Acids and Bases: Properties, Reactions, and Quantitative Analysis

Acids and Bases

Introduction to Acids and Bases

Acids and bases are two fundamental categories of compounds in chemistry, each with distinct properties and behaviors. Their study is essential for understanding chemical reactions, biological processes, and industrial applications.

• Acids are substances that produce hydrogen ions (H+) in aqueous solution.

• Bases are substances that produce hydroxide ions (OH−) in aqueous solution.

Properties of Acids

• Sour taste: Many acids, such as citric acid in lemons, taste sour.

• Reaction with metals: Acids can dissolve many metals, producing hydrogen gas.

• Litmus test: Acids turn blue litmus paper red.

• Examples: Hydrochloric acid (HCl), sulfuric acid (H2SO4), acetic acid (CH3COOH).

• Industrial and biological roles: Used in cleaning, food processing, and as a component of stomach acid.

Example: The sour taste of candies like Sour Patch Kids is due to acids such as citric and tartaric acid, which react with proteins on the tongue to produce a sour sensation.

Properties of Bases

• Bitter taste: Bases often taste bitter (e.g., caffeine in coffee).

• Slippery feel: Bases feel slippery because they react with oils on the skin to form soap-like substances.

• Litmus test: Bases turn red litmus paper blue.

• Examples: Sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonia (NH3).

• Household uses: Found in cleaning products and soaps.

Definitions of Acids and Bases

The Arrhenius Definition

• Acid: Produces H+ ions in aqueous solution.

• Base: Produces OH− ions in aqueous solution.

• Limitation: Only applies to aqueous solutions and does not explain all acid-base behavior.

The Brønsted–Lowry Definition

• Acid: Proton (H+) donor.

• Base: Proton (H+) acceptor.

• Broader scope: Explains acid-base reactions in non-aqueous solutions and substances that do not contain OH−.

• Amphoteric substances: Can act as either an acid or a base (e.g., water).

Conjugate Acid–Base Pairs

• When an acid donates a proton, it becomes its conjugate base.

• When a base accepts a proton, it becomes its conjugate acid.

• Acid–base reactions always involve conjugate pairs.

Reactions of Acids and Bases

Neutralization Reactions

When an acid and a base are mixed, they react to form water and a salt. This process is called neutralization.

• General equation: $\mathrm{H^+ (aq) + OH^- (aq) \rightarrow H_2O (l)}$

• The salt formed contains the cation from the base and the anion from the acid.

Reactions with Metals and Metal Oxides

• Acids react with many metals to produce hydrogen gas and a dissolved salt.

• Acids react with metal oxides to produce water and a dissolved salt.

• Bases can react with certain metals (e.g., aluminum) to produce hydrogen gas.

Quantitative Analysis: Titration

Acid–Base Titration

Titration is a laboratory technique used to determine the concentration of an unknown acid or base solution by reacting it with a solution of known concentration.

• A solution of known concentration (the titrant) is added to the unknown until the reaction reaches the equivalence point (when moles of acid = moles of base).

• An indicator is used to signal the equivalence point, often by a color change.

• Calculation: Use the volume and concentration of the titrant to find the concentration of the unknown.

Example: If 12.54 mL of 0.100 M NaOH is required to neutralize 10.00 mL of HCl, the concentration of HCl is calculated as follows:

• Calculate moles of NaOH: $0.01254 \; \mathrm{L} \times 0.100 \; \mathrm{mol/L} = 0.001254 \; \mathrm{mol}$

• Since the reaction is 1:1, moles of HCl = moles of NaOH

• Concentration of HCl: $0.001254 \; \mathrm{mol} / 0.01000 \; \mathrm{L} = 0.125 \; \mathrm{M}$

Strong and Weak Acids and Bases

Strong Acids and Bases

• Strong acids completely ionize in solution (e.g., HCl, HNO3).

• Strong bases completely dissociate in solution (e.g., NaOH, KOH).

• Strong electrolytes conduct electricity well due to the presence of many ions.

Weak Acids and Bases

• Weak acids only partially ionize in solution (e.g., HF, acetic acid).

• Weak bases only partially react with water to produce OH− (e.g., ammonia, organic amines).

• Weak electrolytes conduct electricity poorly due to fewer ions.

Water: Self-Ionization and the Ion Product Constant

Self-Ionization of Water

• Water can act as both an acid and a base (amphoteric).

• Self-ionization: $2 \mathrm{H_2O (l)} \rightleftharpoons \mathrm{H_3O^+ (aq)} + \mathrm{OH^- (aq)}$

• Ion product constant at 25°C: $K_w = [\mathrm{H_3O^+}][\mathrm{OH^-}] = 1.0 \times 10^{-14}$

Acidic, Basic, and Neutral Solutions

• Neutral: $[\mathrm{H_3O^+}] = [\mathrm{OH^-}] = 1.0 \times 10^{-7} \; \mathrm{M}$

• Acidic: $[\mathrm{H_3O^+}] > [\mathrm{OH^-}]$

• Basic: $[\mathrm{H_3O^+}] < [\mathrm{OH^-}]$

The pH and pOH Scales

Definition and Calculation

• pH: $\mathrm{pH} = -\log [\mathrm{H_3O^+}]$

• pOH: $\mathrm{pOH} = -\log [\mathrm{OH^-}]$

• At 25°C: $\mathrm{pH} + \mathrm{pOH} = 14$

• pH < 7: acidic; pH = 7: neutral; pH > 7: basic

Example: If $[\mathrm{H_3O^+}] = 1.8 \times 10^{-4} \; \mathrm{M}$, then $\mathrm{pH} = -\log(1.8 \times 10^{-4}) = 3.74$ (acidic).

Buffers

Buffer Solutions

• Buffers contain significant amounts of both a weak acid and its conjugate base.

• They resist changes in pH when small amounts of acid or base are added.

• Example: Acetic acid and sodium acetate buffer system.

• Human blood is a buffer, maintaining pH between 7.36 and 7.40.

Applications and Environmental Impact

Acid Rain

• Formed when sulfur oxides and nitrogen oxides react with water in the atmosphere to produce acids.

• Acid rain can dissolve metals and damage buildings and ecosystems.

Health Connection: Ethylene Glycol Poisoning

• Ethylene glycol (antifreeze) is metabolized to glycolic acid, which can overwhelm the blood's buffer system and lower blood pH to dangerous levels.

• Severe acidosis can lead to coma and death if untreated.