Lesson 1 Acids and Bases Pure water has an equal concentration of the hydronium ion H3O+ (aq) and the hydroxide ion OH–(aq) because it self ionizes: H2O(l) ↔ H3O+ (aq) + OH–(aq) Acids and bases are substances that disrupt the balance of the hydronium ion and the hydroxide ion in a water solution. Because of LeChatelier’s Principle, as the concentration of one of the ions increases, the concentration of the other will decrease.

Properties of Acids An acid is any substance that releases hydrogen ions, H+, into water (making more hydronium ions.) So the hydronium ion concentration is greater than the hydroxide concentration in the solution. Blue litmus paper turns red in the presence of hydrogen ions. Blue litmus is used to test for acids. Acids have a sour taste; lemons, limes, and vinegar are acidic. Acids are electrolytic.

Properties of Bases A base is a substance that releases hydroxide ions, OH –, into water, and therefore hydroxide ion concentration is greater than hydronium ion concentration in the solution. Red litmus paper turns blue in the presence of hydroxide ions. Red litmus is used to test for bases. Bases have a slippery, soapy feel. Bases also have a bitter taste. taste; Milk of magnesia is a base. Bases are electrolytic. is a ba

Electrolyte Strength Strong acids and bases are strong electrolytes. Weak acids and bases are weak electrolytes.

Acid/Base Neutralization Recall that an acid and a base react with each other in a neutralization reaction. When an acid and a base react, water and a salt are produced. This a type of double replacement reaction. For example, nitric acid reacts with sodium hydroxide to produce sodium nitrate and water: HNO3(aq) + NaOH(aq) → NaNO3(aq) + H2O(l)

The pH Scale A pH value expresses the acidity or basicity of a solution. Most solutions have a pH between 0 and 14. Acidic solutions have a pH less than 7. As a solution becomes more acidic, the pH decreases. Basic solutions have a pH greater than 7. As a solution becomes more basic, the pH increases.

Acid/Base Classifications of Solutions A water solution can be classified according to its pH. Strongly acidic solutions have a pH less than 2. Weakly acidic solutions have a pH between 2 and 7. Weakly basic solutions have a pH between 7 and 12. Strongly basic solutions have a pH greater than 12. Neutral solutions have a pH of 7.

Arrhenius Acids and Bases Svante Arrhenius proposed the following definitions for acids and bases in 1884: An Arrhenius acid is a substance that ionizes in water to produce hydrogen ions. An Arrhenius base is a substance that dissociates in water to release hydroxide ions. For example, HCl is an Arrhenius acid and NaOH is an Arrhenius base.

Strengths of Acids Acids have varying strengths. The strength of an Arrhenius acid is measured by the degree of ionization in solution. Ionization is the process where polar compounds separate into cations and anions in solution. The acid HCl ionizes into H+ and Cl– ions in solution.

Strengths of Bases Bases also have varying strengths. The strength of an Arrhenius base is measured by the degree of dissociation in solution. Dissociation is the process where cations and anions in an ionic compound separate in solution. A formula unit of NaOH dissociates into Na+ and OH– ions in solution.

Strong and Weak Arrhenius Acids Strong acids ionize extensively to release hydrogen ions into solution. HCl is a strong acid and ionizes nearly 100%. Weak acids only ionize slightly in solution. HF is a weak acid and ionizes only about 1%.

Arrhenius Acids in Solution All Arrhenius acids have a hydrogen atom bonded to the rest of the molecule by a polar bond. This bond is broken when the acid ionizes. Polar water molecules help ionize the acid by pulling the hydrogen atom away: HCl(aq) + H2O(l) → H3O+(aq) + Cl–(aq) (~100%) HC2H3O2(aq) + H2O(l)   H3O+(aq) + C2H3O2–(aq) (~1%) The hydronium ion, H3O+, is formed when the aqueous hydrogen ion attaches to a water molecule.

Strong and Weak Arrhenius Bases Strong bases dissociate extensively to release hydroxide ions into solution. NaOH is a strong base and dissociates nearly 100%. Weak bases only dissociate slightly in solution. NH4OH is a weak base and only partially dissociates

Arrhenius Bases in Solution When we dissolve Arrhenius bases in solution, they dissociate, giving a cation and a hydroxide anion. Strong bases dissociate almost fully, and weak bases dissociate very little: NaOH(aq) → Na+(aq) + OH–(aq) (~100%) NH4OH(aq)   NH4+(aq) + OH–(aq) (~1%)

Chemistry Connection: Svante Arrhenius Svante Arrhenius noted that NaCl solutions conducted electricity; while sugar solutions did not. He also noticed that the freezing point of NaCl solutions were lowered twice as much as sugar solutions at the same concentration. He proposed that NaCl produces ions when dissolved, while sugar was in solution as molecules. It was nearly 20 years before his ideas were accepted by the scientific community.