference between an acid and a base?

3. Acids • What do you think of when you hear the word “acid”? • An acid is a substance that produces hydrogen ions, H+, in a water solution. • It is this ability to produce H+ions that gives acids their characteristic properties. When an acid dissolves in water, H+ions interact with water molecules to form hydronium ions.

4. Properties of Acids • All acids taste sour or tart • Please never taste a substance to see if it’s acidic! • Some acids can burn you. • Corrosive – eat away some metals • When an acid reacts with a metal, hydrogen gas and metallic compounds form • Can cause indicators to change color • An indicator is an organic compound that changes color in acid/base • Example: litmus paper

6. Bases • A base is any substance that forms hydroxide ions, OH-, in a water solution. • Examples of foods / products that are basic: • Egg whites • Baking powder • Antacids (TUMS) • soap

7. Properties of Bases • Feel slippery, like soapy water • Bitter taste • Tasting most bases is HAZARDOUS! • Corrosive • Can burn you • Also cause indicators to change color • When they are not dissolved in water, many bases are solid in the form of crystals.

9. Solutions of Acids & Bases • Many products that contain acids and bases are solutions. • Water is the main solvent for these solutions because water molecules have polarity.

10. What happens when acids dissolve in water? • Acids produce hydrogen ions (H+) in water • When an acid dissolves in water, the negative ends of nearby water molecules attract the positive hydrogen in the acid. The acid separates into ions, which is called dissociation. • What is left after dissociation are negative ions and positive H+ions. The H+ions combine with water molecules to form hydronium ions (H3O+).

11. What happens when acids dissolve in water?

12. What happens when bases dissolve in water? • Bases form hydroxide ions (OH-) in water. • When bases dissolve in water, the positive ends of nearby water molecules attract the OH-ions in the base. • The base dissociates • What is left after dissociation are positive ions and negative OH-ions. • Unlike acid dissociation, the OH-ions do NOT combine with water molecules.

13. What happens when bases dissolve in water?

17. How is ammonia different from other bases? • Ammonia, NH3, is a base that does not contain –OH. • In water, ammonia actually dissociates water molecules. An ammonia molecules attracts a hydrogen ion from a water molecule to form an ammonium ion, NH4+. • The rest of the water molecules is a hydroxide ion, OH-.

18. How is ammonia different from other bases?

19. Identifying Acid & Bases • The BrØnsted-Lowry theory defines an acid as a hydrogen-ion donor, and a base as a hydrogen-ion acceptor.

20. Lewis Acids and Bases • Lewis proposed that an acid accepts a pair of electrons during a reaction, while a base donates a pair of electrons.

21. pH scale • A patient is brought to a hospital unconscious and with a fruity odor on his breath. The doctor suspects the patient has fallen into a diabetic coma. To confirm her diagnosis, she orders several tests, including one of the acidity of the patient’s blood. The results from this test will be expressed in units of pH, not molar concentration.

22. pH of a Solution • If you have a swimming pool or keep tropical fish, you know that the pH of the water must be kept at certain levels. • The pH of a solution is a measure of the concentration of H + ions in the solution. • The pH of a solution is measured on a scale ranging from 0 to 14. • The greater the concentration of H+ ions is, the lower the pH and the more acidic the solution.

23. The figure shows a pH scale. • Solutions with a pH lower than 7 are acidic. The greater the pH value, the more basic the solution is. • A solution with a pH of 7 is called neutral. In a neutral solution, concentrations of H+ and OH- ions are equal. Pure water at 25°C has a pH of 7

24. Measuring pH • People need to be able to measure the pH of the solutions they use. From maintaining a swimming pool to creating soil conditions ideal for plant growth, to making medical diagnoses, pH measurements have valuable applications.

25. Strength of Acids • Strength of acids depends on how many acid molecules dissociate into ions in water. • A strong acid is one in which almost all of the acid molecules dissociate in water. • A weak acid is one in which only a small number of the acid molecules dissociate in water. • Ions in solutions can conduct electric currents. Acids and bases can carry an electric current because they dissociate into ions. The more ions in a solution, the more electric current the solution can conduct. Strong acids can conduct currents well. Weak acids have few ions and therefore do not conduct current well.

26. Strength of Bases • A strong base is one that dissociates completely in solution. • Example: Sodium hydroxide (NaOH) • NaOH(s)  Na+(aq) + OH-(aq) • A weak base is one that does not dissociate completely in solution. • Example: Ammonia (NH3) • NH3 (aq) + H2O  NH4+(aq) + OH-(aq) 

27. Measuring pH • The pH of your blood must remain between 7.0 and 7.8. If your blood pH goes outside of this range, enzymes do not work. Why doesn’t your blood pH change when you eat acidic foods? • Your blood contains compounds called buffers that allow small amounts of acids or bases to be absorbed without changing the pH. • Buffers are solutions containing ions that react with additional acids or bases to decrease their effects on pH. The buffers help your blood stay at an almost constant pH of 7.4. One buffer system in the blood involves biocarbonate ions, HCO3-

28. Neutralization • Neutralization is a chemical reaction between an acid and a base that happens in a water solution. • Example: NaOH neutralizes HCl. Hydroniom ions from the acid combine with hydroxide ions from the base, producing neutral water.

29. How is salt formed? • Antacids are medicines that contain bases or other compounds that neutralized the HCl in your stomach. One of these antacids is sodium biocarbonate – NaHCO3. HCl(aq)+NaHCO3(s) NaCl(aq) + CO2 (g)+ H2O(l) • HCl, shown above, is being neutralized by NaOH. Only half of the ions are shown. The other ions react to form a salt. • A salt is a compound formed when the negative ions from an acid combine with the positive ions from a base. • HCl reacts with NaOH, the salt formed is sodium chloride. Na+(aq) + Cl-(aq)  NaCl(aq)

30. How do acid-base reactions equation look? • The general equation shows acid-base reactions in water: acid + base  water + salt

31. Titration • A titration (ti TRAY shun), a solution of known concentration is used to find the concentration of another solution. • To do a titration on an acid solution, measure the volume of the solution of unknown concentration. Add a few drops of an indicator to the solution. Phenolphthalein (fee nul THAY leen) is an indicator that has no color in an acid but turns pink in a base.

32. Titration • Since the unknown solution is an acid, slowly and carefully add a base solution of known concentration to the acid-and-indicator mixture. Toward the end of the titration, add base solution drop by drop until one drop turns the solution pink and the color stays.

33. Finding the number of moles of an acid in neutralization Sample Problem 19.6 • How many moles of sulfuric acid are required to neutralize 0.50 mol of sodium hydroxide? • Knowns/Unknowns: H2SO4(aq) + 2NaOH(aq) Na2SO4(aq) + 2H2O(l) • molNaOH = 0.50 mol • mole ratio: 1 molH2SO4 : 2 molNaOH • moles H2SO4 = ?? mol

34. Sample Problem 19.6How many moles of sulfuric acid are required to neutralize 0.50 mol of sodium hydroxide? • Knowns/Unknowns: • molNaOH = 0.50 mol • Equation: H2SO4 (aq) + 2NaOH(aq) Na2SO4(aq) + 2H2O(l) • mole ratio of H2SO4 to NaOH = 1:2 • moles of H2SO4 = ?? • Solve for the unknowns: 1 molH2SO4 0.50 molNaOH 0.25 mol H2SO4 = X 2 molNaOH

35. Determining the Concentration of an Acid by Titration Sample Problem 19.7 • A 25-mL solution of H2SO4 is completely neutralized by 18 mL of 1.0MNaOH. What is the concentration of the H2SO4 solution? H2SO4(aq) + 2NaOH(aq) Na2SO4(aq) + 2H2O(l) • molarity base = 1.0M NaOH • Volume acid = 25 mL = 0.025 L • volume base = 18 mL = 0.018 L • molarity acid = ??

36. Sample Problem 19.7A 25-mL solution of H2SO4 is completely neutralized by 18 mL of 1.0MNaOH. What is the concentration of the H2SO4 solution? • molarity base = 1.0M NaOH • Volume acid = 25 mL = 0.025 L • volume base = 18 mL = 0.018 L • molarity acid = ?? H2SO4(aq) + 2NaOH(aq) Na2SO4(aq) + 2H2O(l) 0.018 L NaOH1.0 molNaOH1 mol H2SO4 X X • 1 L NaOH 2 molNaOH • = 0.0090 mol H2SO4 0.0090 mol moles molarity = = = 0.36 M 0.025 L liters

37. Practice Problems • Write equations for the ionization of HNO3 in water and the reaction of CO32- with water. For each equation, identify the hydrogen-ion donor and the hydrogen-ion acceptor. • Identify the Lewis acid and the Lewis base in each reaction. • H+ + H2O  H3O+ • AlCl3 + Cl-  AlCl4- • What is the relationship between H+ and OH- in an aqueous solution? • How many moles of potassium hydroxide (KOH) are needed to completely neutralize 1.56 mol of phosphoric acid (H3PO4)? • How many moles of sodium hydroxide (NaOH) are required to neutralize 0.20 mol of nitric acid (HNO3)? • How many milliliters of 0.45M HCl will neutralize 25.0mL of 1.00M KOH? • What is the molarity of H3PO4 if 15.0mL is completely neutralized by 38.5mL of 0.150M NaOH?