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Acids and Bases: pH Indicators, Properties, Theories, and Reactions

Learn about pH indicators, properties, theories, and reactions of acids and bases. Explore Arrhenius and Bronsted-Lowry theories, amphoteric substances, conjugate acid-base pairs, and neutralization reactions.

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Acids and Bases: pH Indicators, Properties, Theories, and Reactions

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  1. ACIDS and BASES Chapter 19

  2. pH indicators • pH indicators are valuable tool for determining if a substance is an acid or a base. • The indicator will change colors in solution.

  3. Things to use… • pH meter will indicate the numeric value of acid or base based on the pH range • Chemical indicators: phenolphthalein, universal indicator… • Natural indicators: poinsettia, red cabbage juice…

  4. ACIDS Have a sour taste Change the color of many indicators Are corrosive (react with metals) Neutralize bases Conduct an electric current BASES Have a bitter taste Change the color of many indicators Have a slippery feeling Neutralize acids Conduct an electric current Properties of Acids and Bases

  5. The Arrhenius Theory of Acids and Bases

  6. Arrhenius Theory of Acids and Bases: an acid contains hydrogen and ionizes in solutions to produce H+ ions: HCl  H+(aq) + Cl-(aq)

  7. Arrhenius Theory of Acids and Bases: a base contains an OH- group and ionizes in solutions to produce OH- ions: NaOH  Na+(aq) + OH-(aq)

  8. Neutralization • Neutralization: the combination of H+ with OH- to form water. H+(aq) + OH-(aq) H2O (l) • Hydrogen ions (H+)in solution form hydronium ions (H3O+)

  9. In Reality… H+ + H2O  H3O+ Hydronium Ion (Can be used interchangeably with H+)

  10. Commentary on Arrhenius Theory… One problem with the Arrhenius theory is that it’s not comprehensive enough. Some compounds act like acids and bases that don’t fit the standard definition.

  11. Bronsted-Lowry Theory of Acids & Bases

  12. Bronsted-Lowry Theory of Acids & Bases: • An acid is a proton (H+) donor • A base is a proton (H+) acceptor

  13. for example… Proton transfer HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq) Base Acid

  14. Water is a proton donor, and thus an acid. another example… CONJUGATE BASE ACID NH3(aq) + H2O(l) NH4+ (aq) + OH- (aq) BASE CONJUGATE ACID Ammonia is a proton acceptor, and thus a base

  15. Amphoteric Substances A substance that can act as both an acid and a base (depending on what it is reacting with) is termed amphoteric. Water is a prime example.

  16. Conjugate acid-base pairs • Conjugate acid-base pairs differ by one proton (H+) A conjugate acid is the particle formed when a base gains a proton. A conjugate base is the particle that remains when an acid gives off a proton.

  17. Examples: In the following reactions, label the conjugate acid-base pairs: • H3PO4 + NO2- HNO2 + H2PO4- • CN- + HCO3- HCN + CO32- • HCN + SO32- HSO3- + CN- • H2O + HF  F- + H3O+ acid base c. acid c. base base acid c. acid c. base acid base c. base c. acid c. base c. acid base acid

  18. SUMMARY OF ACID-BASE THEORIES

  19. Strength of Acids and Bases • A strong acid dissociates completely in sol’n: • HCl  H+(aq) + Cl-(aq) • A weak acid dissociates only partly in sol’n: • HNO2 H+(aq) + NO2-(aq) • A strong base dissociates completely in sol’n: • NaOH  Na+(aq) + OH-(aq) • A weak base dissociates only partly in sol’n: • NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

  20. Acid-Base Reactions • Neutralization reactions: reactions between acids and metal hydroxide bases which produce a salt and water. • H+ ions and OH- ions combine to form water molecules: • H+(aq) + OH-(aq) H2O(l)

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