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Ch. 18: Acids & Bases

Ch. 18: Acids & Bases. Sec. 18.1: Acids & Bases: An Introduction. Objectives. Identify the physical & chemical properties of acids & bases. Classify solutions as acidic, basic, or neutral. Compare the Arrhenius, Brønsted-Lowry, and Lewis models of acids & bases. Acids Taste sour

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Ch. 18: Acids & Bases

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  1. Ch. 18: Acids & Bases Sec. 18.1: Acids & Bases: An Introduction

  2. Objectives • Identify the physical & chemical properties of acids & bases. • Classify solutions as acidic, basic, or neutral. • Compare the Arrhenius, Brønsted-Lowry, and Lewis models of acids & bases.

  3. Acids Taste sour May sting or burn on contact Turn blue litmus paper pink Are electrolytes React with most metals to form H2 gas* React with carbonates to form CO2 gas** Bases Taste bitter Feel slippery Turn pink litmus paper blue Are electrolytes Properties

  4. Metal* & Carbonate** Acid Reactions • Recall that acid and metal reactions are single replacement reactions: • Mg + HNO3 ? • Al + H2SO4  ? • Recall that acid and carbonate reactions are double replacement reactions with the immediate decomposition of H2CO3: • HBr + CaCO3  ? • HCl + KHCO3  ?

  5. Aqueous Solutions • All aqueous solutions contain H+ and OH- ions because water does dissociates slightly. The process is called self-ionization. H2O H+ and OH- • If [H+] = [OH-], the solution is neutral. It is not acidic or basic. Pure water is neutral because [H+] has to equal [OH-]. • In an acidic solution, the [H+] > [OH-]. • In a basic solution, [OH-] > [H+].

  6. The Hydronium Ion • Since H+ ions will bond to water molecules in a solution, the solution does not really contain H+ ions. It contains H3O+ ions. • We will use the symbols H+ and H3O+interchangeably to represent a hydrogen ion in a solution.

  7. The Arrhenius Model • An acid is a substance that contains hydrogen and produces H+ ions in aqueous solution. • A base is a substance that contains a hydroxide group and produces OH- ions in aqueous solution.

  8. An Arrhenius acid HCl --> H+(aq) + Cl-(aq) HCl is an acid. When dissolved in water, the resulting solution is acidic. A Arrhenius base NaOH --> Na+(aq) + OH-(aq) NaOH is a base. When dissolved in water, the resulting solution is basic. Examples

  9. Brønsted-Lowry Model Two chemists independently proposed a new model of acids & bases - one that would recognize that some bases (like NH3, for example) did not contain hydroxide yet produced OH- ions in water.

  10. Brønsted-Lowry Model According to Bronsted-Lowry, ammonia was considered a base: NH3 + H2O  NH4+ + OH-

  11. Brønsted-Lowry Model • An acid is a hydrogen ion (proton) donor. • A base is a hydrogen ion (proton) acceptor. Look at this general equation: HX (aq) + H2O H3O+ + X- the acid the base

  12. Practice Problems • Identify the acid and base in the following reactions: • H3O+ + OH- H2O + H2O • HCl + NH3 NH4+ + Cl- • S-2 + H2O  HS- + OH- • HS- + H2O  S-2 + H3O+ • H2O + HC2H3O2 C2H3O2-+ H3O+ • C2H3O2-+ H3O+ H2O + HC2H3O2

  13. Brønsted-Lowry Model HX (aq) + H2O H3O+ + X- • On accepting the H+ ion, H2O becomes H3O+ (which is an acid). WHY?? Because in the reverse reaction, the H3O+donates its H+ ion to X-. • On donating its H+ ion, HX becomes X- (which is a base). WHY?? Because in the reverse reaction, the X-accepts the H+ ion.

  14. Brønsted-Lowry Model • The forward and reverse reactions are BOTH reactions of an acid and base. • The acid and base that react in the reverse reaction are called the conjugate acid and base. • The conjugate acid is the substance produced in the forward reaction when the base accepts H+ from an acid. • The conjugate base is the substance leftover after the acid has donated H+ to a base in the forward reaction.

  15. Practice Problems • Identify the conjugate acid and the conjugate base in the reactions from before: • H3O+ + OH- H2O + H2O • HCl + NH3 NH4+ + Cl- • S-2 + H2O  HS- + OH- • HS- + H2O  S-2 + H3O+ • H2O + HC2H3O2 C2H3O2-+ H3O+ • C2H3O2-+ H3O+ H2O + HC2H3O2

  16. Brønsted-Lowry Model HX (aq) + H2O H3O+ + X- • The hydronium ion is the conjugate acid of the base water. • The X- ion is the conjugate base of the acid HX. • A conjugate acid-base pair are two substances that are related to each other by the donating & accepting of a H+ ion.

  17. Examples HCO3- & H2CO3 are a conjugate acid-base pair. H2O & OH- are also a conjugate acid-base pair.

  18. Examples

  19. Water • Water (& other substances) that can act as both an acid & a base are said to be amphoteric. • HF + H2O H3O+ + F- BASE • NH3 + H2O NH4+ + OH- ACID

  20. Practice Problems Identify the conjugate acid-base pairs in the following reactions: • HSO4- + H2O H3O+ + SO4- • CO3-2 + H2O HCO3- + OH- • NH4+ + OH- NH3 + H2O • OH- + HC2H3O2 C2H3O2- + H2O

  21. Monoprotic and Polyprotic Acids An acid that can donate only one hydrogen ion is a monoprotic acid. Acids that can donate more than one hydrogen ion are polyprotic acids. See Table 1 on p. 641. Which acids are monoprotic? polyprotic?

  22. The Lewis Model According to the Lewis model, a Lewis acid is an electron-pair acceptor and a Lewis base is an electron pair donor. The Lewis model includes all the substances classified as Brønsted-Lowry acids and bases and many more.

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