Module 11: Chemical Reactions - Acid-Base Reactions

Module 11: Chemical Reactions - Acid-Base Reactions

Overview of Acid-Base Reactions

• Definition: In acid-base reactions, an acid reacts with a base, yielding a salt and water.

Strong Acid and Strong Base Reactions

• When a strong acid reacts with a strong base, the reaction is known as:

  • Titration Reaction (also referred to as Neutralization Reaction).

• Strong acids and strong bases are classified as strong electrolytes:

  • They fully dissociate in water.

Example 1: Hydrochloric Acid and Sodium Hydroxide

• Molecular Equation:

  • \text{HCl (aq)} + \text{NaOH (aq)} \rightarrow \text{H}_2\text{O (l)} + \text{NaCl (aq)}

• Complete Ionic Equation:

  • \text{H}^+ (aq) + \text{Cl}^- (aq) + \text{Na}^+ (aq) + \text{OH}^- (aq) \rightarrow \text{Na}^+ (aq) + \text{Cl}^- (aq) + \text{H}_2\text{O (l)}

• Spectator Ions:

  • Sodium ion ($\text{Na}^+$) and chloride ion ($\text{Cl}^-$) are spectator ions.

• Net Ionic Equation:

  • \text{H}^+ (aq) + \text{OH}^- (aq) \rightarrow \text{H}_2\text{O (l)}

• Clarification:

  • Note that $\text{H}^+$ cannot exist in water and forms hydronium ions: \text{H}_3\text{O}^+ .

• Adjusted Net Ionic Equation:

  • H3O+(aq)+OH−(aq)→2H2O (l)

• pH of Reaction:

  • At the stoichiometric point (equivalence point), the pH is neutral, i.e., pH = 7 .

  • Resulting salt is neutral (neither acidic nor basic).

Example 2: Nitric Acid and Barium Hydroxide

• Balanced Molecular Equation:

  • 2HNO3(aq)+Ba(OH)2(aq)→Ba(NO3)2(aq)+2H2​O (l)

• Complete Ionic Equation:

  • 2H+(aq)+2NO3−(aq)+Ba2+(aq)+2OH−(aq)→Ba2+(aq)+2NO3−(aq)+2H2​O (l)

• Spectator Ions:

  • Barium ion ($\text{Ba}^{2+}$) and nitrate ion ($\text{NO}_3^-$) are spectator ions.

• Net Ionic Equation:

  • Remaining equation is simplified:

  • \text{H}^+ (aq) + \text{OH}^- (aq) \rightarrow \text{H}_2\text{O (l)}

  • Final Net Ionic Equation:

  • H3O+(aq)+OH−(aq)→2H2O (l)

• pH at Equivalence:

  • pH remains 7 (neutral).

Neutralization of Weak Acids by Strong Bases and Vice Versa

• Key Concept:

  • Weak acids/bases react with strong acids/bases to complete reactions.

  • Strong acids/bases ensure that reactions go to completion.

Reaction of Weak Acid with Strong Base

• Example: Acetic Acid and Sodium Hydroxide

• Molecular Equation:

  • CH3COOH (aq)+NaOH (aq)→CH3COONa (aq)+H2​O (l)

• Complete Ionic Equation:

  • CH3COOH (aq)+Na+(aq)+OH−(aq)→CH3COO−(aq)+Na+(aq)+H2​O (l)

• Spectator Ion:

  • Sodium ion ($\text{Na}^+$) is the only spectator ion.

• Net Ionic Equation:

  • CH3COOH (aq)+OH−(aq)→CH3COO−(aq)+H2​O (l)

• pH at Endpoint:

  • Solution consists of sodium acetate (weak base); pH > 7 (slightly basic), approximately 8.

Reaction of Weak Base with Strong Acid

• Example: Ammonia and Hydrobromic Acid

• Molecular Equation:

  • NH3(aq)+HBr (aq)→NH4Br (aq)

• Complete Ionic Equation:

  • NH3(aq)+H+(aq)+Br−(aq)→NH4+(aq)+Br−(aq)

• Spectator Ion:

  • Bromide ion ($\text{Br}^-$) is the spectator ion.

• Net Ionic Equation:

  • NH3(aq)+H3O+(aq)→NH4+(aq)+H2O (l)

• pH at Endpoint:

  • Solution consists of ammonium ion (weak acid); pH < 7 (slightly acidic).

Titration Example: Methylamine with HCl

• Molecular Equation:

  • CH3NH2(aq)+HCl (aq)→CH3NH3Cl(aq)

• Complete Ionic Equation:

• CH3NH2(aq)+H+(aq)+Cl−(aq)→CH3NH3+(aq)+Cl−(aq)

• Spectator Ion:

  • Chloride ion ($\text{Cl}^-$) is the spectator ion.

• Net Ionic Equation:

  • CH3NH2(aq)+H3O+(aq)→CH3NH3+(aq)+H2O (l)

Stoichiometric Calculation

• When 25 mL of 0.05 M HCl is added to 25 mL of 0.05 M methylamine:

  • Calculation of Moles:

  • Moles of HCl = Concentration × Volume = 0.05 imes 0.025 = 0.00125 moles.

  • Moles of methylamine = 0.05 imes 0.025 = 0.00125 moles.

  • Stoichiometry:

  • 1:1 ratio, thus both reactants are fully consumed.

  • Remaining in solution: Methyl ammonium chloride, with a pH slightly acidic (below 7).

Example of Acid-Base Gas Formation Reaction

• Example: Sodium Carbonate and HCl

• Molecular Equation:

  • Na2CO3+2HCl→2NaCl+H2CO3
    (Will decompose to water and CO$_2$)

• **Balanced Equation After Decomposition: **

  • Na2CO3+2HCl→2NaCl+H2O+CO2(g)

• Complete Ionic Equation and Net Ionic Equation:

  • Utilize strong electrolytes for sodium carbonate and HCl, cancel out spectator ions for clarity.

Reaction of Potassium Sulfite with HBr

• Molecular Equation:

  • K2SO3+2HBr→SO2(g)+H2O+2KBr

• Complete Ionic Equation:

  • Dissociate all strong electrolytes in the equation to identify spectator ions.

• Net Ionic Equation:

  • SO32−(aq)+2H+(aq)→SO2(g)+H2​O

Stoichiometric Calculations for Gas Volume

1. Volume Calculation:

   • Using the equation PV = nRT ; rearranging gives volume.

2. Using given parameters:

   • Temperature in Kelvin: 298.15 K

   • Pressure in Torr is converted to Atmosphere for use in calculations.

3. Final Volume of Gas Released:

   • After plugging in the calculated values, the volume of sulfur dioxide released is 0.196 L or 196 mL.

Conclusion of Module 11

• Students are encouraged to practice questions in activity 11 and review textbook materials in preparation for upcoming examinations.