Section 18.1: Introduction to Acids and Bases

Essential Questions and Model Comparison

• What are the physical and chemical properties of acids and bases?
• How are solutions classified as acidic, basic, or neutral?
• How do the Arrhenius, Brønsted-Lowry, and Lewis models of acids and bases compare?

Vocabulary Reference

• Acidic solution: A solution containing more hydrogen ions than hydroxide ions.
• Basic solution: A solution containing more hydroxide ions than hydrogen ions.
• Arrhenius model: States that an acid contains hydrogen and ionizes to produce hydrogen ions ($H^+$) in aqueous solution; a base contains a hydroxide group and dissociates to produce a hydroxide ion ($OH^-$) in aqueous solution.
• Brønsted-Lowry model: An acid is a hydrogen-ion donor; a base is a hydrogen-ion acceptor.
• Conjugate acid: The species produced when a base accepts a hydrogen ion.
• Conjugate base: The species that results when an acid donates a hydrogen ion.
• Conjugate acid-base pair: Consists of two substances related to each other by the donating and accepting of a single hydrogen ion.
• Amphoteric: A substance, such as water, that can act as both an acid and a base.
• Lewis model: An acid is an electron-pair acceptor and a base is an electron-pair donor.
• Lewis structure: A model using electron-dot structures to show electron arrangement in molecules.

Properties of Acids and Bases

• Everyday Contexts:   - Acids are recognized by the tart taste of beverages and foods (stomach acid aids digestion).   - Ants emit formic acid to alert the colony of danger.   - Bases are found in household cleaners (ammonia), soaps, and antacid tablets.

• Physical Properties:   - Taste: Acidic solutions taste sour; basic solutions taste bitter.   - Feel: Basic solutions feel slippery (e.g., wet bar of soap).   - Touch/Taste Warning: One should never identify acids or bases in the lab by taste or feel.   - Indicator Reaction: Acids turn blue litmus red; bases turn red litmus blue.   - Biological Indicators: Rhododendrons flourish in moderately acidic ("sour") soil, while sempervivum ("hen and chicks") grow best in slightly basic (alkaline) soil.

• Electrical Conductivity:   - Pure water is a non-conductor.   - The addition of an acid or base produces ions that cause the resulting solution to become a conductor of electricity.

• Chemical Properties (Acids):   - Reaction with Metals: Magnesium ($Mg$) and zinc ($Zn$) react with aqueous acid solutions to produce hydrogen gas ($H_2(g)$).     - Example: $Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$   - Reaction with Carbonates: Metal carbonates/hydrogen carbonates react with aqueous acids to produce carbon dioxide ($CO_2$) gas.     - Example: Baking soda ($NaHCO_3$) + vinegar (acetic acid, $HC_2H_3O_2$):       $NaHCO_3(s) + HC_2H_3O_2(aq) \rightarrow NaC_2H_3O_2(aq) + H_2O(l) + CO_2(g)$     - Geologists use a hydrochloric acid solution ($HCl$) to identify limestone (primarily $CaCO_3$) by testing for CO2 bubbling.

Hydrogen and Hydroxide Ions

• Relative Amounts: The concentrations of hydrogen ions ($H^+$) and hydroxide ions ($OH^-$) determine if a solution is acidic, basic, or neutral.   - Acidic: $[H^+] > [OH^-]$   - Basic: $[OH^-] > [H^+]$   - Neutral: $[H^+] = [OH^-]$
• Self-Ionization of Water: Water molecules react to form hydronium and hydroxide ions through a process where one water molecule donates a proton to another:   $H_2O(l) + H_2O(l) \rightleftharpoons H_3O^+(aq) + OH^-(aq)$
• Interchangeability: The symbols $H^+$ and $H_3O^+$ represent the hydrogen ion and hydronium ion (a hydrogen ion covalently bonded to a water molecule) and are used interchangeably:   $H_2O(l) \rightleftharpoons H^+(aq) + OH^-(aq)$

History and Evolution of Acid-Base Theory

• 1865: Antiseptic spray containing carbolic acid begins modern antiseptic surgery.
• 1869: Nucleic acids (DNA/RNA) are discovered in cell nuclei.
• 1883: Svante Arrhenius proposes that acids produce $H^+$ and bases produce $OH^-$ in water.
• 1909: Development of the pH scale.
• 1923: Refinement of definitions currently in use (Brønsted-Lowry and Lewis models).
• 1933-1934: Portable pH meters are developed.
• 1953: Watson, Crick, and Franklin study DNA structure.
• 1963: Discovery of acid rain in North America; polluted rain found to be $100 \times$ more acidic than unpolluted rain.
• 1980s: Silicon-chip pH meters introduced (no glass component) for food, cosmetics, and pharmacy.
• 2005: Development of super-acids (more acidic than $100\%$ sulfuric acid) for plastic and gasoline production.
• 2010: National Institute of Standards and Technology (NIST) develops techniques to monitor nanoparticle stability via acidity changes to target tumor cells.

The Arrhenius Model

• Definition: An acid is a substance containing hydrogen that ionizes in aqueous solution to produce $H^+$ ions. A base contains a hydroxide group ($OH$) and dissociates to produce $OH^-$ ions.
• Examples:   - Hydrogen chloride gas in water: $HCl(g) \rightarrow H^+(aq) + Cl^-(aq)$   - Sodium hydroxide: $NaOH(s) \rightarrow Na^+(aq) + OH^-(aq)$
• Shortcomings: It cannot explain the alkalinity of substances like ammonia ($NH_3$) or sodium carbonate ($Na_2CO_3$) which do not contain hydroxide groups but still produce hydroxide ions in solution. (Example: Lake Natron in Tanzania contains high alkalinity from dissolved $Na_2CO_3$).

The Brønsted-Lowry Model

• Definition: Focuses on hydrogen ion ($H^+$) transfer. Acid = hydrogen-ion donor; Base = hydrogen-ion acceptor.
• Conjugate Pairs Mechanism:   - Equation: $HX(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + X^-(aq)$   - Forward reaction: $HX$ (acid) and $H_2O$ (base).   - Reverse reaction: $H_3O^+$ (conjugate acid) and $X^-$ (conjugate base).
• Metaphor: A father throwing a ball to his son. The father (acid) has the ball ($H^+$) to throw; the son (base) is ready to catch it. Once the son has the ball, he becomes the conjugate acid (now able to donate).
• Ammonia Example: $NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$   - $NH_3$ is the base; $H_2O$ is the acid.   - $NH_4^+$ is the conjugate acid; $OH^-$ is the conjugate base.

Ionizability and Bonding

• Ionizable Hydrogens: Not all hydrogens in a molecule can be donated. Ionization depends on bond polarity.   - Acetic Acid ($HC_2H_3O_2$): Only the hydrogen bonded to the highly electronegative oxygen atom is ionizable. The other three C-H hydrogens are in nonpolar bonds.   - Benzene ($C_6H_6$): Bonds are nonpolar (little electronegativity difference between C and H); therefore, benzene is not an acid.
• Monoprotic vs. Polyprotic:   - Monoprotic: Can donate only one $H^+$ (e.g., $HCl$, $HNO_3$, $HBr$, $HC_2H_3O_2$, $HCN$, $HF$).   - Polyprotic: Contain more than one ionizable hydrogen.     - Diprotic: Two $H^+$ (e.g., $H_2SO_4$, $H_2CO_3$).     - Triprotic: Three $H^+$ (e.g., $H_3PO_4$, $H_3BO_3$).
• Stepwise Ionization: Polyprotic acids ionize in distinct stages.   - Example: Phosphoric Acid ($H_3PO_4$):     1. $H_3PO_4(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + H_2PO_4^-(aq)$     2. $H_2PO_4^-(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + HPO_4^{2-}(aq)$     3. $HPO_4^{2-}(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + PO_4^{3-}(aq)$

The Lewis Model

• Definition: The most general model. Lewis acid = electron-pair acceptor (vacant orbital). Lewis base = electron-pair donor (lone pair).
• Mechanism Examples:   - H+ and Fluoride: $H^+$ (Lewis acid with vacant $1s$ orbital) accepts an electron pair from $F^-$ (Lewis base with lone pair) to form $HF$.   - Boron Trifluoride and Ammonia: $BF_3$ (Lewis acid with vacant $2p$ orbital) reacts with gaseous $NH_3$ (Lewis base with lone pair on N).   - Sulfur Trioxide and Magnesium Oxide: $SO_3(g) + MgO(s) \rightarrow MgSO_4(s)$. The $SO_3$ molecule (Lewis acid) accepts an electron pair from the oxide ion $O^{2-}$ (Lewis base) to form the sulfate ion ($SO_4^{2-}$).

Questions & Discussion

• 1. Balanced Equations Practice:   - a. aluminum and sulfuric acid: $2Al(s) + 3H_2SO_4(aq) \rightarrow Al_2(SO_4)_3(aq) + 3H_2(g)$   - b. calcium carbonate and hydrobromic acid: $CaCO_3(s) + 2HBr(aq) \rightarrow CaBr_2(aq) + H_2O(l) + CO_2(g)$
• 2. Challenge Question: Write the net ionic equation for the reaction of calcium carbonate and hydrobromic acid.
• 3. Conjugate Pairs Identification:   - a. $NH_4^+(aq) + OH^-(aq) \rightleftharpoons NH_3(aq) + H_2O(l)$: Acid ($NH_4^+$), Base ($OH^-$), Conjugate Base ($NH_3$), Conjugate Acid ($H_2O$).   - b. $HBr(aq) + H_2O(l) \rightarrow H_3O^+(aq) + Br^-(aq)$: Acid ($HBr$), Base ($H_2O$), Conjugate Acid ($H_3O^+$), Conjugate Base ($Br^-$).   - c. $CO_3^{2-}(aq) + H_2O(l) \rightleftharpoons HCO_3^-(aq) + OH^-(aq)$: Base ($CO_3^{2-}$), Acid ($H_2O$), Conjugate Acid ($HCO_3^-$), Conjugate Base ($OH^-$).
• 4. Challenge Reaction Construction: Products are $H_3O^+$ and $SO_4^{2-}$. Identify reactant species and pairs.   - Reaction: $HSO_4^-(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + SO_4^{2-}(aq)$   - Acid ($HSO_4^-$), Base ($H_2O$), Conjugate Acid ($H_3O^+$), Conjugate Base ($SO_4^{2-}$).