👩🏽‍🔬Honors Chemistry Unit 10 – Acids, Bases, and pH

Key Concepts

• Acids donate protons $\ce{(H+)}$ while bases accept protons in aqueous solutions
• Arrhenius theory defines acids as $\ce{H+}$ donors and bases as $\ce{OH-}$ donors
  • Limited to aqueous solutions and does not explain all acid-base behaviors
• Brønsted-Lowry theory expands the definition of acids and bases
  • Acids are proton donors and bases are proton acceptors
  • Includes a wider range of substances and reactions (ammonia, amino acids)
• Lewis theory further broadens the definition of acids and bases
  • Acids are electron pair acceptors and bases are electron pair donors
  • Encompasses more substances and reactions than Brønsted-Lowry theory (boron trifluoride, silver ion)
• Conjugate acid-base pairs consist of a species and its corresponding acid or base formed by the loss or gain of a proton
  • Example: $\ce{HCl}$ (acid) and $\ce{Cl-}$ (conjugate base), $\ce{NH3}$ (base) and $\ce{NH4+}$ (conjugate acid)
• Autoionization of water produces $\ce{H+}$ and $\ce{OH-}$ ions with a constant equilibrium at 25°C
  • $\ce{H2O + H2O <=> H3O+ + OH-}$, $K_w = [\ce{H+}][\ce{OH-}] = 1.0 \times 10^{-14}$

Acid-Base Theories

• Arrhenius theory recognizes acids as substances that increase $\ce{H+}$ concentration and bases as substances that increase $\ce{OH-}$ concentration in aqueous solutions
• Brønsted-Lowry theory defines acids as proton donors and bases as proton acceptors
  • Allows for the classification of substances without $\ce{OH-}$ as bases (ammonia, amines)
  • Explains the basic properties of substances like $\ce{NH3}$ and their reactions with acids
• Lewis theory identifies acids as electron pair acceptors and bases as electron pair donors
  • Includes a broader range of reactions beyond proton transfer (formation of complex ions)
  • Explains the acidity of substances without dissociable protons (boron trifluoride)
• Conjugate acid-base pairs are related by the loss or gain of a proton
  • A strong acid has a weak conjugate base, while a weak acid has a strong conjugate base
  • Example: $\ce{HCl}$ (strong acid) and $\ce{Cl-}$ (weak base), $\ce{CH3COOH}$ (weak acid) and $\ce{CH3COO-}$ (strong base)
• Water acts as both an acid and a base, dissociating into $\ce{H+}$ and $\ce{OH-}$ ions
  • Amphiprotic nature allows water to react with both acids and bases
  • Equilibrium constant $K_w = [\ce{H+}][\ce{OH-}] = 1.0 \times 10^{-14}$ at 25°C

Properties of Acids and Bases

• Acids taste sour, react with metals to produce hydrogen gas, and turn blue litmus paper red
  • Examples: citric acid in lemons, acetic acid in vinegar, hydrochloric acid in stomach
• Bases taste bitter, feel slippery, and turn red litmus paper blue
  • Examples: sodium hydroxide in drain cleaner, ammonia in cleaning products, baking soda
• Strong acids and bases completely dissociate in aqueous solutions, while weak acids and bases only partially dissociate
  • Strong acids: $\ce{HCl}$, $\ce{HNO3}$, $\ce{H2SO4}$; strong bases: $\ce{NaOH}$, $\ce{KOH}$
  • Weak acids: $\ce{CH3COOH}$, $\ce{HF}$; weak bases: $\ce{NH3}$, $\ce{CH3NH2}$
• Acids and bases conduct electricity due to the presence of ions in solution
  • Strong acids and bases are strong electrolytes, while weak acids and bases are weak electrolytes
• Acids react with bases to form salt and water in neutralization reactions
  • Example: $\ce{HCl + NaOH -> NaCl + H2O}$
• Acids react with carbonates and bicarbonates to produce carbon dioxide gas
  • Used in baking (baking soda + vinegar) and in fire extinguishers

pH Scale and Calculations

• pH is a logarithmic scale that measures the acidity or basicity of a solution
  • Defined as the negative logarithm of the hydrogen ion concentration: $\text{pH} = -\log[\ce{H+}]$
  • Lower pH values indicate higher acidity, while higher pH values indicate higher basicity
• The pH scale ranges from 0 to 14, with 7 being neutral (pure water at 25°C)
  • Acidic solutions have pH < 7, while basic solutions have pH > 7
  • Each unit change in pH represents a tenfold change in $\ce{H+}$ concentration
• pOH is the negative logarithm of the hydroxide ion concentration: $\text{pOH} = -\log[\ce{OH-}]$
  • pH and pOH are related by the equation: $\text{pH} + \text{pOH} = 14$
• Calculating pH involves using the negative logarithm of the $\ce{H+}$ concentration
  • For strong acids, $[\ce{H+}]$ equals the acid concentration
  • For weak acids, use the acid dissociation constant $K_a$ and the initial acid concentration
• Buffer solutions resist changes in pH when small amounts of acid or base are added
  • Consist of a weak acid and its conjugate base, or a weak base and its conjugate acid
  • Example: acetic acid $\ce{(CH3COOH)}$ and sodium acetate $\ce{(CH3COONa)}$

Strength of Acids and Bases

• Strength of acids and bases depends on their ability to dissociate in aqueous solutions
  • Strong acids and bases completely dissociate, while weak acids and bases partially dissociate
• Acid strength is measured by the acid dissociation constant $K_a$
  • Higher $K_a$ values indicate stronger acids, while lower $K_a$ values indicate weaker acids
  • Example: $\ce{HCl}$ $(K_a \approx 10^7)$ is a strong acid, $\ce{CH3COOH}$ $(K_a = 1.8 \times 10^{-5})$ is a weak acid
• Base strength is measured by the base dissociation constant $K_b$
  • Higher $K_b$ values indicate stronger bases, while lower $K_b$ values indicate weaker bases
  • Example: $\ce{NaOH}$ $(K_b \approx 10^7)$ is a strong base, $\ce{NH3}$ $(K_b = 1.8 \times 10^{-5})$ is a weak base
• The strength of an acid or base determines the pH of its solution
  • Strong acids and bases produce solutions with extreme pH values (pH < 3 or pH > 11)
  • Weak acids and bases produce solutions with moderate pH values closer to neutral
• Acid and base strength also influence the strength of their conjugate pairs
  • A strong acid has a weak conjugate base, while a weak acid has a strong conjugate base
  • Example: $\ce{HCl}$ (strong acid) and $\ce{Cl-}$ (weak base), $\ce{CH3COOH}$ (weak acid) and $\ce{CH3COO-}$ (strong base)

Neutralization Reactions

• Neutralization reactions occur when an acid and a base react to form salt and water
  • The $\ce{H+}$ ions from the acid combine with the $\ce{OH-}$ ions from the base to form $\ce{H2O}$
  • The remaining ions from the acid and base form a salt
• The general equation for a neutralization reaction is: $\ce{Acid + Base -> Salt + Water}$
  • Example: $\ce{HCl + NaOH -> NaCl + H2O}$
• Titration is a technique used to determine the concentration of an acid or base solution
  • A solution of known concentration (titrant) is gradually added to the unknown solution (analyte)
  • The endpoint is reached when the acid and base have completely reacted, often indicated by a color change
• Stoichiometry is used to calculate the concentrations of the acid and base in a neutralization reaction
  • The mole ratio of acid to base is determined by balancing the chemical equation
  • Example: For $\ce{HCl + NaOH -> NaCl + H2O}$, the mole ratio is 1:1
• Neutralization reactions have many practical applications
  • Used in the production of salts, pH control in industrial processes, and wastewater treatment
  • Example: Antacids (calcium carbonate, magnesium hydroxide) neutralize excess stomach acid

Buffers and Applications

• Buffers are solutions that resist changes in pH when small amounts of acid or base are added
  • Consist of a weak acid and its conjugate base, or a weak base and its conjugate acid
  • Example: Acetic acid $\ce{(CH3COOH)}$ and sodium acetate $\ce{(CH3COONa)}$
• Buffer capacity is the amount of acid or base that can be added before the pH changes significantly
  • Depends on the concentrations of the weak acid and its conjugate base (or weak base and its conjugate acid)
  • Higher concentrations provide greater buffer capacity
• The Henderson-Hasselbalch equation relates the pH of a buffer solution to the $pK_a$ of the acid and the ratio of the concentrations of the acid and its conjugate base
  • $\text{pH} = pK_a + \log \frac{[\text{Base}]}{[\text{Acid}]}$
  • Useful for preparing buffer solutions with a specific pH
• Buffers play crucial roles in biological systems, maintaining the pH within optimal ranges
  • Example: Bicarbonate buffer system in blood maintains pH between 7.35 and 7.45
  • Enzymes and other proteins require specific pH ranges for proper function
• Buffers are also used in various industrial and laboratory applications
  • pH control in fermentation processes, food production, and drug manufacturing
  • Maintaining stable pH in analytical chemistry experiments and chromatography

Real-World Examples and Lab Work

• Acids and bases are encountered in many aspects of daily life
  • Foods and beverages: citric acid in citrus fruits, acetic acid in vinegar, lactic acid in yogurt
  • Cleaning products: hydrochloric acid in toilet bowl cleaner, ammonia in window cleaner
  • Personal care products: citric acid in shampoo, sodium hydroxide in soap
• Environmental examples of acids and bases include:
  • Acid rain caused by sulfuric and nitric acids from fossil fuel combustion
  • Ocean acidification due to increased absorption of atmospheric carbon dioxide
  • Alkaline soils in arid regions due to the presence of calcium and magnesium carbonates
• Laboratory experiments demonstrate the properties and reactions of acids and bases
  • Measuring pH using pH paper, pH meters, or acid-base indicators (phenolphthalein, methyl orange)
  • Titration experiments to determine the concentration of an acid or base solution
  • Observing neutralization reactions and the formation of salts
• Acid-base chemistry has numerous applications in industry and research
  • Production of fertilizers, detergents, and pharmaceuticals
  • Water treatment and purification processes
  • Developing new materials and catalysts for chemical reactions
• Understanding acid-base concepts is essential for many scientific disciplines
  • Biology: pH regulation in cells, enzyme function, and physiological processes
  • Environmental science: water and soil chemistry, pollution control
  • Materials science: synthesis and characterization of new compounds and materials