Sec. 18.1: Acids & Bases: An Introduction 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.

Properties 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

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  ?

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+].

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.

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.

Examples An Arrhenius acid A Arrhenius base 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.

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.

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

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

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

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.

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.

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

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.

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

Examples

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

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

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?

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.